The Study of Elementary Electricity and Magnetism by Experiment

FUN WITH MAGNETISM. A book and complete outfit for Sixty-One Experiments.

FUN WITH ELECTRICITY. A book and complete outfit for Sixty Experiments.

FUN WITH PUZZLES. A book and complete outfit for Four Hundred Puzzles.

FUN WITH SOAP-BUBBLES. A book and complete outfit for Fancy Bubbles and Films.

HUSTLE-BALL. An American game. Played by means of magic wands and polished balls of steel.

JINGO. The great war game, including JINGO JUNIOR.

HOW TWO BOYS MADE THEIR OWN ELECTRICAL APPARATUS. A book containing complete directions for making all kinds of simple apparatus for the study of elementary electricity.

THE STUDY OF ELEMENTARY ELECTRICITY AND MAGNETISM BY EXPERIMENT. This book is designed as a text-book for amateurs, students, and others who wish to take up a systematic course of simple experiments at home or in school.

Don't fail to make at least a part of your own apparatus; there is a great deal of satisfaction and pleasure in home-made apparatus.

Don't experiment in all parts of the house, if working at home. Fit up a small room for your den, and carry the key.

Don't begin an experiment before you really know what you are trying to do. Read the directions carefully, then begin.

Don't rush through an experiment to see what happens at the end of it. See what happens at each step, and notice every little thing that seems unusual.

Don't try to do all parts of an experiment at the same time. Understand one part, then proceed.

Don't fail to ask yourself questions, and form an opinion about the results of an experiment before you read what the author has to say about it.

Don't fail to keep a note-book. Keep all the data and arithmetical work for future reference.

Don't leave the apparatus around after you have finished the day's work.

EXPERIMENT 1. To study steel.

[A] NOTE. Each piece of apparatus used in the following experiments has a number. See "Apparatus list" at the back of this book for details. The numbers given under "Apparatus," in each experiment, refer to this list.

3. Directions. (A) Bend a sewing-needle until it breaks. Is the steel brittle?

(B) If you have a file, test the hardness of the needle.

EXPERIMENT 2. To find whether a piece of hard steel can be made softer.

Fig. 1.

5. Directions. (A) Stick the point of the needle into Ck, Fig. 1, then hold the needle in the flame until it is red-hot. (The upper part of the flame is the hottest.)

(B) Allow the needle to cool in the air.

(C) Test the brittleness of the steel by bending it. Test its hardness with a file (Exp. 1).

EXPERIMENT 3. To find whether a piece of annealed steel can be hardened.

7. Directions. (A) Heat the bent portion of the needle in the candle flame (Exp. 2) until it is red-hot, then immediately plunge the needle into the water.

(B) Test its brittleness and hardness, as in Exp. 2.

EXPERIMENT 4. To test the hardening properties of soft iron.

9. Directions. (A) Test the wire by bending and filing.

(B) Heat the wire in the candle flame as you did the needle (Fig. 1), then cool it suddenly with the water. Study the results.

EXPERIMENT 5. To study the horseshoe magnet.

12. Directions. (A) Remove the armature, A, from the magnet, then move A about upon H M to see (1) if the curved part of H M has any attraction for A, and (2) to see if there is any attraction for A at points between the curve and the extreme ends of H M.

EXPERIMENT 6. To ascertain the nature of substances attracted by a magnet.

14. Directions. (A) Try the effect of H M upon the above substances, and upon any other substances thought of.

EXPERIMENT 7. To study the action of magnetism through various substances.

17. Directions. (A) Place the needle upon the paper and move H M about immediately under it.

(B) In place of the paper, try wood, glass, etc.

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(C) Invent an experiment to show that magnetism will act through your hand.

(D) Invent an experiment to show that magnetism will act through water.

EXPERIMENT 8. To find whether a magnet can give magnetism to a piece of steel.

19. Note. You have seen that the horseshoe magnet can lift nails, iron filings, etc.; you have used this lifting power to show that the magnet was really a magnet, and not merely an ordinary piece of iron painted red. Can we give some of its magnetism to another piece of steel? Can we pass the magnetism along from one piece of steel to another?

20. Directions. (A) Test the needles for magnetism with the iron filings, and be sure that they are not magnetized.

(B) Remove the armature, A, from H M, then touch the point of one of the needles to one pole of H M.

(C) Lay H M aside, and test the point of the needle for magnetism.

(D) If you find that the needle is magnetized, rub its point upon the point of the other needle; then test the point of the second needle for magnetism.

EXPERIMENT 9. To make small magnets.

22. Directions. (A) Hold H M (Fig. 3) in the left hand, its poles being uppermost. Grasp the point of the needle with the right hand, and place its point upon the N or marked pole of H M.

(B) Pull the needle along in the direction of its length (see the arrow), continuing the motion until its head is at least an inch from the pole.

(C) Raise the needle at least an inch above H M, lower it to its former position (Fig. 3), and repeat the operation 3 or 4 times. Do not slide the needle back and forth upon the pole, and be careful not to let it accidentally touch the S pole of H M.

(D) Test the needle for magnetism with iron filings, and save it for the next experiment.

EXPERIMENT 10. To find whether a freely-swinging bar magnet tends to point in any particular direction.

23. Note. An oily sewing-needle may be floated without the cork by carefully lowering it to the surface of the water. All magnets, pieces of iron and steel, knives, etc., should be removed from the table when trying such experiments. Why?

24. Directions. (A) Place the little bar magnet (the needle) upon the floating cork, turn it in various positions, and note the result.

EXPERIMENT 11. To study the action of magnets upon each other.

27. Directions. (A) Float each little bar magnet (needles) separately to locate the N poles.

(B) Leave one magnet upon the cork, and with the hand bring the N pole of the other magnet immediately over the N pole of the floating one. Note the result.

(C) Try the effect of two S poles upon each other.

(D) What is the result when a N pole of one is brought near a S pole of the other?

EXPERIMENT 12. To study the action of a magnet upon soft iron.

28. Directions. (A) Test the wire for magnetism with filings.[12] Be sure that it is not magnetized. If it shows any magnetism, twist it thoroughly before using. (Exp. 19.)

(B) Float the magnetized needle (Exp. 10), then bring the end of the wire near one pole of the needle and then near the other pole.

(C) Place the wire upon the cork, hold the needle in the hand and experiment.

EXPERIMENT 13. To learn how to produce a desired pole at a given end of a piece of steel.

30. Directions. (A) Magnetize a sewing-needle (Exp. 9) by rubbing it upon the N pole of H M from point to head. Float it and locate its N pole.

(B) Take another needle that has not been magnetized, and rub it on the same pole (N) from head to point. Locate its N pole.

(C) Magnetize another needle by rubbing it from point to head upon the S pole of H M; locate its N pole. Can you now determine, beforehand, how the poles of the needle magnet will be arranged?

33. Review; Magnetic Problems. To be sure that you understand and remember what was learned in Exp. 11, do these problems:

1. Using the S pole of the horseshoe magnet, magnetize a needle so that its head will become a N pole. Test with floating cork, as in Exp. 11.

2. Using the N pole of the horseshoe magnet, magnetize a needle so that its head shall be a S pole. Test.

3. Magnetize two needles, one on the N and one on the S pole of the horseshoe magnet, in such a way that the two points will repel each other. Test.

If the student cannot do these little problems at once, and test the results satisfactorily to himself, he should study the previous experiments again before proceeding.

EXPERIMENT 14. To find whether the poles of a magnet can be reversed.

34. Directions. (A) Magnetize W N so that its point shall be a S pole. Test with O C to make sure that you are right.

(B) Swing W N in the paper (Fig. 6), then slowly bring the S pole of H M near its point. Note result.

(C) Quickly bring the S pole of H M near the point. Is W N still repelled? Has its S pole been reversed?

EXPERIMENT 15. To find whether we can make a magnet with two N poles.

36. Note. You have already learned that the polarity of a weak magnet can be changed (Exp. 14). Can you think of any method by which two N poles can be made in one piece of steel?

37. Directions. (A) Place the needle upon H M, as in Fig. 7.

(B) Keeping the part, C, in contact with the N pole of H M, and using the N pole of H M as a pivot, turn the needle end for end so that its head will be in contact with the S pole of H M.

(C) Pull the needle straight from H M, being careful not to slide it in either direction.

(D) Test the polarity of the ends with O C (Fig. 5), and save it for the next experiment.

EXPERIMENT 16. To study the bar magnet with two N poles.

38. Directions. (A) Sprinkle filings over the whole length of the needle and then raise it (Fig. 8).

(B) Break the needle at its center, and test, with O C, the two[15] new ends produced at that point. Remember that repulsion is the test for polarity.

EXPERIMENT 17. To study consequent poles.

40. Directions. (A) Let w, x, y, and z stand for four places along the body of the needle, w being at its point and z at its head.

(B) Touch w with the N pole of H M, x with the S pole, y with the N pole, and z with the S pole. Do not slide H M along on the needle, just touch the needle as directed.

(C) Cover the needle with filings, then lift it.

EXPERIMENT 18. To study the theory of magnetism.

Fig. 9.

41. Directions. (A) Sprinkle some iron filings upon a sheet of paper.

(B) Bring one pole of B M in contact with the filings (Fig. 9), and lightly sweep it through them several times, always in the same direction. Are the filings simply pushed about?

(C) Do the same with a stick, and compare the result with that produced with B M.

EXPERIMENT 19. To find whether soft iron will permanently retain magnetism.

43. Directions. (A) Magnetize the wire (Exp. 9). Notice that the wire clings strongly to H M.

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(B) Test the lifting power of the little wire magnet by seeing about how many iron filings its poles will raise.

(C) Test the pointing power (§ 25) of the wire by floating it on F C (Fig. 4).

(D) Holding one end of the wire in the hand, thoroughly jar it by striking the other end several times against a hard surface.

(E) Test the lifting and pointing powers, as in B and C.

EXPERIMENT 20. To test the retentivity of soft steel.

45. Directions. (A) With H M magnetize the nail; this is made of soft steel.

(B) Test the lifting and pointing powers of W N (Exp. 19).

(C) Strike W N several times with a hammer to jar it.

(D) Again test its lifting and pointing powers.

EXPERIMENT 21. To test the retentivity of hard steel.

47. Directions. (A) Magnetize the needle with H M.

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(B) Test its lifting and pointing powers (Exp. 19).

(C) Hammer the needle and test again as in (B).

EXPERIMENT 22. To test the effect of heat upon a magnet.

48. Directions. (A) Test the needle for magnetism.

(B) Stick the needle into the cork (Fig. 1), and heat it until it is red-hot.

(C) Test the needle again for magnetism.

(D) See if you can again magnetize the needle.

EXPERIMENT 23. To test the effect of breaking a magnet.

Fig. 10.

50. Directions. (A) Break the little bar magnet (needle), and test the two new ends produced for magnetism, with the iron filings. (Fig. 10).

(B) Touch the two new poles together to see whether they are like or unlike.

(C) Test the nature of the poles with O C (Fig. 5)

(D) Break one of the halves and test its parts.

EXPERIMENT 24. To find whether we can magnetize a piece of iron without touching it with a magnet.

52. Directions. (A) Hold the armature of the magnet in a vertical position (Fig. 11), its lower end being directly in a little pile of iron filings.

(B) Bring the N pole of H M near the upper end of A, but do not let them touch each other.

(C) Keeping A and the pole of H M the same distance apart, lift them. Do any filings cling to A?

(D) Without moving or jarring A, take H M away from it and note result upon the filings.

EXPERIMENT 25. To find whether a piece of steel can be permanently magnetized by induction.

54. Directions. (A) Test the needle for magnetism.

(B) Place the unmagnetized needle upon the paper, then move H M about immediately under it, so that the needle will be attracted.

(C) Test the needle again for permanent magnetism.

EXPERIMENT 26. To study the inductive action of a magnet upon a piece of soft iron.

55 Directions. (A) Test the lower end of I W for magnetism with I F.

(B) Leaving I W upon the N pole of H M, test the pole at the lower end of I W with O C, to determine whether it is N or S.

(C) Jar I W (Exp. 19), then place it upon the S pole of H M, and again test the polarity of the lower end.

EXPERIMENTS 27–30. To study pole pieces.

57. Directions. (A) Suspend two wires, each about an inch long (Fig. 13) from one pole of H M. Do their lower ends attract or repel each other?

EXPERIMENT 28.

58. Directions. (A) Place the two wires just used so that one shall cling to the N pole of H M, and the other to the S pole of H M (Fig. 14).

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(B) Bring the lower ends of the wires near each other. Do they attract or repel each other?

EXPERIMENT 29.

59. Directions. (A) Bend a 2-inch iron wire, as in Fig. 15, and place it upon the poles of H M.

(B) See if its central part, marked X, will strongly attract filings.

EXPERIMENT 30.

60. Directions. (A) Bend the wire just used a little more, and place its ends upon one pole of H M (Fig. 16).

(B) See if the iron filings and small wires will cling to its central part.

EXPERIMENT 31. To study the space around a magnet, in which pieces of iron become temporary magnets by induction.

61. Directions. (A) Lay the paper upon the table, and place the compass over the center of the line, C L, previously drawn.

(B) Place the eye directly over the compass-needle, then turn the paper until the line is N and S; that is, until the line is parallel to the length of the needle. Pin the paper to the table to hold its center line N and S.

(C) Place B M upon the paper, as shown (Fig. 16½), its N pole to the north, and its center at the cross line, E W.

Fig. 16½.

(D) Slowly move the compass entirely around and near B M, and note the various positions taken by the needle. Note especially the way in which its N pole points. This is to get a general idea of the action of the needle.

(E) Place the compass in the position marked 1, which is on E W, about 1 in. from the line, C L. Press the wooden support down firmly upon the paper to show, by the dent made in the paper by the pin-head, the exact place on the paper that is under the center of the compass-needle. Before removing the compass from this position, look down upon it again, and make a dot on the paper with a pencil directly under each end of the needle. Remove the compass, and draw a line through the dent and the two dots just made. This will show a plan of the exact position of the needle.

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(F) Repeat this for the various points marked 2, 4, 6 in. from C L, always marking on the plan the position of the N pole of the needle. Do the same with the other points marked on Fig. 16½ by dots, and study the resulting diagram.

EXPERIMENT 32. To study the magnetic field of a bar magnet.

63. Directions. (A) Place B M upon the table, and lay the paper over it.

(B) With the sifter sprinkle some filings upon the paper directly[25] over B M, then tap the paper gently, to assist the particles to take final positions. Study the results.

EXPERIMENTS 33–37. To study the magnetic fields of various combinations of bar magnets.

65. Note. The student will find it very helpful to make the magnetic figures of the combinations given. Thoroughly magnetize the bar magnets upon an electro-magnet, or upon a strong horseshoe magnet, and mark their N poles in some way. The N poles may be marked by sticking a small piece of paper to them.

66. Directions. (A) Arrange the two magnets, B M, as in[26] Fig. 18, with their unlike poles about an inch apart. (The dotted circle indicates the iron ring to be used in the next experiment. About a quarter, only, of the magnets are shown.)

(B) Place the paper over the magnets, and sift filings upon it immediately over the unlike poles. Note particularly the lines of filings between N and S.

(C) Make a sketch of the result. (See experiments with electromagnets, and the illustrations of magnetic figures with them.)

EXPERIMENT 34.

67. Directions. (A) Leaving the opposite poles an inch apart, as in Exp. 33, place the iron ring, I R (No. 23), between them (Fig. 18, dotted circles).

(B) Place the paper over it all, and sprinkle filings upon it to get the magnetic figure.

(C) Make a sketch of the resulting figure, and compare it with the figure made in Exp. 33. Why do the lines of force appear indistinct in the center of the ring and around it? (See §74.)

EXPERIMENT 35.

68. Directions. (A) Arrange the two bar magnets, as in Exp. 33, but with their two N poles an inch apart.

(B) Make the magnetic figure of the combination. Do the lines of force flow from one N pole directly to the N pole of the other? Do the particles of filings reaching out from one B M attract or repel those from the other B M?

EXPERIMENT 36.

69. Directions. (A) Place the two bar magnets side by side, so that their unlike poles shall be arranged as in Fig. 19.

(B) Make the magnetic figure.

EXPERIMENT 37.

70. Directions. (A) Turn one B M end for end, so that their like poles shall be near each other, but otherwise arranged as in Fig. 19.

(B) Make and study the magnetic figure.

EXPERIMENTS 38–39. To study the lifting power of combinations of bar magnets.

71. Directions. (A) Find out about how many filings you can lift with the N pole of one magnet.

(B) Place the two magnets together (Fig. 20), their like poles being in contact; then see whether the two N poles will lift more or less filings than one pole.

EXPERIMENT 39.

72. Directions. (A) Remove all filings from the two magnets just used, and hold them tightly together (Fig. 20), with their unlike poles in contact.

(B) Compare the amount of filings you can lift at one end of this combination with that lifted in Exp. 38 (A) and (B).

EXPERIMENTS 40–42. To study the magnetic field of the horseshoe magnet.

75. Directions. (A) Place H M, with its armature removed, flat upon the table, and cover it with the paper; then make the magnetic figure. (Exp. 32.)

(B) Compare the number of well-defined curves at the poles with the number at the equator.

EXPERIMENT 41.

76. Directions. (A) Make the magnetic figure of H M with its armature in place.

(B) Is the attraction for outside bodies increased or decreased by placing the armature on H M?

EXPERIMENT 42.

77. Directions. (A) Lay H M flat upon the table, and place one or two matches between its poles and the armature; cover with paper as before, and make the magnetic figure. Do lines of force still pass through the armature?

EXPERIMENT 43. To show that lines of force are on all sides of a magnet.

Fig. 21.

79. Directions. (A) Sprinkle iron filings upon the paper, and study the resulting magnetic figure.

(B) Place O C upon the paper in different positions. Does the magnetic needle always come to rest about parallel to the lines of filings?

EXPERIMENT 44. To study a horseshoe magnet with movable poles.

81. Directions. (A) Magnetize the spring steel, S S.

(B) Bend S S until its poles are about ¼ in. apart, then using it as a horseshoe magnet, and keeping its poles the same distance apart, see about how many filings you can lift.

(C) Clean the poles of S S, press them tightly together, then again test its lifting power with filings.

EXPERIMENT 45. To study the lines of force above and below a bar magnet placed horizontally.

85. Directions. (A) Lay B M upon the table and place O C upon its center. Note the position of the compass-needle.

(B) Slide O C along from one end of B M to the other, and study the effect upon its needle. Do lines of force curve over B M as well as around its sides, as shown in Exp. 31?

(C) Place O C upon the table. Hold B M horizontally above O C, and move O C back and forth under B M. Does the needle remain horizontal, or does it show that lines of force pass under B M on their way from its N to its S pole?

EXPERIMENT 46. To study the dip or inclination of the magnetic needle, due to the action of the earth.

87. Directions. (A) Place O C upon the table, and mark upon a piece of paper the height of the N pole of its needle above the table. (Fig. 25.) The paper should be held in a vertical position, and near the pole.

Fig. 25.

(B) With H M reverse the poles of the compass-needle (Exp. 13), so that its former N pole shall become a S pole.

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(C) Place the needle upon its pivot again, and mark upon the paper, as before, the height of its new N pole above the table. Does the needle remain horizontal?

(D) Remagnetize the needle, and reverse its poles so that it will again balance.

EXPERIMENTS 47–48. To study the inductive influence of the earth.

Apparatus for Exps. 47–48. Compass, O C, (No. 18); an iron stove poker, or other rod of iron; a hammer. (The iron and hammer are not furnished.)

89. Note. You have seen (Exp. 24), that iron becomes magnetized by induction when placed near a magnet. As the earth acts like a huge magnet, having poles, lines of force, etc., will it magnetize pieces of iron which are in the air or upon its surface?

90. Directions. (A) Test the poker for poles with O C, remembering that repulsion is necessary to prove that it is polarized.[34] If the poker has very weak poles, proceed; but if it shows some strength, hold it in an east and west direction, and hit it several sharp blows on the end with the hammer. Test for polarity again.

(B) With one hand hold the poker in the N and S line, give it a dip toward the north, and strike it several times with the hammer to thoroughly stir up its molecules.

(C) Test again for poles with O C, and note especially whether the lower end (of the poker) became a N or a S pole.

EXPERIMENT 48.

91. Directions. (A) Turn the poker end for end (See Exp. 47); repeat the striking, and test again the pole produced at the lower and north end of it.

(B) Now hold the poker horizontally in the east and west line, and pound it.

(C) Test for poles. Has this strengthened or weakened the poker magnet?

EXPERIMENT 49. To test the effect of twisting a wire held north and south in the earth's magnetic field.

Note. You have seen that we can pound magnetism into or out of a piece of iron at will. Can we twist it into a wire and out again without the use of magnets?

94. Directions. (A) Test the wire for poles with O C.

(B) Hold the wire in a N and S direction, dipping it at the same time, as directed in Exp. 47 for the poker, and twist it back and forth.

(C) Test again for poles with O C. As the poles of the wire may be very weak, bring them slowly toward the compass-needle (see Exp. 14), and note the first motions produced upon the needle.

(D) Hold the wire horizontally east and west, twist and test again. Has its magnetism become weaker or stronger than before?

EXPERIMENT 50. To test for magnetism in bars of iron, tools, etc.

95. Directions. (A) With the compass test the ends of the above for magnetism, and note which ends are S.

EXPERIMENTS 51–52. To study electrification by friction.

101. Directions. (A) Examine E S. Note that its surface is not smooth, like that of ordinary hard-rubber combs. Can you think of any reason for this?

(B) Hold its flat surface near your face, then near the back of your hand. Do you feel anything unusual?

(C) Lay E S upon a flat board, or uncovered wooden table, and slide it about. Can you easily pick it up?

[40]

(D) Place E S flat upon the table again; keep it from sliding about with your left hand, and rub it vigorously for a minute with F C. Does E S act exactly as it did before in (B) and (C)?

(E) Repeat the experiment in a dark room.

(F) Thoroughly electrify E S, and see if it will cling to the wall strongly enough to support its own weight.

EXPERIMENT 52.

105. Directions. Repeat Exp. 51, but in place of the ebonite, use hot tissue-paper, hot brown paper, hot newspaper, or a hot silk handkerchief. Rub your hand vigorously over them. Do these become charged?

EXPERIMENTS 53–54. To study electrical attractions.

106. Directions. (A) Thoroughly electrify E S as before, then lift and hold it in the air. (Fig. 28.)

(B) See what the paper and thread will do when held loosely near E S.

EXPERIMENT 54.

108. Directions. Charge a sheet of hot paper by friction; lift it, by its opposite ends, and lower it over small pieces of tissue-paper placed on the table. What happens to the little pieces?

EXPERIMENT 55. To study mutual attractions.

Tie one end of S T to W S, Fig. 29; tie the other end of S T to S W; adjust W S by bending, if necessary, so that it will securely hold E R. It will be found convenient to use a rubber band instead of S T; if you do, let W S straddle one end of R B (Fig. 33), and hang the other end of R B upon S W.

109. The support consists of a support base (S B, Fig. 56), a support rod (S R, Fig. 56), and a support wire (S W, Fig. 29). There is a small hole in one end of S R to receive the wire, S W, and a large hole in the other end to take the short ebonite which holds the insulating table (Fig. 32). A little paper should be wound around the lower end of S R, so that it will stand solidly in the spool which forms a part of the base.

110. Directions. (A) Electrify E R with F C, and place E R in the swing, W S (Fig. 29).

Fig. 29.

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(B) Bring your finger near one side of the rubbed end of E R, then near the unrubbed end, and compare the results.

EXPERIMENT 56. To study electrical repulsions.

112. Directions. (A) Charge E R, and place it in W S, Fig. 29.

(B) Charge E S, and bring it slowly near one side of the charged end of E R.

EXPERIMENT 57. To study electrical repulsions.

113. Directions. (A) Heat the paper, then place it flat upon the table and electrify it by rubbing it with your hand. You must rub away from the uncut part, or you will break the legs.

(B) Raise T P, holding it by the uncut part. Note the action of legs, and make a sketch of them.

EXPERIMENT 58. To study electrical repulsions.

115. Directions. (A) Electrify E R, then hold it near the carbon of the electroscope.

(B) Bring the charged rod near little pieces of damp tissue-paper.

EXPERIMENT 59. To study the electrification of glass.

117. Directions. (A) Vigorously rub the hot glass with the silk (or flannel), also heated.

(B) Test G for electrification by means of little pieces of tissue-paper and the carbon electroscope, Exp. 58.

EXPERIMENT 60. To compare the electrification produced by ebonite and flannel with that produced by glass and silk.

119. Directions. (A) Electrify E R, and place it in W S, Fig. 29.

(B) Bring the uncharged glass near E R, noting the action of E R.

(C) Heat and electrify G; bring it near E R, and carefully note whether the attraction between them is stronger or weaker than before, or whether they repel each other.

EXPERIMENT 61. To study insulators.

122. Directions. (A) Holding one end of E R in the hand, charge the other end by rubbing it with F C.

(B) With bits of the T P test each end of E R for a charge, and compare the results.

EXPERIMENT 62. To study insulators.

123. Directions. (A) Thoroughly electrify E S (Exp. 51, D), then lift and hold it in the air, as in Fig. 28.

(B) By moving your rounded knuckle about near the surface of E S, see if you can get more than one spark from it.

EXPERIMENT 63. To study insulators.

124. Directions. (A) Electrify the comb with F C.

(B) Move the nail along near the teeth of the comb, and listen carefully.

EXPERIMENT 64. To study conduction.

127. The Insulating Table consists of a tin box (exactly like that used for the electrophorus cover), and an ebonite rod about 1¾ in. long. See § 139 for full details about fitting the rod into the box, etc. The lower end of the short rod fits into the large hole in one end of the support rod, S R. Arrange as in Fig. 32. B F B should swing about very easily.

128. Directions. (A) Charge E S, then rub it upon I T, as shown, noting the action of B F B.

EXPERIMENT 65. To study conduction.

130. Telegraph Line. To have our telegraph line using frictional electricity complete, we must have: (1) Some way of generating or making the electricity; (2) Some means of getting it or its effects to the other end of the line; (3) Some way of showing that it has been taken there.

The charged E S will be the source of the electrification. New York will represent the end at which we send the message, so at N. Y. we must have a sending instrument. See Fig. 33, which explains itself. R B or a silk thread must be used to insulate the sender. Around one leg of W S is twisted one bare end of the conductor, C W.

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Boston will represent the end of the line at which the message is received, and there we need a receiving instrument. This is similar to the apparatus described in Exp. 69, Fig. 37. In addition to this, tie the middle of a moist cotton thread that is 6 in. long, to B C (Fig. 37), and let its two free ends lie over the top and reach down against the bottom of the tin; that is, on the left-hand side. Fig. 42 will give you an idea in regard to the looks of the thread; at first, however, it should be close to the bottom of the tin. Twist the other bare end of the copper wire around B C.

When the line is properly constructed and ready for use, both instruments and C W are entirely insulated. Do not let any part of C W touch the table or your clothing.

131. Directions. (A) Touch the insulated sending instrument with the charged ebonite sheet, and watch for any motion in the receiving instrument.

Note. Better results will be obtained by using the charged electrophorus cover as the source of electrification, instead of E S. (Exp. 68.)

EXPERIMENT 66. To study the effect of moisture upon an insulator.

135. Directions. (A) See if a charge can be sent through the thread, in the same manner as it was through the copper. Is dry silk a conductor?

(B) Thoroughly wet the thread, being careful not to wet the rubber band insulator (Fig. 33); see if wet silk is a conductor.

EXPERIMENT 67. To test the effects of moisture upon bodies to be electrified.

137. Directions. (A) Heat one piece to make it thoroughly dry, and leave the other cold.

(B) Stroke each, say 10 times, with your hand, pressing them upon the table; then place them upon the wall at the same time, being careful not to let them touch your clothing. See which will cling to the wall the longer.

139. Our Electrophorus is shown in Fig. 34. For the insulating handle use the ebonite rod, E R (No. 28); for the plate, use the ebonite sheet, E S (No. 26). The electrophorus cover, E C (No. 42), furnished, is a tin box with a fancy top. A hole has been punched in the center of its top, and into the hole has been riveted a short tube, so that the handle, E R, can be firmly held. The hole has been made a little larger than E R for convenience. To make E R fit tightly in the hole, so that you can lift E C, wrap a small piece of paper around the end of E R before pushing it into the hole. You can easily find out how much paper to use to make a good fit. With a knife cut away all loose points of paper that stick out of the hole around E R; this is important. The top and bottom of E C should be pressed firmly together.

First learn how to use the electrophorus. With the large amount of electrification produced we can then find out how it works.

EXPERIMENT 68. To learn how to use the electrophorus.

140. Directions. (A) Place E S upon a flat, uncovered, wooden table, and rub it vigorously for a minute with the warm flannel, F C, to thoroughly charge it. Do not let E S slide about, and do not lift it from the table.

(B) With the right hand grasp E R at its extreme end, and place E C upon E S.

(C) Touch E C for an instant with a finger of your left hand (Fig. 35).

(D) Remove your finger entirely from E C, then lift E C by its insulating handle, E R, at the same time holding E S down to the table, if it tries to follow E C.

Fig. 35.

Fig. 36.

(E) Bring your left hand near E C (Fig. 36). You should get a good spark from E C.

(F) It is not necessary to immediately rub E S again. You have discharged E C by taking a spark from it. To recharge it, simply place it upon E S again; let it remain there while you count 5; touch it as before, and then lift by E R.

141. Extra Notes. You may repeat the above operation many times. As soon as the sparks begin to get small, electrify E S again. The charge on E C is +, although that on E S is -. You will understand, later, why this is so.

If you do not get a good spark from the electrophorus, read the directions again. The ebonite must be well electrified; the cover[54] must be lifted by the end of its handle; you must touch the cover and withdraw your finger from it before lifting. You must allow the cover to remain upon the ebonite 3 or 4 seconds each time. The board, or table, upon which E S rests, must be flat, and not warped, so that E C will fit down perfectly upon E S.

EXPERIMENT 69. To study "charging by conduction."

142. Directions. (A) Charge E C (Exp. 68), and bring it near B F B (Fig. 37). Note the spark.

(B) Repeat (A) twice, noting the relative sizes of the sparks. Does B F B continue to be attracted by E C?

(C) Bring your knuckle slowly towards the charged disk, B F B.

EXPERIMENT 70. To study potential; electro-motive force.

143. Directions. (A) Pass a spark from the thoroughly charged E C (Exp. 68) to I T.

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(B) Recharge E C, and see how many times I T will take good sparks from it, and note the relative sizes of the sparks.

(C) As soon as I T refuses to take more sparks from E C, touch E C to see if it is completely discharged.

(D) Touch I T.

145. Theories about Electrifications. The "One-Fluid" Theory suggests that neutral bodies have a certain amount of electrification, and that they have a certain potential called zero[56] potential. If the potential of a body becomes greater than that of the earth, the body is said to be positively electrified; if the potential of the body is less than that of the earth, it is said to be negatively electrified. If we fill a bottle with sea water, we have a great deal of water when we compare it with the bottle, but a very little water when we compare it with the sea. The earth is so large that small amounts of electrification taken from it or added to it do not affect its potential to any extent.

146. The "Two-Fluid" Theory suggests that there are two absolutely different kinds of electrification, one called positive (+), and the other negative (-). When these two are equal in quantity, the body is said to be neutral. If the body contains more + than -, the body is said to be charged positively.

It is evident then, if the two-fluid theory be accepted, that no matter how strongly a body is charged positively there must be in it some negative electrification; that is, we may charge a neutral body + by adding + electrification to it, or by taking - electrification from it. There must always be, then, some + and - electrifications in a body.

These theories do not require much consideration by the student of elementary electricity. The best thing he can do is to learn what electricity can do, and how it can be used.

EXPERIMENT 71. To study some methods of discharging an electrified body.

147. Note. You have seen sparks pass from E C to your rounded knuckle, and to other conductors. In all of these cases the discharge was sudden, one spark doing the work. Can we slowly discharge E C, or discharge it without sounds?

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148. Directions. (A) Thoroughly charge E C, and test it with your knuckle to be sure that it is working properly.

(B) Charge E C again; hold the pin in your left hand (Fig. 39), and slowly bring its head toward E C; listen for sparks.

(C) Recharge E C, and bring the point of the pin slowly toward it. Touch E C to see whether it has been discharged or not.

EXPERIMENT 72. To study intermittent or step-by-step discharges.

150. Directions. (A) Charge E C, then hold your hand on one side of the carbon (Fig. 40), and hold E C upon the opposite side. What should the carbon do?

EXPERIMENT 73. To ascertain the location of the charge upon an electrified conductor.

Fig. 41.

152. Directions. (A) Charge E C; pass a spark to T B, and note the action of both parts of C T.

EXPERIMENT 74. To study the effect of points upon a charged conductor.

154. Directions. (A) Charge E C; test its charge with your knuckle. Be sure that you get a good spark.

(B) Charge E C again, and hold it by its insulating handle, E R, long enough to count 10 before discharging it with your knuckle. Be sure that it holds its charge during this time.

(C) While E C is upon E S (Fig. 34), lay the bent pin upon E C, so that its point will project into the air. The point should stick out about ¼ in. from the edge of E C.

(D) Touch E C; raise it by E R; count 10 as before; then test with your knuckle to see if E C is still charged.

157. Note. You have seen that we can charge an insulated conductor by touching it with the charged cover, or by allowing a spark to pass to the conductor. What effect, if any, has a charged body upon an insulated conductor before they touch each other, and before any spark passes to the conductor?

EXPERIMENT 75. To study electric induction.

158. Directions. Part 1.—(A) Pass a spark from the charged E C to T B, and note the action of the thread, which will be our electroscope. Remove E C.

(B) Touch the charged T B with the finger, watching C T.

Part 2. (C) Bring the re-charged E C near the neutral T B, and parallel to its end surface; but keep them at least an inch apart, so that a spark cannot pass. Watch C T.

(D) Withdraw E C, and try to explain the action of C T.

160. Note. Figs. 43 and 44 may aid the student. In Fig. 43, T B is supposed to be neutral. The "double sign" means that the + and - electrifications are united; and, as there are an equal number of both kinds, none are left free to tell the tale. Fig. 44 shows what happens when the + E C is near.

What would happen if we could cut into T B at the middle with an insulated knife while it is polarized by E C?

EXPERIMENT 76. To learn how to charge a body by induction.

161. Directions. (A) Bring the charged E C within an inch of the bottom of T B, and as soon as C T is repelled, showing that T B is polarized (Exp. 75), touch T B with your finger; then remove your finger while you still hold E C in place.

(B) Withdraw E C and its inductive action. Explain the motions of C T during the experiment. Is it still repelled by T B after E C is removed?

EXPERIMENT 77. To show that a neutral body is polarized before it is attracted by a charged one.

163. Directions. (A) Place the bits of dry T P upon a board or table, and convince yourself that they are attracted equally by the charged E C.

(B) Slightly moisten one piece of T P only. See if one is attracted by E C more readily than the other.

EXPERIMENT 78. To find whether electric induction will act through an insulator.

165. Directions. (A) Charge the electrophorus cover, E C (Exp. 68), move it about a little above the glass, and see if the carbon, etc., are attracted.

EXPERIMENT 79. To find whether a polarized conductor can act inductively upon another conductor.

167. Directions. (A) Hold E C, charged, near and under I T, then bring your finger, F, near T P. Explain the action of T P.

EXPERIMENT 80. To study the action of the electrophorus.

170. Directions. (A) Thoroughly electrify E S, Fig. 34, and place E C upon it by its handle, E R.

(B) Touch E C, as directed in Exp. 68, and listen for a small spark which should pass from E C to your finger.

(C) Again, place a little piece of T P upon E C before lowering it upon E S. Do not touch E C, but bring your finger near T P. What does T P do? Now, touch E C and see, when you bring your finger near it, if T P acts as it did before.

(D) Again, place several pieces of T P upon E C (E S being thoroughly charged); touch E C, then lift it by its handle. Note action of T P, which should be slightly moist.

172. Details of Action. The different steps in the action of the electrophorus are shown graphically in Figs. 47 to 51. Fig. 47 shows E S negatively charged. E C is neutral at first, Fig. 48; that is, it is supposed to contain both + and -, as shown by the "double sign" (§ 160). Fig. 49 shows that E C has been polarized by the inductive action of E S. The repelled - escapes to the finger (this escaping is what gave the small spark to the finger and charged the T P in the last experiment), leaving the top uncharged, while the + is bound (Fig. 50). As soon as E C is[67] lifted (Fig. 51) the + spreads all over E C, which is then charged. The +, upon going to the top, charged the pieces of T P (Exp. 80, D), causing them to be repelled. The charge of - upon E S has not been removed, so the operation may be repeated many times before E S must be again electrified.

The - electrification on the ebonite acts inductively through E S, drawing up + electrification from the earth. To make this action easier a "sole," or metal conductor, is often placed under the ebonite.

EXPERIMENT 81. To see, hear, and feel the results of inductive influence and polarization.

173. Directions. (A) Thoroughly charge E S with F C. With the right hand bring E S near and parallel to the top surface of I T, but do not let them touch each other.

(B) Remove E S, then touch I T to see if it is charged.

(C) Repeat (A), and while you hold E S about ½ inch from I T, their flat surfaces being parallel, touch I T. Watch for any sparks, and note any peculiar actions of E S.

(D) Remove your finger from I T, then withdraw E S; finally touch I T with your knuckle.

EXPERIMENT 82. To find whether a large surface will hold more electrification than a small one.

175. Directions. (A) Test the electrophorus and be sure that it is working properly.

(B) As in Exp. 70, see how many good sparks I T will take from E C (which should be recharged at each trial) before the potential of I T is raised so that it equals the potential of E C.

(C) Carefully set the basin or pan upon I T, then count the number of good sparks you can pass to it from E C (recharged at each trial). Compare the number of sparks necessary to raise the potential of the large surface until it equals that of E C, with the number found in part (B).

EXPERIMENT 83. To find whether the capacity of a given conductor can be increased without increasing its size.

177. Directions. (A) See how many good sparks I T will take from E C. Re-charge E C at each count, and note the relative sizes of the sparks.

(B) Discharge I T by touching it with your knuckle.

179. The Leyden Jar consists of a wide-mouthed glass jar, with tin-foil pasted upon the inside and outside to within 2 or 3 inches of the top. The inner coat or conductor is connected to a knob or ball at the top by means of a chain. To charge the jar, the outer coat is connected with the earth by holding it in the[70] hand, or by resting it upon a table while the electrification is passed to the knob. A Leyden Battery consists of 2 or more connected jars, the object being to increase the area of the surface. The jar is discharged by touching one end of a discharger (§ 188) to the outer coat, and swinging its other end over to the knob, when a bright spark will pass between the knob and discharger. (See Exp. 86.)

180. Fulminating Panes, or Franklin's Plates, are practically the same as a Leyden Jar. The tin-foil, however, is pasted upon the opposite sides of a pane of glass, a margin of about an inch being left all around. One side of the pane is charged, and takes the place of the inside coat of the jar. The other side is grounded. The pane is discharged by connecting the two sheets of foil.

181. Induction Coil Condensers consist of sheets of tin-foil separated by sheets of paraffined paper, which act as the dielectric. (See Induction Coils.)

182. Submarine Cables, with the surrounding water, act like condensers, the result being that the condensing effect slows up the electric current and retards the signals. These make a condenser of enormous capacity. The wires inside form one conductor, and the water the other, while the insulation around the wires forms the dielectric.

EXPERIMENT 84. To study the condensation of electrification.

183. Directions. (A) Pass good sparks to I T from the charged E C until something happens. Watch the side where I T and F B are near each other.

EXPERIMENT 85. To study the action of the condenser.

185. Directions. (A) See that your electrophorus works properly, then find out how many good sparks you can pass from E C to F B, recharging E C each time. Note the relative sizes of the sparks, and compare the result with the number taken by the condenser in the last experiment.

(B) When F B seems to be fully charged, touch I T with your knuckle. (From your study of induction what should be the result?)

(C) Now see if F B will again take good sparks from the charged E C. Pass sparks to F B until it seems fully charged.

(D) Again touch I T, then repeat (A) and (B) several times, until a bright spark passes from F B over the edge of E S to I T.

E C was positively charged (Exp. 80). This also charged F B positively by contact. F B acted inductively through the dielectric, E S, drawing up some of the - in I T, and repelling some of the +. As I T was insulated, this free + electrification could not escape. Before we touched I T, its + and - electrifications, although partially separated, were struggling against this inductive action; and, on account of their strong attraction for each other, our efforts to charge the condenser were retarded. Upon touching I T the free + escaped to the earth. (This was the cause of the spark.) This left some - electrification bound on the underside of E S, and some + bound on the upperside of E S. The capacity of F B was increased by this process, as the +[72] already put into it was very much occupied by the attractions of the induced - just under E S. As more + was given to F B, more - was drawn up under E S and more + was pushed out of I T. This action went on until the two conductors were strongly and oppositely charged. This action goes on continuously when the lower conductor is grounded. The spark between the tins was due to the rushing together of the + and - electrifications; it showed that there was a momentary current of electricity.

EXPERIMENT 86. To study the effect of electrical discharges upon the human body.

187. Directions. (A) Charge the condenser (Exp. 83) with 10 good sparks from E C, then touch I T (Fig. 52).

(B) Recharge the condenser with 10 sparks, then touch F B. Discharge it by again touching I T as in (A).

(C) Recharge with 10 sparks; then place your thumb against F B, and quickly swing the first finger of the same hand over to I T, and get a slight shock.

(D) Recharge with as many sparks as you think you can stand.

(E) Instead of using your hand to discharge the condenser, try the bent hairpin. Keeping one end against F B, swing the other end over near I T.

EXPERIMENTS 87–88. To show the strong attraction between the opposite electrifications in the condenser.

189. Directions. (A) Charge the condenser with 15 or 20 good sparks from E C.

(B) Lift the condenser by one corner of G (Fig. 54), being careful not to discharge it. Explain why the lower conductor follows the glass.

EXPERIMENT 88.

190. Directions. (A) Charge and lift the condenser as just explained (Exp. 87). Fig. 54.

(B) With your right hand touch the upper tin alone, then the lower tin alone.

(C) Touch both tins at the same time, and note the action of the lower one.

EXPERIMENT 89. To show how the condenser maybe slowly discharged.

192. Directions. (A) Charge the condenser by means of the[74] electrophorus; then hang the carbon so that it can swing between the upper conductor and E C placed as shown.

EXPERIMENT 90. To ascertain the location of the charge in the condenser.

194. Directions. (A) Charge the condenser with 15 or 20 good sparks from E C.

(B) Lift I T away from E S by its insulating handle, and set it upon the table. (It may be necessary to hold E S down.)

(C) Lift E S directly up and away from F B. (Lift by 2 corners; do not scrape E S along on F B; do not allow E S to touch your clothing.)

(D) Replace E S and then I T by its handle quickly, making the condenser complete again.

(E) With H P D see if the condenser still holds a charge. Touch F B first (Exp. 86).

EXPERIMENT 91. To find whether any electrification remains in the condenser after it has once been discharged.

196. Directions. (A) Thoroughly charge the condenser.

(B) Discharge it with H P D, being sure to touch F B first, and to touch I T for an instant while H P D is against F B.

(C) After a few moments use H P D again, and see if you get a slight spark.

EXPERIMENT 92. To study successive condensation; the chime cascade.

198. Directions. (A) Pass 15 or 20 good sparks from E C to the under side of I T (Fig. 56), noting the action of H P.

(B) Hold E C in the hand, and, with its insulating handle, poke H P away from the condensers. Do not discharge them.

(C) With H P D test the lower condenser for a charge, touching T B first.

(D) With H P D touch T F B first (why?), and discharge the upper condenser.

201. Our Leaf Electroscope (Fig. 57) is made with aluminum-leaf. Gold-leaf is too delicate for unskilful handling, and aluminum will do for all ordinary experiments. To cut it into any desired shape, place it between two sheets of paper, then cut through paper and all.

202. Construction. Bend one leg of a hairpin, H P, as in Fig. 57, and slide it onto I T. Hang a wire, W, or another hair pin straightened, then bent, from the horizontal leg of H P. This is to support the "leaves," L, which are made from a strip of[78] aluminum-leaf about 4 in. long and ¾ in. wide. Moisten the under side of the horizontal part of W with paste or mucilage; press it upon the middle of the strip laid flat upon the table, and then lift W. The leaves should cling to W. Each leaf should be, then, 2 in. long. They should hang close together when not in use. A large chimney, or fruit-jar, may be used to surround the leaves, and to keep currents of air from them. The leaves should not touch the side of the jar when spread.

EXPERIMENT 93. To study the leaf electroscope; charging by conduction.

203. Directions. (A) Thoroughly charge E R, then scrape it along upon I T, noting the action of the leaves, L.

(B) See if the leaves will remain spread for some time.

(C) Touch I T to discharge it, and note the action of L.

EXPERIMENT 94. To charge the leaf electroscope by induction.

205. Directions. (A) Charge E S with F C, then hold E S above I T (Fig. 57), their surfaces being kept parallel and about 2 or 3 inches apart. Watch the leaves.

(B) Withdraw E S. Do the leaves remain spread?

(C) Repeat (A), and before removing E S, touch I T.

(D) Remove your finger from I T, then withdraw E S. Do the leaves now remain spread?

EXPERIMENT 95. To learn some uses of the electroscope.

207. Directions. (A) With the charged E R charge the electroscope negatively by conduction (Exp. 93). Note the amount of permanent divergence of the leaves.

(B) Electrify the glass, which will be +, (or use the + E C), and slowly lower it over I T, noting the effect upon L. Raise and lower G or E C several times. Does G, which has an opposite charge to the electroscope, make L diverge more or less?

(C) Discharge the electroscope and recharge as in (A).

(D) Slowly lower the charged E S over I T.

(E) Slowly lower the palm of your hand over I T.

Note. If the + G is brought too near the -ly charged electroscope, L will first collapse and then instantly diverge again with a + charge by contact. The first motions should be observed.

EXPERIMENT 96. To show that friction always produces two kinds of electrifications.

210. Directions. (A) Vigorously rub E S with F C (folded as in Fig. 58). See if you can discover any attraction between them.

(B) Rub E S again, but do not lift F C from it with the hand alone. Slip E R under the top fold in F C (Fig. 58), and lift F C straight up from E S. Do not let F C touch the table or your hand.

(C) See if F C is charged, using 2 or 3 different tests.

(D) Charge the electroscope with F C until the carbon is strongly repelled.

(E) Bring the positively charged E C slowly near the carbon, and note the result.

(F) Slowly bring the negatively charged E S near the carbon that has been charged by contact with F C.

EXPERIMENT 97. To show "successive sparks."

212. Directions. (A) Thoroughly charge the electrophorus cover.

(B) Place your finger upon the coin marked A, to "ground" it, then quickly touch the coin C with the charged cover, at the same time watching for sparks between the coins. If you cannot see the sparks, darken the room a little, and look at the center coin, B, while doing the experiment.

EXPERIMENT 98. To show to the eye the strong attraction between a charged and a neutral body.

213. Directions. (A) Stand F B upon its edge upon a level table, then bring the charged electrophorus cover near it.

(B) Instead of the above, use light hoops made of paper, eggshells, feathers, sawdust, etc.

EXPERIMENT 99. To feel the strong attraction between a charged and a neutral body.

214. Directions. (A) Hold F B in the left hand, as shown, then slowly bring near it the charged cover, at the same time looking between them so that you can keep them the same distance apart all the way round.

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(B) Bring them near enough to allow a spark to pass from E C to F B.

EXPERIMENT 100. The human body a frictional electric machine.

215. Directions. (A) Scrape your feet along upon the carpet, then quickly touch your finger to some conductor, as, for example, a friend's nose.

(B) It is possible to light the gas by the above process. Have a friend turn on the gas just before you bring your finger to the jet, and be sure that the spark from your finger passes through the gas on its way to the conductor, the jet.

(C) Bring your finger quickly near a small piece of tissue paper after you have scraped your feet along to charge your body.

Note.—Before taking up the study of cells and the electric current, let us perform a few experiments in order to understand the construction and use of some of the apparatus needed for such study. A dry cell will be used as the source of the electricity for these first experiments, because it is convenient. You will understand its action later. Use this cell only as directed; improper use of it might spoil it.

EXPERIMENT 101. To study the effect of the electric current upon the magnetic needle.

226. Electrical Connections. One must constantly join wires, connect wires with apparatus, or connect one piece of apparatus to another, to make the proper electrical connections. A very simple method of connections has been used in all the apparatus described in these experiments.

A little arrangement which we shall call a spring connector, S C, (Fig. 61), gives us a means of quickly making connections; that is, it does away with expensive binding-posts. It is made of brass, nickel plated, and may be used anywhere without affecting the magnetic needle.

Six or eight wires, about No. 24 gauge, each about 1½ ft. long, should be prepared with a connector at each end. You may use wire furnished (No. 53). Scrape the insulation from the ends of the wires for about 1½ inches, then twist the bare ends around[90] the connectors as shown in Fig. 61. The wire should pass around tightly at least 4 or 5 times and then be twisted a little, as shown, to help tighten it. Do not put it on so poorly or in such quantity that the part, B, will spread.

227. Fig. 62 shows how the connector should be slipped upon a thin piece of metal, M, like that on the galvanoscope, for example. The wire, W, from the apparatus itself is permanently fastened under the head of the screw, S, while the wire from any other apparatus is one of those kept on hand as above mentioned and connected with S C.

Fig. 62.

Fig. 63.

Fig. 64.

228. Fig. 63 shows how several wires may be quickly joined, electrically, by slipping the connectors at their ends upon a thin metal plate, M P, which may be a piece of tin, zinc, copper, etc. M P should not be too thick. In case the connectors become too much spread to pinch the plate, squeeze the part, A, a little more together.

229. Fig. 64 shows the method of connecting with the special form of binding-post used, for example, upon the resistance coil, R C. The end, C, of S C, is pressed down into the tube, T, until you feel, by moving it, that it springs firmly against the sides of the tube. In case you wish S C to fit very tightly in T, one of the legs may be slightly bent outwards.

Fig. 65.

230. The connector may be used in still another way; that is, by pushing the part, A, into the hole of an ordinary binding-post, (Fig. 65), and using it just the same as a thick wire.

231. Directions. (A) Stand the compass and D C near each other (Fig. 66). Attach one end of an insulated copper wire, C W, to the binding-post, C, which is on the carbon plate of the cell. Do not join the other end to the other binding-post, Zn, of the zinc plate.

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(B) With the left hand hold C W above and near the compass-needle, and in the N and S line, so that it will extend over the entire length of the needle.

(C) Take the free end of C W in the right hand and touch binding-post, Zn, for an instant only, watching the needle. Repeat.

EXPERIMENT 102. To study the construction and use of a simple "key."

Fig. 68.

233. A key is merely a piece of apparatus by which the circuit can be conveniently and rapidly opened and closed at the will of the operator; that is, by it the electricity can be quickly turned on or off. Fig. 68 shows a simple form of key. To the base, B, are fastened two metal pieces or straps, the upper one, L, being the lever or key proper. The front end of L is raised[92] above O, so that the two do not touch each other unless L is firmly pressed down. A screw, S, keeps L from springing too far above O. For convenience we shall suppose that the wire leading to the key joins it at I (in); the wire from the key is joined to O (out), by means of connectors (§ 226).

The key may be put into any circuit by first cutting a wire and then joining the ends to I and O. Spring connectors make the best connections with this form of key. (For Home-Made Keys see Apparatus Book.)

234. Directions. (A) The magnetic needle being directly under the wire, press L down for an instant only and note the action of the needle.

(B) Press L again, hold it down for 3 seconds, not over that, and watch the needle.

EXPERIMENT 103. To study the construction and use of a simple "current reverser."

235. The Current Reverser. (No. 57.) To the wooden base (Fig. 69) are fastened four metal straps, each turned up at the end so that spring connectors (§ 227) can be slipped on to make electric connections with other pieces of apparatus.

Suppose that at C and Z connections are made with the carbon and zinc of the cell, by[93] means of wires and spring connectors (§ 226). The current comes from the cell to C. As the two straps, 2 and 3, press firmly up against strap 4, and do not touch 1, it is evident that no current can pass from 1 to 2 or to 3 until they are pressed down upon 1. Two wires are joined by spring connectors to 2 and 3 at their turned up ends, X and Y, and these wires lead to any desired instrument.

Fig. 70.

236. Directions. (A) Press down lever 2 (Fig. 70), for an instant only, at the same time noting carefully in which direction the N pole of the needle is deflected.

(B) After allowing lever 2 to spring up again, and after the needle comes to rest, press down lever 3 for an instant, watching the needle. Is the N pole of the needle deflected in the same direction as it was in (A)?

EXPERIMENT 104. To study the simple current detector.

238. Directions. (A) Join the ends of the wire to X and Y of the reverser, C R, as in the last experiment. Coil up C W so that you can hold the coil with your left hand, as shown in Fig. 71, the magnetic needle being inside of it and parallel to it.

(B) Press lever 2 of the reverser for an instant only. Is the needle deflected more or less than it was when the wire simply passed over or under it once?

(C) Reverse the current through C W by pressing lever 3, and note the result.

(D) Get clearly in mind which way the N pole of the needle is deflected when the current enters C W at X, also when it enters at Y.

EXPERIMENT 105. To study the construction and use of the simple galvanoscope.

240. The Galvanoscope (Fig. 72) is more than a mere detector of electricity. With it we shall be able to study, more fully, cells, currents, etc., etc. We must first understand its construction.

[95]

241. The Coil-support, C S, is fastened to the cross-piece, C P, on which are the 3 binding-posts or coil-ends, L, M and R (left, middle, right). The legs, G L, are screwed to C P in such a way that C P is held a little above the table: this allows C S to be tipped to the front or rear to adjust it vertically. On account of the peculiar arrangement of the legs, the galvanoscope can be made to stand firmly, even upon uneven surfaces. The screws holding G L should not be put in far enough to tear the threads in the wood, C P.

242. The Galvanoscope Coils, G C, are two in number, both being fastened to the coil-support, C S. The first coil has five turns of wire, its ends being fastened to L and M; the other coil has ten turns, with ends at M and R. The current can, at will, be made to pass through 5, 10 or 15 turns of wire by making the proper connections.

Suppose that we have two wires direct from a cell, or from the current reverser, with spring connectors on them so that we can slip them onto L, M or R, which stand for left, middle or right. When the wires are joined to L and M the current can pass through but 5 turns; when joined to M and R it will go through 10 turns; and when to L and R it will pass through the entire 15 turns. When the current enters the galvanoscope at L and passes out at M or R, it will pass through the turns of wire from left to right, at the top; that is, it will pass in a "clockwise" direction.

243. The Compass-needle, furnished with O C (No. 18), will do also for this galvanoscope. It should be placed upon the pin-point after fixing on the pointers (§ 246). The length of the needle should be parallel to the plane of the coil when no current[96] passes; that is, the coil and coil-support should be in the N and S line.

The needle can be centered in regard to right and left, and in regard to up and down, by properly adjusting the position of the pin-point support, P P S; this is held firmly to C S by two spring-connectors. By removing S C, the support, P P S, may be raised or lowered.

244. To place the coil in the N and S line, simply swing the galvanoscope bodily around, at the same time looking down upon the needle, until the length of the needle becomes parallel to the coil-support. When once carefully adjusted N and S, a line may be drawn upon the table as a guide for its position in future experiments. The coil should stand in a vertical plane, and this straight up and down position can be easily adjusted.

To place the coil in the E and W line, turn it until the pointers are at the 90° (90 degree) marks,—the 0° (zero degree) marks remaining, of course, as described above.

245. The Degree-Card, G D C (Fig. 72) has a dot at its center, to show where to make a pin-hole for the pin that supports the compass-needle. With this you can tell how many degrees the needle is deflected when the current passes. This card, G D C, should be pressed down over the pin-point. The zero points of G D C should be N and S, also, when the coil is in that position; that is, they should be in the plane of the coil. The pointers on the needle (§ 246) will then be at O, when the needle is at rest, no current passing through the coils. (See Apparatus Book § 272 for Home-Made Degree-Card.) G D C may be held permanently in position after it is adjusted, by sticking a short pin through it into P P S. Do not let this pin interfere, however, with the swinging of the needle.

Fig. 73.

246. Pointers (Fig. 73) should be fastened to the needle, in order to make the readings of degrees accurate. Fasten to the compass-needle a piece of No. 30 insulated copper wire, as shown. It may be cut to the proper length after it is wound around the needle. See that the wire does not touch the pin when needle is[97] in place; balance needle by cutting a little from the heavier end of wire with shears; bend the ends of wire so that they are at opposite sides of the degree-card, both pointing at O, for example. The needle must swing freely, be nicely balanced, and the wire must not touch pin or degree-card.

Fig. 74.

247. Directions. (A) Arrange as in Fig. 74, the coil being N and S (§ 244). Join the ends of the wires, 2 and 3, with the 5-turn coil of G V as shown. Wire, 2, is connected to L (Fig. 72). Press lever 2 of C R (Fig. 69) for an instant, watching the compass-needle and noting how many degrees it swings the first time. Get thoroughly in mind the direction in which the N pole of the needle is deflected when the current passes around G C in a "clockwise" direction. There must be no magnets, iron, or pieces of steel within 3 feet of A G.

(B) Press lever, 3, for an instant, watching the needle. The current will now pass in an "anti-clockwise" direction. Is the needle deflected about the same number of degrees as in (A)?

(C) Change the ends of the wires, 2 and 3, to the 10-turn coil (§ 242) and repeat (A) and (B).

(D) Change 2 and 3 to L and R (Fig. 72), thus allowing the current to pass around 15 turns; then repeat (A) and (B).

249. Note of Caution. It has already been stated that the compass-needle should be in the center of the coil (§ 243), and that the coil should be in the N and S line (§ 244). In addition to the above, see that there are no magnets near G V, when using it; tap G V occasionally to be sure that the needle swings freely, hold the eye directly over the pointers when reading degrees; the pointers should be at zero when no current passes through G V; be sure that the electrical connections are good.

There are several sources of error in taking readings, and in all the quantitative experiments given. The author takes it for granted that such errors will be looked out for by the teacher.

EXPERIMENT 106. To study the construction and use of a simple astatic needle.

250. Directions. (A) Draw each needle across the N pole of H M five times from point to head (Exp. 9). This should make them of nearly equal strength, both points being N poles.

(B) Stick the needles through the paper as shown, the N poles being at the same end of the paper. Balance the paper upon the pin-point. Has this combination a strong or weak pointing-power?

(C) Turn one of the needles end for end. Again test the pointing-power.

EXPERIMENT 107. To study the construction and use of a simple astatic galvanoscope.

252. Construction of the Astatic Galvanoscope. When not to be used for a long time, or for shipping, the legs, A (Fig. 77) may be removed, and the whole packed inside of the box, B.

The Coil, C, has a resistance of about 5 ohms, and is fastened to the coil-support, C S. The ends of the coil are permanently fastened to the binding-posts, L and R (left and right). The ends are so arranged that when the current enters at L it will pass around the coil in a clockwise direction.

253. The Astatic Needle (Exp. 106) is supported by a small thread, T, which is tied to the thread-wire, T W. This T W springs into an eyelet, E, which, in turn, rests in a hole made in the end of B. E should turn easily in the hole, but it should not wabble.

[100]

Fig. 78 shows a sectional view of the coil and needle. The wire, W, should be bent, as shown, so that the magnets can be as near the center-line of C as possible. Fig. 79 shows a front view of the needle. As a matter of convenience it will be best to arrange the poles of the needles, as shown, to agree with the descriptions of the experiments.

To keep the needle from being affected by air currents, the glass plate (No. 38) may be placed in front of the box, B. Stand it upon the legs, A, and tie a string around it, and B, to hold it in place.

Fig. 78.

Fig. 79.

254. Adjusting the Needle. As T is tied to T W, the needle may be swung completely around by turning T W. This should be done until the length of the needle is parallel to the turns of C. The up and down position of the needle should be fixed as nearly as possible when fastening T to T W, the exact place being finally fixed by raising or lowering T W through E. The spring in T W should hold it firmly in E after adjustment. The wire, W, joining the needle-magnets should not touch the coil. It may be made to just swing free from C by tilting the box forward or backward a little. The construction of the legs, etc., makes it possible to tilt the box, and to make it stand firmly upon an irregular surface.

Fig. 80.

255. Directions. (A) See that there are no magnets within 3 feet of A G. Test the astatic needle, after you have it properly suspended, to convince yourself that it does not try to swing around in a N and S line. In case the needle-magnets have been in contact with other magnets, or are not equally magnetized, remagnetize them as directed in Exp. 106. They must remain in any position given them by turning T W. Finally, bring them parallel to the turns of the coil. (See § 254.) Arrange as in Fig. 80.

[101]

(B) Press lever 2 of C R (§ 235) for an instant only. This allows the current to enter A G at L. Repeat several times until you thoroughly fix in your mind the direction in which the right-hand end of the needle is deflected. Does the needle jump suddenly when the current passes?

(C) Press lever 3 for an instant only. Study the result.

EXPERIMENT 108. To study the effect of dilute sulphuric acid upon carbon and various metals.

257. To amalgamate one of the above zinc strips is to coat it with mercury. Remove all jewelry from the hands before proceeding. Wash the zinc with water, and with a cloth remove all dirt from its surface. Amalgamated zinc is very brittle, so lay it flat upon a piece of board or upon a plate, after dipping it for an instant in the dilute acid. Place a small drop or two of mercury upon the strip, and rub the mercury about upon both sides of the zinc with a cloth made wet with the dilute acid.

Mercury clings strongly to zinc or tin, so you may use a narrow piece of either as a spoon to carry a small drop from the supply to the zinc. Tap it upon the zinc to dislodge the drops. Do not get on too much mercury, just enough to coat it, or, at least, that part of it that will be under the acid. Be careful not to break the thin zinc when amalgamating it, as it gets very brittle. It should look bright. (See Apparatus Book § 32, 33.)

Note. Should any mercury get upon copper plates it may be removed by heating them in a flame.

258. Dilute sulphuric acid, for these experiments, should be made by mixing 1 part, by volume, of concentrated acid, with 20 parts of water. Do not let the acid get upon clothes or carpets. Do not add water to acid. Mix by slowly adding the acid to the water in a glass or earthen dish, stirring at the same time. Mix over a sink or out of doors. (For fuller details see App. Book; § 21, 22, 23, 24, 25.) To save time, make at least a quart of the mixture, bottle, and label it for use.

259. Directions. (A) Bend over one end of each of the wires and metal strips, and hang them upon the edge of the tumbler[103] (Fig. 81), so that their lower ends shall be in the acid. Do not let them touch each other. Stand the carbon rod in the acid.

If there is no visible action upon any of the above substances, add a few drops of concentrated acid to the tumbler.

Note carefully what takes place in the tumbler, and state which of the substances are dissolved, which simply made brighter, and which not acted upon at all.

EXPERIMENT 109. To study the effect of dilute sulphuric acid upon various combinations of metals.

261. Directions. (A) Twist one end of the clean copper wire around the small piece of amalgamated zinc (Fig. 82). Hold one end of the wire in the hand and dip the combination into the acid. What takes place? Watch the surface of the copper, remembering that each, alone, was not acted upon by the acid (Exp. 108).

(B) Use the clean iron wire in place of the copper wire, and repeat (A). Watch the surface of the iron.

(C) With a string or thread tie a small piece of well amalgamated zinc to the carbon rod (Fig. 82), then dip the combination into the acid. Watch the surface of the carbon.

EXPERIMENT 110. To study the construction of a simple Voltaic or Galvanic cell.

Fig. 83.

263. Directions. (A) Holding the amalgamated zinc strip in one hand and the copper strip in the other (Fig. 83), dip them into the acid, but do not let them touch each other. Note any chemical action.

(B) Touch the copper and zinc together, below the surface of the acid. Watch the copper.

(C) Separate the lower ends of the strips, then touch them together above the acid. Anything still happen to the copper?

(D) Slip one spring connector with the attached wire upon the zinc strip, then stand the strips in the tumbler, so that they can not touch each other. Now touch the copper strip with the free end of the wire, at the same time watching the copper.

(E) Raise the wire from Cu, touch it to Cu again, and repeat several times until you are sure that something takes place every time the wire touches Cu.

270. Chemical Action in the Simple Galvanic Cell. The chemical formula of sulphuric acid is H₂SO₄ (read H, 2, S, O, 4). This means that it is a compound of hydrogen (H), sulphur (S), and oxygen (O). The H₂SO₄ stands for molecule of acid, and the small figures show that the molecule is made up of 2 atoms of hydrogen (H₂), one of sulphur (S), and 4 of oxygen (O₄). The atoms are held together by chemical attraction or affinity.

There is a stronger chemical affinity between zinc (Zn) and SO₄ than between H₂ and SO₄; so, as soon as the Zn gets a [107]chance, as it does in the cell, it drives out the H₂, and it takes its place in the molecule. This chemical reaction may be shown by the following chemical equation:

Zn + H₂SO₄ = ZnSO₄ + H₂.

Zinc and sulphuric acid produce zinc sulphate and hydrogen.

The zinc sulphate produced weakens the effect of the acid; in fact, the acid has to be renewed occasionally, as it is changed to the sulphate which remains in solution.

271. Action in Cell Using Impure Zinc. The above action takes place in the cell when impure zinc is used, even when no current passes, heat being produced by the reaction instead of useful electricity. (See Local Currents.)

Action in Cell Using Pure Zinc. When pure zinc (or impure zinc that has been properly amalgamated) is used in the cell, it dissolves, or is eaten away, only when the current passes. It should be noted that the bubbles of hydrogen do not even then appear at the zinc; they are not seen throughout the body of the liquid. There seems to be an unseen transfer of hydrogen from the zinc, through the liquid, to the copper (or other - plate used), and it appears there in the shape of bubbles. The larger the quantity of pure zinc dissolved, the stronger the current, and the larger the amount of hydrogen produced.

As the zinc dissolves it parts with its latent energy, and this energy forces the electric current through the circuit. While the hydrogen of the decomposed acid makes its way towards the - plate, the SO₄ part of it travels towards the Zn plate, where the ZnSO₄ is formed. (See § 270.)

EXPERIMENT 111. To see what is meant by "local currents" in the cell.

272. Directions. (A) Hold the nail in the acid for a few seconds, and note result.

(B) Rub the copper sulphate upon the zinc simply in one spot, then place the zinc in the acid, noting the result at the spot.

EXPERIMENT 112. To study the "single-fluid" Galvanic cell.

275. The Simple Cell should be arranged so that the plates will be held firmly in position. The zinc, Zn (No. 60), and copper, Cu (No. 67), should be fastened to the wooden cross-piece, W C P (No. 70), as shown in Fig. 85. Care should be taken not to use longer screws than those provided for (No. 72). If the screws touch each other they will short circuit the cell. Partly fill the tumbler (No. 65) with dilute sulphuric acid (§ 258), join wires with connectors to the plates. The free ends of the wires are then ready to join to apparatus. The ends of wires may be fastened under the screw-heads instead of using connectors on the plates. Do not put the plates into the acid until you read the "directions."

[109]

276. Directions. (A) Arrange as in Fig. 86. Place the coil of G V, N and S (§ 244). Before putting the plates in the acid join them to the 15-turn coil of G V (§ 242). The compass-needle should point to zero. See that the needle swings freely.

(B) Place the plates in the acid, and quickly bring the needle to rest with the aid of the hand, so that you can take the reading at once before the hydrogen bubbles entirely cover the copper plate. Watch the action of the needle for a few minutes. Make a note of the reading, in degrees, at the beginning of the experiment and at the end of five minutes. (See Note.)

Fig. 85.

Fig. 86.

Note. If no change takes place in the position of the needle, the change beginning inside of 10 seconds after the plates are let down into the acid, withdraw the plates, then clean and thoroughly dry the copper to remove all traces of hydrogen. This may be done by heating the copper over a gas flame. Let the copper remain in the air 15 minutes, then try again. In taking the first reading you must work quickly. Catch the needle during its first swing. If you allow it to swing back and forth until it comes to rest, your first reading will not be what it should be.

(C) After the needle has remained in one position, without change, for 2 or 3 minutes, take hold of the wooden cross-piece, move the plates back and forth in the acid to dislodge the hydrogen bubbles, and note carefully the action of the needle. Does the current seem stronger when the plates are moved? Can you get the needle back to the first reading?

(D) Remove the plates from the acid, dry and clean the copper, let them stand in the air for 15 minutes, then take another quick reading and compare it with the first.

EXPERIMENT 113. To study the "two-fluid" Galvanic cell.

Fig. 87.

281. Setting Up the Two-Fluid Cell. Fig. 87. Stand the amalgamated zinc rod, Zn, in P C, then place P C in the tumbler, G T; put in the copper plate as shown. Pour diluted acid (§ 258) into P C until it stands about 2½ in. deep; then at once pour the copper solution (§ 283) into G T, on the outside of P C, until it stands at the same height as the acid in P C. As soon as the[112] liquids have soaked into P C the cell will be ready for use; but it is better to connect it with the galvanoscope at once and note the increase of current during the first few minutes while the liquids soak through P C. A crystal of copper sulphate should be put outside of P C to keep the solution full strength. This is a form of the well-known Daniell cell. Fig. 87 shows a form of home-made two-fluid cell as described in Apparatus Book. If you have the one furnished, use the rod instead of sheet zinc, and use connectors on the plates.

282. Care of Two-Fluid Cell. This experimental cell should be taken apart when not in use. It should not be left in open circuit, even for half an hour. Even after the plates are removed, copper may be deposited upon P C in case there are any metallic impurities on it. Remove the plates and P C, and thoroughly wash them. The copper solution should be put into a bottle to prevent evaporation; the dilute acid may be thrown away to be replaced by fresh acid for the next experiment.

283. Copper Sulphate Solution is made by adding the blue crystals to water until no more will dissolve—that is, the solution should be "saturated," extra crystals always being left in the stock bottle. An ounce of the crystals to half a tumbler of water will be about right, but a pint or so should be made at a time and be kept bottled to save time.

284. Directions. (A) In case you have access to a pair of scales that weigh to within 0.1 gram, carefully weigh both copper and zinc before proceeding. They should be washed and dried with a cloth. See that there are no drops of mercury upon the zinc that may fall off during the experiment.

(B) Replace the plates in the cell, and connect them with the 15-turn coil of G V, placed N and S. Allow the circuit to remain closed for half an hour, and record the position of the needle every 5 minutes.

(C) Again wash, dry with a cloth without rubbing, and weigh both the zinc and copper. Compare the new weights with those found in (A).

287. The Leclanché Cell is an open circuit cell in which carbon and zinc are the plates. The carbon is surrounded with dioxide of manganese, a depolarizer; the two are either packed together in a porous cup, or the latter is compressed into blocks, which are fastened to the carbon. The porous cup stands in a jar containing a solution of sal ammoniac (ammonium chloride),[114] which acts as the exciting fluid, and in which stands a zinc rod. The zinc is not acted upon when the circuit is open. The hydrogen given off by the decomposition of the ammonium chloride is destroyed by the oxygen contained in the manganese dioxide. The E. M. F. is nearly 1.5 volts.

288. Dry Cells are for open circuit work. Sheet zinc forms at the same time the active plate and the outside cylinder or case. A carbon plate acts as the inactive or - plate. The exciting fluid is kept from spilling by its being absorbed by one of the various substances used for that purpose.

289. The Bichromate of Potash Cell is a very common one for laboratory use. It gives a strong current, and although a single fluid cell, it does not readily polarize. Zinc and carbon plates are used. In the sulphuric acid, which is the exciting fluid, is dissolved bichromate of potash. This cell is used for running small motors, induction coils, etc. It has, with fresh solutions, an E. M. F. of about 2 volts. (See Apparatus Book, Chapter I., for Home-Made Batteries.)

290. The Daniell Cell, of which the two-fluid cell used in Exp. 112 is a form, is noted for its constant current. The E. M. F. is a little over 1 volt, and it should be kept working through a resistance when not in regular use; it should not be left in open circuit. The porous cup keeps the two fluids from mixing, but it does not stop the current.

291. The Gravity Cell is a form of the above. As one of the fluids is heavier than the other, no porous cup is needed. Gravity, together with the action of the current, tends to keep the fluids separated. A copper plate is placed in the bottom of the jar, and upon this is put the copper sulphate solution. The zinc plate is supported by the top of the jar and rests in a solution of zinc sulphate, which is lighter than the blue solution below. An insulated wire extends from the copper through the liquids. This cell is used for telegraph and similar work. (See Apparatus Book for Home-Made Gravity Cell, its Regulation, etc.)

EXPERIMENT 114. To see what is meant by "divided circuits" or "shunts."

Fig. 88.

292. Directions. (A) Without A G in place, take the reading of G V. The current now passes from Cu through 1, M P-B, 2, G V, 3, M P-A, 4 to Zn.

(B) Connect wires 5 and 6 to the plates, as shown by the dotted lines. Again take reading of G V, and compare it with the first reading. Does some of the current pass through A G?

EXPERIMENT 115. To see what is meant by "short circuits."

294. Directions. (A) With the current passing as described in Exp. 114 (A), take the reading of G V.

(B) Lay the ends of the metal, or other thick conductor, upon M P-A and M P-B. Compare the new reading of G V with that in part (A).

(C) Remove the conductor used to short circuit G V, take the reading in degrees, then touch M P-A to M P-B; watch G V.

EXPERIMENT 116. To see if the E. M. F. of a cell depends upon the materials used in its construction.

[118]

Fig. 90.

298. Directions. (A) With Zn and Cu connected to G V as shown (Fig. 90), take the reading in degrees, and note in which direction (east or west) the N end of the needle is deflected. Tabulate results, as shown in Fig. 91, filling in each column of your table made out on paper.

(B) In like manner try the following combinations in the order given, in each case connecting the first-mentioned plate with the left-hand binding-post, L, of G V. For (B) use zinc-iron.

(C) Use zinc-lead; (D) iron-copper; (E) iron-lead; (F) lead-copper; (G) copper-carbon.

Fig. 91.

299. Note. Some of the combinations produce but slight currents. In case G V is not delicate enough to show clearly which way the current passes, use the astatic galvanoscope in its place for such combinations.

301. Electromotive Series. All metals are not acted upon to the same degree by dilute acid. From the results of Exp. 116 it is seen, part (B), that iron is electronegative to zinc; that is, the[119] current in the cell flows from zinc to iron. Part (D) showed that iron is electropositive to copper, as the current flowed from iron to copper in the cell. It is possible to arrange the metals in a series, one below the other, in such a way that any one will be electronegative to those above it and electropositive to those below it; that is, the list should have the most electropositive metal at the top, and the one least acted upon by the acid at the bottom. Make such a list from your results. The farther the metals used are apart in the list, the greater will be the E. M. F. of the cell. Good carbon is acted upon the least of all, so zinc and carbon are better than zinc and copper.

EXPERIMENT 117. To see whether the electromotive force of a cell depends upon its size.

302. Directions. (A) Join the large and small cells with G V so that their currents will oppose each other. To do this, join the two zinc plates by means of a wire and connectors. With two other wires connect the two copper plates with the galvanoscope binding-posts, and watch for any indication of current. Does one cell oppose the other?

EXPERIMENT 118. To study the general effect of "resistance" upon a current.

306. Directions. (A) Take the reading of G V in degrees, the current passing through the entire length of R C. (See § 310.)

(B) Change the end of wire 4 from binding-post R to M, on R C, so that the current will pass through one-half only of R C. Note the reading of G V.

(C) Remove R C entirely and connect wires 3 and 4 by means of a metal plate. Compare the readings of (A), (B) and (C). What do they show?

310. Simple Resistance Coil. Fig. 94 shows a simple form of coil, R C (No. 79). The total resistance is 2 ohms, L (left) and R (right) being binding-posts to which the ends of the coil, C, are joined. M (middle) connects with the middle of the wire, at which point the wire is doubled. The coil is fastened to a stiff pasteboard base, B.

[123]

Connections. When 2 ohms resistance are wanted, let the current enter at L and leave at R (or the reverse). When 1 ohm is wanted, let the current leave or enter at M, the other wire being joined to L or to R. Connections should be made with spring connectors. See § 229.

EXPERIMENT 119. To test the power of various substances to conduct galvanic electricity.

311. Directions. (A) Make tests with the above substances, and with any others at hand, and note which are conductors and which are not.

EXPERIMENT 120. To find the effect of sulphuric acid upon the conductivity of water.

[124]

313. Directions. (A) Put a little pure water in S, and see if enough current can pass through it to deflect the needle of G V. The ends of the wires, 1 and 2, should be gradually moved toward each other, the needle being watched.

(B) Put 4 or 5 drops of concentrated acid into the water; stir it, then repeat the test. What effect has the acid?

EXPERIMENT 121. To find what effect the length of a wire has upon its electrical resistance.

315. Note. This experiment is usually done with a reverser in the circuit, first taking readings with the current passing in one direction, and then in the opposite direction. Considerable time will be saved by taking all the readings for one direction of the current at a time, simply using different lengths of German-silver wire, and allowing the current to flow constantly during each part. This obviates all danger of poor contacts in the reverser, etc.; it saves the trouble of handling the reverser, and much of the time needed for the needle to come to rest.

316. Directions. (A) With the circuit arranged as in Fig. 96, and with G V properly placed, take the reading of G V, the current passing through 200 cm. of No. 30 G-S W. Record your[126] results in a diagram made like Fig. 97. The row of figures across the top shows the length of the circuit. The table is started with results from one experiment. Your results will probably be different from these.

(B) Get the deflection with the current passing through 180 cm. of wire. To do this press a piece of copper (O, Fig. 96) upon the wire at the mark 10 cm. from Z, another thin piece of metal, U, having been slipped under the wire. This will allow the current to pass across from one wire to the other. Record the deflection in the col. marked 180.

(C) Record the deflections for the lengths, 160 cm., 140, 120, 100, 80, and 60; then repeat (A) to be sure that the cell has been working uniformly. This deflection should agree with that in (A).

(D) Change the direction of the current through G V; to do this, change wire, 1, from M P 2 to M P 1, and wire 5 to M P 2. This must be done without disturbing G V.

(E) Repeat (A), (B), and (C), and record the deflections for the different lengths.

(F) Get the average deflections.

(G) Take, for future use, the deflection produced without G-S W being in the circuit. Swing the end of wire, 3, that is joined to Y, around to M P 2. The current will then pass simply through G V. Record deflection in col. marked O.

Note. It is best to do the next experiment at once with the same cell, so that the results of the two experiments can be compared. In case this is impossible, get your cell to produce the same deflection when you use it again, as shown in col. O, Fig. 97. You can regulate the deflection of the needle of G V by varying the strength and quantity of the acid in P C.

EXPERIMENT 122. To find what effect the size (area of cross-section) of a wire has upon its electrical resistance.

318. Directions. (A) See that your cell is in the same condition as for Exp. 121; that is, it should produce the same deflection of the needle of G V as before, when the two, only, are in the circuit. (See Exp. 121, G.) The deflection may be changed by changing the strength and quantity of the dilute acid and copper solution.

(B) Find the average deflection of the needle with the 2 meters of No. 28 G-s wire in the circuit, arranged as in Fig. 96.

(C) Compare this average deflection with the results obtained in Exp. 121, in order to find what length of the No. 30 wire has the same resistance as 2 meters of No. 28 wire. To find how many times greater one length is than another, we divide the larger length by the smaller; hence, to find the relation between the two lengths of wire that gave the same deflection,—lengths of equal resistance,—we divide the 200 centimeters (the length of the No. 28) by the length of No. 30 found as directed.

(D) From the wire tables it will be found that the area of cross-section of No. 28 wire is about 1.59 times that of No. 30 wire. How does this quotient, or ratio, compare with that found in part (C)? What is the relation between the area of cross-section of a wire and its resistance? (See § 319, also Exp. 136.)

EXPERIMENT 123. To compare the resistance of a divided circuit with the resistance of one of its branches.

320. Directions. (A) Note the deflection of the needle when the current passes through 1 meter of G-s wire, as shown. This will be considered as one branch of the divided circuit.

(B) Still allowing the current to pass as in part (A), press a piece of copper firmly across the binding-posts X and Y, to electrically connect them, and note the reading of the needle. In this case the current divides at Z through the two branches. What is learned from the results of (A) and (B)?

(C) See if you can show the same results with apparatus arranged as in Fig. 99.

EXPERIMENT 124. To study the effect of decreasing the resistance in one branch of a divided circuit.

322. Directions. (A) Take the reading of G V with 2 ohms resistance in the lower branch; that is, with the whole of R C in circuit.

(B) Take the reading of G V with one ohm in circuit; that is, with the end of wire, 5, connected to M instead of to R.

(C) Cut out R C from the lower branch by replacing it with a metal plate, thus joining wires 3 and 5. Compare the results from (A), (B), and (C), and explain.

EXPERIMENT 125. To study the construction and use of a simple "Wheatstone's Bridge."

Fig. 103.

324. The Simple Wheatstone's Bridge (Fig. 103) consists of a wooden base, W, at the ends of which are fastened two aluminum conductors, 1 and 3. At one side of W is fastened another conductor, 2. In Fig. 104 are side views of the conductors. These[131] are used merely for convenience in making connections, and take the place of the metal plates used in previous experiments. A German-silver wire, G-s W, is stretched between 1 and 3, and under this is a scale, S, divided into 100 small parts, these being tenths of the larger divisions. The ends of G-s W are held between eyelets, as shown at E, Fig. 104.

Fig. 104.

Reading the Scale. The value of part A can be read directly from the scale, using the lower row of figures. The point marked P, for example, would be read 3.7 (three and seven-tenths large divisions); B would be 6.3, found by subtracting 3.7 from 10. The sum of A and B must always equal 10. The 6.3 may also be read directly by using the upper row of figures for the whole numbers, counting the tenths to the left. Try to divide the smallest divisions into halves, at least; that is, if A = 3.75, B = 6.25. Take the readings carefully.

325. Directions. (A) Touch the free end of wire, 3, to the point, C, which has a higher potential than M P. Press down K for an instant only. Some current should pass through A G, as a shunt. Should it pass from C to M P or the reverse? Note in which direction the right-hand end of the astatic needle is deflected.

(B) Swing the end of 3 around and touch it to the point, Z, which has a lower potential than M P. Press K for an instant, watch the needle, and compare with the results in (A).

(C) Move the free end of 3 along on G-s W, touching K at intervals, until a point is found at which the needle of A G is not deflected. How does the potential of this point compare with that of M P?

327. Use of Wheatstone's Bridge. It will be found, upon trial, if we put a resistance of 2 ohms in place of R, Fig. 102, and 2 ohms in place of X, that the free end of wire 3 will have to be at the center of the bridge-wire in order to get a "balance"; that is, to find the place where A G is not affected. No matter what the resistance of R and X are, provided they are equal, this will be true. The value of both A and B, on the scale, will be 5 whole spaces, no tenths. From this we see that A: B:: R: X, which reads A is to B as R is to X; this means that A × X = B × R. Supplying the values of the letters, we have 5 × 2 = 5 × 2. If we did not know the value of X, that is, if we were measuring the resistance of a coil of wire, using a 2-ohm coil as the standard, or R, we could find the value of X, knowing the other 3 parts of the proportion. 5 × X = 5 × 2, which means that 5 times the value of X is 10; hence the value of X is 10 ÷ 5 = 2 ohms.

Suppose that we have R = 2 ohms, which is the standard resistance coil (No. 79), and are trying to find the resistance of a coil, X. We slide the end of wire, 3, along on the bridge-wire until the correct place is found. (See Exp. 125, 126, for details.) Take the values of A and B (§ 324), supply them in the equation given, and work out the value of X.

328. EXAMPLE. R = 2 ohms; A = 3.7; B = 6.3; to find the value of X in ohms.

A: B:: R: X, which means that A × X = B × R, or 3.7 × X = 6.3 × 2. X must equal, then (6.3 × 2) ÷ 3.7 = 3.405 ohms.

Note. In practice it is most convenient to make connections as shown in Fig. 105 when measuring resistances (Exp. 126). The arrangement given in Fig. 102 is simply for explanation. It will be seen that the smaller A is, compared with B, the larger the unknown resistance compared with your standard.

EXPERIMENT 126. To measure the resistance of a wire by means of Wheatstone's Bridge; the "bridge method."

329. Directions. (A) Touch the free end of wire, 3, to the left-hand side of the bridge-wire, press the key for an instant, only, and note the direction taken by the right-hand end of the needle. Move the end of wire, 3, to the right-hand side of the bridge-wire, touch key, watching needle. Does the needle move more or less than before? In the same or opposite direction? If the deflections are opposite, the point that has the same potential as binding-post, 2, must be between the two points touched.

(B) Be sure that all connections are good. Find the point on G-s W, at which there is no deflection, as directed in Exp. 125 (C). Note the readings on the scale, as explained in § 324.

(C) Make the proper calculation, § 327, 328, and find the resistance of the coil of G V, the resistances of the wires joining R C and G V to the bridge being neglected.

(D) Make proper allowances for the resistances of the wires[134] just mentioned (see § 330), and compare them with the results found in part (C).

330. Allowances for connections. It should be remembered that the wires joining R C and G V to the bridge also have some resistance. Such connections, in regular instruments, are made by heavy copper straps or by thick, short wires, so that their resistances can be neglected. In case you use the ordinary No. 24 copper wire, as directed, the resistances of the pieces can be measured by means of the bridge, or you can calculate their resistances from the wire tables. The resistances should be allowed for. It is evident that your standard resistance is 2 ohms plus the resistance of the connecting wires, and that the resistance of the coil, X, is found by deducting the resistance of its connecting wires from that found from the proportion previously used.

Example. We see from the table that the resistance of about 39 ft. 1 in. of No. 24, B and S copper wire is 1 ohm. This equals 469 in. If 469 in. have a R (resistance) of 1 ohm, 1 in. will have a R of one-469th of an ohm; that is 1 divided by 469, which equals a little over .002 ohm. For every inch of No. 24 wire used, then, for connections, we may allow .002 ohm. This will be near enough for our purposes.

Suppose that each connection is 18 in. long, the regular wires with connectors being used. The R of the 36 in. joined to R C will then be 36 times .002 = .072 ohm. Our standard R must then be considered as 2.072 ohm. If we substitute this in the example, as stated in § 328, we have 3.7 × X = 6.3 × 2.072. X must equal (6.3 × 2.072)/3.7 = 3.528 ohm, which includes the unknown resistance and 36 in. of connections, the R of which is .002 ohm; 3.528 - .072 = 3.456, the resistance of X alone. Compare this with the answer to example, § 328. Make allowances according to length of connectors used.

EXPERIMENTS 127–137. To measure the resistances of various wires, coils, etc., by the "bridge method."

Note. Make proper allowances for connections (§ 330) in all experiments in measuring resistances.

EXPERIMENT 127.

331. Directions. (A) As explained in Exp. 126, measure the resistance of the 10-turn coil of G V, allowing for connections (§ 330). Read the bridge-scale carefully.

(B) Use one-half of the 2-ohm coil as standard and repeat.

EXPERIMENT 128.

332. Directions. (A) Measure the resistance of the 5-turn coil of G V (see Exp. 126, etc.), using 2 ohms as standard.

(B) Use 1 ohm as standard, repeat, and compare results.

(C) Add the resistances of the 5 and 10-turn coils, and compare the sum with the resistance of the 15-turn coil, as found in Exp. 126, D. The difference should be but a few hundredths of an ohm.

EXPERIMENT 129.

333. Directions. (A) Measure the resistance of the coil of No. 24 copper wire (No. 89). This coil is used for later experiments. Spring connectors are fastened to the ends of this coil, allowing it to be directly connected to the conductor on the bridge, so no allowance should be made for its connecting wires. (See Exp. 126 for details.) Mark the resistance upon the coil for future use. (See Note.)

Note. The student will be surprised, perhaps, to find that different results are obtained for the resistance of a given wire in case he uses different standard resistances in the various tests; that is, he will probably get a different result in Exp. 127 (A) from the result of Exp. 127 (B). The difference here, however, may not be large. The best results are obtained by making the standard resistance as nearly equal as possible to the resistance to be measured, so that a balance can be found when the end of wire 3 (Fig. 105) is near the center of the bridge-wire. If R, Fig. 105, is much larger or smaller than X, the point desired on G-s W will be near one of its ends, and large errors thereby produced. The approximate resistance of X can be found by trial, then more or less resistance can be used for R to suit. The student should make several coils as explained in Apparatus Book, Chapter XVII. The resistance of the different coils furnished should be measured and marked. These can be used to vary the value of R.

EXPERIMENT 130.

334. Directions. (A) Measure the resistance of the coil of No. 25 copper wire (No. 90). (See Exp. 126 for details and the Note, Exp. 129.)

EXPERIMENT 131.

335. Directions. (A) Measure the combined resistance of the two coils used in Exps. 129 and 130, when they are joined in "series"; that is, when one end of one coil is joined to one end of the other by means of a metal plate, the free ends being connected to the bridge (Exp. 126). The current has to travel through the entire length of both coils.

(B) Compare this result with the sum of their separate resistances found in Exps. 129 and 130. (See Exp. 129, Note.)

EXPERIMENT 132.

336. Directions. (A) Measure the resistance of the two coils (Exp. 131) when they are joined "in parallel." (See § 293.) They may be joined in parallel by connecting them both to the bridge at the same time, one end of each being slipped onto 2 (Fig. 103), the other end of each being joined to 3. In this way the current has two paths, side-by-side, to get from 2 to 3. (See Exp. 129, Note.)

(B) Compare this resistance with that of Exp. 131.

EXPERIMENT 133.

337. Directions. (A) Measure the resistance of 1 meter of No. 28 German-silver wire. Use the wire as arranged on a board, Exp. 122 (Figs. 96 and 98), making the connections with the bridge from binding-posts, X and Z. (See Exp. 129, Note.) The wires connecting the bridge with the ends of the G-s wire will each have to be about 2 ft. long. In making deductions (§ 330) figure according to the length used.

(B) Divide the total resistance by 100 to get the resistance of 1 cm. of the wire, and carefully mark off the board into cm. This will give 100 parts between X and Z.

EXPERIMENT 134.

338. Directions. (A) Using the No. 28 G-s wire on the board, as arranged for Exp. 122, measure the resistance of the 2 meters in series, the connections being made with the bridge from X and Y, Fig. 98.

[137]

(B) Compare the result with that of Exp. 133. What is the relation between the length of a wire and its resistance? See Summary of Laws. (See Exp. 129, Note.)

EXPERIMENT 135.

339. Directions. (A) Measure the resistance of the above two meters of No. 28 G-s wire when joined in parallel. (§ 293.) The binding-posts, X and Y, can be joined by a short wire with connectors on its ends, or by clamping a thin strip across by means of spring connectors. Use the 2-ohm coil as the standard, and make proper allowances. (§ 330.)

(B) From the results of Exps. 132 and 135 what can be said about the resistances of parallel circuits as compared with the resistances of the separate branches?

EXPERIMENT 136.

340. Directions. (A) Arrange the 2 meters of No. 30 G-s wire on the table or board, again (Exp. 121, Fig. 96).

(B) Measure the resistance of one meter. Find the value of X approximately, and use a resistance for R that will suit. (See Exp. 129, Note.)

(C) Divide the result by 100 to get the resistance of 1 cm. of the wire.

(D) Compare the resistance of one meter of No. 28 G-s wire, found in Exp. 133, with the resistance of 1 meter of No. 30 G-s wire. What is the relation, then, between the size (area of cross-section) of a wire and its resistance? (See the results of Exp. 122, and § 319, also Summary of Laws.)

EXPERIMENT 137.

341. Directions. (A) Measure the resistance of 2 meters of No. 30 copper wire, arranged on a board as in Fig. 96. (See Exp. 129, Note.) Get the resistance of 1 meter.

(B) Compare the conductivities of copper and German silver by studying the results of Exps. 136 and 137. Which has the greater resistance? To find out how many times greater one resistance is than the other, divide the larger by the smaller.

EXPERIMENT 138. To study the effect of heat upon the resistance of metals.

342. Directions. (A) Measure the resistance of the coil as before, Exp. 129. The result should nearly agree with that of Exp. 129, provided connections, etc., are the same.

(B) Remove the coil from the bridge, hold it about a foot above a lamp or stove, to warm it thoroughly, but do not heat it enough to injure the covering. It will take a minute or so to warm it so that the heat will get to the inside also.

(C) Replace the coil, measure its resistance, and compare the result with its resistance when cold. Does heat increase or decrease the resistance of a copper wire?

EXPERIMENT 139. To measure the resistance of a wire by the method of "substitution."

Fig. 106.

345. Directions. (A) Be sure that 2-F C gives a constant current, shown by the uniform deflection at G V, when arranged as in Fig. 106. Do not use a cell that quickly polarizes. The coil, X, forms a part of the circuit; it is joined to wires, 1 and 2, by means of metal plates, so that it may be quickly removed without disturbing either G V or 2-F C. Carefully read the deflection at G V.

(B) Remove X from the circuit, and join the free end of wire, 2, to binding-post, X, and the free end of wire, 1, to a small piece of sheet copper, which can be firmly pressed upon the G-s wire to make a contact. Move this along on the G-s wire until the deflection produced equals that of part (A), remembering that the longer the G-s wire in the circuit the less the deflection. Make two or three trials, as one or two cm. difference in length make but a little difference in the deflection. Note the number of cm. of G-s wire used, the resistance of which must equal that of the coil, X.

(C) Find the resistance of X by multiplying the length just found by the resistance of each cm., and compare the result with the value found by using the bridge method directly.

EXPERIMENT 140. To measure the E. M. F. of a cell by comparison with the two-fluid cell.

346. Directions. (A) Be sure that 2-F C gives a constant current. Take the reading of G V without the rheostat in the circuit; that is, with wires, 2 and 1, joined directly. The deflection should be 50 or 60 degrees at least, and be constant.

(B) Attach a small piece of copper to the end of 1, and firmly rub it along upon the G-s wire, thus introducing resistance into the circuit, until the deflection is, say, 60° (50 or 55 degrees will do). Note the length of G-s wire used and call it (B).

(C) Gradually add more resistance by moving the end of 1 along until the deflection is 50°, 10 degrees less than before. (If the original was 50° make the new 40°). Call the number of cm. of wire used (C).

(D) Replace 2-F C with the dry cell D C. Add resistance, as before, until G V indicates a deflection of 60°, being careful not to keep the circuit closed long enough to partially polarize D C. Make 2 or 3 trials, allowing D C to rest a few minutes between each. Call the number of cm. of G-s wire used (D).

(E) Again add more resistance, as in (C), until the deflection is reduced to 50°. Call the length used (E).

347. Calculation. It is known that resistances that are able to reduce the strength of the currents equally are proportional to the electromotive forces; that is, the electromotive forces of the two cells are to each other as the two resistances necessary to produce equal changes in the deflections, which, of course, indicate equal changes in the strength of the currents. Since the resistances used in the two cases are directly proportional to the lengths used, we have:

Length (C-B): Length (E-D):: E. M. F. of 2-F C: E. M. F. of D C.

Substitute the values found and find the E. M. F. of D C.

EXPERIMENT 141. To measure the internal resistance of a cell by the "method of opposition."

348. Directions. (A) Connect the two cells in opposition, so that no current will be generated by them, and so that the two[141] can be treated as a dead resistance. Do this by joining the two zinc plates by a wire with connectors, and use wires to connect the copper plates to the bridge like any other unknown resistance.

(B) Measure the resistance of the two by the regular bridge method, allowing for wires used for connections. One-half of the resistance found will give the internal resistance of one cell. (See Note.)

Note.—The standard resistance will have to be arranged to suit each particular case to make the calculations even approximately correct. (See Exp. 129, Note.) The standard resistance may be increased by adding the various coils and rheostat wires, their values being known.

EXAMPLE. If 39.1 ft. of No. 24 copper wire has a resistance of 1 ohm, 78.2 ft. will have a resistance of 2 ohms, because 78.2 is twice 39.1; 70.38 ft. will have a resistance of 1.8 ohms, as (70.38 ÷ 39.1 = 1.8) it is 1.8 times 39.1.

EXAMPLE. A No. 30 wire has a diameter of about .01 inch, while the diameter of a No. 24 wire is about .02 in.; that is, the No. 24 has twice the diam. that the No. 30 has. The area of cross-section of the No. 24, however, is four times that of the No. 30, so its resistance is but ¼ that of the No. 30, the lengths, etc., being the same. (See Wire Tables.)

3. The resistance of a wire depends upon its material, as well as upon its length, size, etc.

4. The resistance of a wire depends upon its temperature. (See Elementary Electrical Examples.)

EXPERIMENT 142. Having a cell with LARGE PLATES, to find how the strength of the current is affected by changes in the position of the plates, the external resistance being small.

359. Directions. (A) Connect the wires with the 5-turn coil of G V, which has but little resistance. Have the tumbler nearly full of dilute acid to get the effect of large plates; that is, the current has a large liquid conductor to pass through in the cell, and the internal resistance will be small. G V should be properly placed N and S.

(B) Place the copper and zinc plates as far apart as possible in the acid, and press them against the bottom of the tumbler. Note the reading of G V. It is not necessary to take readings with reversed current.

(C) Still pressing them against the bottom of the glass, to keep the same amount of surface under acid, slowly bring them near each other and watch the needle.

(D) Hold the plates about an inch apart, and against the bottom, and note the reading of G V. Slowly raise the plates, keeping them the same distance apart until they are out of the acid. Watch the action of the needle.

Make a note of your readings in degrees and write your conclusions. Does a change in internal resistance affect the strength of the current?

EXPERIMENT 143. Having a cell with SMALL PLATES to find how the strength of the current is affected by changes in the position of the plates, the external resistance being small.

360. Directions. (A) Repeat (B) and (C) of Exp. 142, recording the reading of G V in each case.

(B) Compare the results with those of Exp. 142, remembering that the internal resistance is larger than before. Is the current[146] as strong with small plates as with large plates when the external resistance is small? When the external resistance is small (the 5-turn coil, for example), should the cell have a high or low internal resistance to produce the greatest effect upon the needle?

EXPERIMENT 144. To find whether the changes in current strength, due to changes in internal resistance, are as great when the external resistance is large, as they are when the external resistance is small.

361. Directions. (A) Arrange as in Fig. 109, the external resistance being 2 meters of No. 30 G-s wire in series with G V. The 2-F C in the Fig. is replaced, however, by the simple cell as in Exp. 143.

(B) Find the effect upon the strength of the current of moving the plates about when but 1 in. of acid is in the tumbler.

(C) Nearly fill the tumbler with acid and repeat (B), taking readings with plates near each other and as far apart as possible. Lift them nearly out of the acid and take the reading.

(D) Still increase the external resistance of the circuit by adding coils of wire or the meter of No. 28 G-s wire and repeat. Is the strength of the current greatly affected by slight changes in the internal resistance when the external resistance is large?

EXAMPLE. Let us take a circuit (A) when the external resistance, R, is small, and (B) when R is large compared with r, E being taken as 1 volt in both cases.

(A) Let R = 1, and r = 2; substituting these values in the formula above, we have:

C = 1 / (1 + 2) = 1 / 3 = .33+ ampere.

Now let the internal resistance, r, be slightly increased from 2 to 3 ohms; the value of C then becomes ¼ ampere, as R + r = 4. The change in C, then, is the difference between ⅓ and ¼; and this expressed in decimals becomes .33 - .25 = .08 ampere.

(B) Let R = 200 ohms, and r = 2 ohms as in (A). Substituting these values we have,

C = 1 / (200 + 2) = 1 / 202 = .00495 ampere.

Increasing r from 2 to 3, as before, etc., we find that C = 1 divided by 203 = .00492 ampere.

The above shows clearly (A) that the value of C is changed considerably by changes in r when R is small, and (B) that changes in r produce very slight changes in C when R is large. Review your results of Exps. 142–144. (See Elementary Electrical Examples.)

EXPERIMENT 145. To find the best way to join two similar cells when the external resistance is small.

366. Directions. (A) Partly fill the tumblers with the acid. Join the cells in series (Fig. 110), then connect wire 1 (Fig. 110) with the left-hand binding-post of G V, and wire 2 with the middle one, thus putting the 5-turn coil into the circuit. Take the reading of G V.

(B) Join the cells in multiple arc (Fig. 111), connecting them as in (A) with G V. Write down the reading, and compare it with that found in (A).

(C) Take the reading with but 1 cell joined to G V.

EXPERIMENT 146. To find the best way to join two similar cells when the external resistance is large.

367. Directions. (A) Take the reading of G V when the two cells are in series (Exp. 145), the external resistance being the 15-turn coil and G-s wire.

(B) Join the cells in parallel and take the reading, using the same external resistance as in (A).

(C) Increase the external resistance by adding coils of wire or 2 metres of No. 28 G-s wire and repeat (A) and (B). What does the experiment show?

(D) Take the reading with 1 cell and large external resistance.

EXPERIMENT 147. To study the electrolysis of water.

371. Directions. (A) Allow the current to pass, and note (1) whether gas is set free at both electrodes, A and C, and (2) at[152] which the quantity of gas is the greater. If very little gas is produced use more cells.

(B) Remove A and C from the liquid, to remove the gas, then watch the action of the needle of G V as the water is again decomposed.

EXPERIMENT 148. To coat iron with copper.

374. Directions. (A) Clean the nail with sandpaper, then hold it in the copper solution for a few seconds. Machinists often cover iron or steel tools with a thin coating of copper in this way.

EXPERIMENT 149. To study the electrolysis of a solution of copper sulphate.

375. Directions. (A) The carbon being clean, allow the current to pass, C and Cu being kept about ½ in. apart. Watch the surface of C, and note the beautiful color of the deposited copper. Save the coated rod for the next experiment. Has the Cu plate been acted upon?

EXPERIMENT 150. To study the chemistry of electroplating.

377. Directions. (A) Allow the current to pass as before. Is copper still deposited? Does anything occur now at the surface of the anode? Is the copper deposited as rapidly as before?

(B) Try the effect of the two simple cells joined in series, Instead of the two-fluid cell.

(C) After a fair coating of copper has been deposited upon the carbon cathode, reverse the direction of the current through the copper solution; that is, use the coated rod for the anode. Allow the current to pass until a change takes place in the anode.

EXPERIMENT 151. To study the construction and action of a simple "storage" cell.

381. Directions. (A) Get clearly in mind the direction in which the right-hand end of the astatic needle is deflected when the current passes, remembering that it passes into A G at L and leaves at R. Allow the current to flow for 10 or 15 minutes through the circuit, at the same time watching the needle to see whether the strength of the current remains constant.

(B) Remove the connector from Cu, swing it over into the position of the dotted line (Fig. 115), slip the connector upon M P and watch the needle. This cuts the cells out of the circuit; but, if you desire, also remove wire 3 from M P. Does the storage cell, S C, produce any current? Does it pass through A G in the same direction as that which came directly from the two cells?

(C) Try the dry cell in place of the two simple cells. Try 2 other cells in series if you have them.

EXPERIMENT 152. To study the lines of magnetic force about a straight wire carrying a current.

384. Directions. (A) Arrange the wire so that the current will flow through it from N to S over the compass-needle as soon as the circuit is closed (Fig. 117, A). Press K for an instant only, and note the direction in which the N pole is deflected. Repeat two or three times until you get clearly in mind the direction taken[159] by the needle. Sketch the result in your note-book, and compare with Fig. 118, A. The arrow shows the direction of the current.

(B) Let the current pass for an instant from N to S and under the needle, as shown in Fig. 117, B. Sketch result.

(C) Let the current pass for an instant from S to N above the needle (Fig. 117, C). Sketch result.

(D) Let it pass from S to N under the needle (Fig. 117, D). Sketch result.

(E) Let it pass through the wire from east to west (Fig. 117, F) above the needle, then under it, and note result. Compare the results with those indicated in Fig. 118.

EXPERIMENT 153. To study the lines of force about a coil of wire like that upon the galvanoscope.

Fig. 120.

Fig. 121.

389. Directions. (A) Hold the compass in the various places marked with a dot (Fig. 120) and note the directions taken by its N pole. Make a circle similar to the one shown to represent the coil, and sketch upon it the way in which the lines of force pass around it according to your observations.

(B) Make a diagram like Fig. 121, which represents a cross-section of the coil through the center. Imagine that you have removed the top half of the coil and that you are looking down upon the ends of the wire of the lower half. Draw curved arrows about the coil at W and E to show which way the lines of force are passing. Compare your results with those in Fig. 119, remembering that at E, Fig. 121, the current is going away from you.

(C) Move O C back and forth on the center-line that runs N and S through the coil, and note the positions of the compass-needle. Does the coil seem to have poles?

(D) Reverse the current through the coil and repeat your observations.

EXPERIMENT 154. To study the magnetic field about a small coil of wire.

390. Coils of wire for some of the following experiments should be wound upon wooden spools that have been turned down thin, so that the wire will be as near the central hole as[162] possible. They should be wound with a winder. (See Apparatus Book, Chapter X.)

For convenience we shall call the starting end of the coil, that is, the end that comes from the wire that is near the center, the inside end, I E. The end of the last layer of the coil we shall call the outside end, O E. These letters should be noted in the diagrams. See Apparatus List for details of the special coils used in these experiments.

Fig. 122.

391. Directions. (A) Arrange as in Fig. 122, so that the axis of the coil will lie in the E and W line. Place O C about 2 in. from the E end of the coil. Press one lever of C R so that the current will pass around the coil for an instant in a clockwise direction; that is, so that it will enter the coil at O E. Note the action of the needle. If the needle is not affected move it nearer the coil and press the lever again. Get clearly in mind the connections, the direction in which the N end of the needle is deflected, etc. Is the E end of the coil a N or a S pole?

(B) Reverse the current through the coil. What effect has it upon the polarity of the E end of the coil?

(C) Place O C at the west end of the coil and repeat (A) and (B).

(D) Place O C in various positions about the coil and note the action of the needle when the current passes. Does this coil act like a magnet, having poles, magnetic field, etc.?

EXPERIMENT 155. To test the attracting and "sucking" power of a magnetized coil or helix.

Fig. 124.

393. Directions. (A) Arrange the coil, etc., as described in Exp. 154. The coil need not lie in the E and W line, however, and a key may be used instead of the current reverser.

(B) Magnetize the needle so that its point will be a N pole.

(C) Tie a thread about the center of the magnetized needle, hold the thread in the hand so that the S pole of the needle will swing freely at the hole at the right-hand end of the coil (Fig. 124). If the current passes as directed, the right-hand end of the coil will be a N pole. What happens to the needle when the key is pressed for an instant.

(D) Change the needle to the left end of the coil and repeat.

(E) Try a nail, pen, iron, etc., instead of the needle.

EXPERIMENT 156. To find whether a piece of steel can be permanently magnetized by an electric current.

394. Directions. (A) Be sure that the needle is not magnetized. It should attract both ends of the compass-needle. How can any magnetism in the needle be removed?

(B) Place the needle inside of the coil with its point to the east; that is, with its point at the N pole of the coil, and its head at[164] the S pole. Close the circuit for an instant. Test the needle again for poles. Is the point a N or a S pole?

(C) Turn the needle end for end in the coil, and see whether its polarity can be reversed.

(D) Experiment with iron wire, nails, steel pens, spring steel, etc.

EXPERIMENT 157. To study the effect of a piece of iron placed inside of a magnetized coil of wire.

395. Directions. (A) Arrange first as for Exp. 154, Fig. 122, with the coil in the E and W line, no core being used, and place O C about 6 in. from the right-hand end of the coil.

(B) Press the lever for an instant to see whether the field of the coil is strong enough to move the compass-needle at that distance. Move O C a little nearer or farther from the coil until the needle just moves, when the circuit is closed.

(C) Place I C inside of the coil (Fig. 125), and repeat (B) to see whether the magnetic field of the coil is stronger or weaker than before.

(D) Study the location of the poles. Can they be reversed?

EXPERIMENTS 158–163. To study straight electromagnets.

EXPERIMENT 158. Lifting power.

398. Directions. (A) Join the cell, key, and coil, as explained in Exp. 154, so that the current will pass only when the key is pressed. Place the core inside of the coil(Fig. 125). Two good cells in series can be used to advantage.

(B) Hold the coil in a vertical position near small nails, iron filings, tin boxes, etc.; then press the key and raise coil; carry the clinging iron to another place, break the circuit at the key, and explain the result. Why do nails cling more strongly to the core than filings after the circuit is broken?

EXPERIMENT 159. Residual magnetism of core.

399. Directions. (A) After the current has passed through the coil with the core in place, remove the core and test it for magnetism with the compass. Will the small end of the core attract both poles of the compass-needle, or is it slightly magnetized?

(B) If there is any residual magnetism, strike the core with a hammer and test again.

(C) Use a soft steel wire nail for the core, and repeat (A) and (B). Why does soft iron make a better core than steel for electromagnets? Which should be the more easily magnetized?

EXPERIMENT 160. Magnetic tick.

400. Directions. (A) Join the electromagnet with the cell and key as before (Exp. 154). Hold one end of the core firmly against the top of a tin box which should stand upon the table and which should act as a sounding-board. The flat boxes used in the experiments on static electricity are good for this, or use the tin[167] box, No. 94, for a base. Rapidly open and close the circuit by means of the key and listen for any clicks made by the core.

(B) Listen for this sound in telegraph sounders, electric bells, etc., if you have them. The armature should be held, of course, so that slight sounds can be heard.

EXPERIMENT 161. Magnetic figures.

402. Directions. (A) Arrange as in Fig. 126. The key should be used in case a dry cell acts as the source of the current. Two good cells joined in series can be used to advantage. Lay the coil flat upon the table and place on it a piece of stiff, smooth paper, or a sheet of glass.

(B) Sprinkle a few iron filings upon the glass, which may be held in place by books. Gently tap the glass with a pencil while you close the circuit at the key. Do the filings arrange themselves as in the case of permanent magnets? Make a sketch of the field, remembering that you have both N and S poles, and compare it with previous results.

EXPERIMENT 162. Magnetic figures.

403. Directions. (A) Arrange as in Fig. 126, but stand the coil on end, using the base as directed in § 407, to hold it firmly in position. Join the ends, O E and I E, to the key as before. Fig. 127 shows a top view of the coil and base.

(B) With books, etc., fix a piece of[168] stiff, smooth paper, or glass just over the top of the core, and proceed as in Exp. 161 to study the field. See § 417 for making permanent pictures of magnetic fields.

EXPERIMENT 163. Magnetic field.

404. Directions. (A) Use same arrangement as for Exp. 162, except filings and glass, which are replaced by the compass.

(B) Hold the compass about 2 in. from the top pole of the electromagnet, close the circuit for a second or two and note action of needle. Is the top N or S, when the current enters the coil at O E? Compare result with § 392.

(C) Move the compass quickly about the pole, the circuit being closed, and note action of needle. Compare result with directions taken by particles of iron filings in Exp. 163.

(D) Reverse the direction of the current through the coil and test the nature of the pole at the top.

407. Experimental Magnets are quickly joined to a tin base (No. 94), which has 3 holes punched in, through which screws can be put to hold the cores in place. Fig. 127 shows plan of tin. Fig. 130 shows how removable cores are fastened to the base, the coils being on the spools, and Fig. 131 shows how home-made coils on bolts can be used. The coils on bolts should be wound as directed in Apparatus Book, Chapter X. The tin base also serves as the yoke.

Removable Cores. Fig. 130. These are of soft iron (No. 92, 93). In one end of each is a hole for the screws, S. Part of the tin has been cut away in the Fig. The copper washer, C W,[170] should be used. (See § 408.) Connectors are fastened to the ends of the coils (§ 226–230).

Bolt Cores. Fig. 131. After winding on the coils, as directed in Apparatus Book, remove the nut and put on an extra washer, E W, so that the ends of the coils will not be pressed against the tin, but come out between the two washers. Push the screw-end of the bolt through holes (about 2 in. apart) punched in the tin, then put on the nut, as shown. Do not force the nut on too far,—just far enough to hold the cores in place. The ends of the wires are not shown in Figs. 130, 131. Connectors are fastened to them (§ 408).

Fig. 132.

408. Method of Joining Coils. To produce the best results the poles of the horseshoe electromagnet should be unlike. As the coils are wound alike, their ends must be joined in such a manner that the current will pass around them in opposite directions; that is, if the current enters one coil at the outside end, O E, it must enter the other coil at the inside end, I E. Fig. 132 shows a plan of the connections, spring connectors being fastened to the coil-ends, to allow rapid and easy changes in the arrangement. L, M, and R are pieces of metal fastened to a strip of wood (No. 95), used to make connections from cells or other apparatus. They are turned up at each end as in Fig. 104, 3. Care should be taken not to get short circuits by allowing two wires to touch the tin base.

By changing the ends of the coils upon L, M, and R (left, middle, and right), and by changing the direction in which the current enters the "combination connecting plates" (No. 95), it is evident that the nature of the poles can be regulated to suit.

EXPERIMENTS 164–173. To study horseshoe electromagnets.

EXPERIMENT 164. To test the poles.

409. Directions. (A) Arrange as in Fig. 126, but use the experimental magnets and combination connections (Fig. 132) in place of the single coil shown in Fig. 126. Join O of the key with L, and Zn of the cell with R of Fig. 132. When the key is pressed the current will enter the magnets from L and leave at R.

(B) With the compass test the polarity of the cores as in Exp. 163, B, C. Make a sketch of the arrangement, and note which pole is N and which S.

(C) See which way the current must pass around each coil, by the way it is wound, and compare the results of (B) with Exp. 154, Fig. 123.

EXPERIMENT 165. To test the poles.

410. Directions. (A) Arrange as in Exp. 164, but reverse the direction of the current through the coils. Do this by joining O of the key (Fig. 126) with R of Fig. 132, and Zn of the cell with L.

(B) Repeat (B) and (C) of Exp. 164 and study results.

EXPERIMENT 166. To test the poles.

411. Directions. (A) Arrange all connections as in Exp. 164, then reverse the positions of O E and I E of coil A; that is, join O E to M, and I E to L, Fig. 132. This will make unlike ends come together at M; in other words, when the current enters at L and leaves at R it will pass around both coils in the same direction.

(B) Study the nature of the poles, as in Exps. 164, 165, and note results.

Note.—Fig. 133 shows simply the two cores of a horseshoe electromagnet with arrows to indicate in which direction the current is passing in each coil to produce N and S poles.

EXPERIMENT 167. To study the inductive action of one core upon the other.

412. Directions. (A) Arrange as for Exp. 164, but join the[172] wire from Zn of the cell to M (Fig. 132). In this way coil B will be cut out of the circuit. Place the coils in the E and W line.

(B) Find about how far the residual magnetism of the core of B can act upon the compass-needle, holding the compass on the side away from coil A, no current passing.

(C) Press the key for an instant, and note whether the magnetism of coil B has been made stronger or weaker. Explain the action of core A on core B.

EXPERIMENT 168. Magnetic figures.

413. Directions. (A) Arrange as in Exp. 164. With books, etc., fix a piece of smooth, stiff paper, or a sheet of glass, just above the poles of the electromagnets.

(B) Sprinkle iron filings upon the glass, and gently tap it while the circuit is closed at the key for a few seconds. Make a sketch of the magnetic figure produced. Do the lines of force from the opposite poles attract or repel each other? See § 417 for making permanent figures. (See "Things a Boy Should Know About Electricity" for drawings of magnetic figures.)

Note.—If possible, use two or three good cells in series for making magnetic figures, as a fairly strong field is best.

EXPERIMENT 169. Magnetic figures.

414. Directions. (A) Arrange apparatus as for Exp. 165, and make the magnetic figure for this combination, as directed in Exp. 168. Sketch and study the results.

EXPERIMENT 170. Magnetic figures.

415. Directions. (A) Arrange the apparatus and connections as in Exp. 166, and make the magnetic figure of this combination as directed in Exp. 168. In this case the poles are alike. Sketch and study the results.

EXPERIMENT 171. Magnetic figures.

416. Directions. (A) Arrange apparatus and connections as in Exp. 167, and make the magnetic figure of the combination as directed in Exp. 168. Compare the figure produced with that of Exp. 168. In this case the current passes through but one coil.

417. Permanent Magnetic Figures can be made in several ways for future study and comparison.

(A) Paraffine paper figures. Make paraffine paper as directed in Apparatus Book, page 135. For this purpose smooth, stiff, white[173] paper is best, so that the filings will show plainly, and but a thin coating of paraffine should be given. Place the magnets upon the table, lay over them a piece of unparaffined paper, and fix the paraffine paper directly over this. This is necessary, as the coated paper sticks when heated. For electromagnets it will be necessary to support the edges of the paper with books, etc. Sprinkle on the filings and tap the paper to make them arrange themselves while the circuit is closed. After the lines of force show plainly, the current need not be used again, provided the paper be kept perfectly still. Pass the flame of a Bunsen burner over the paper to melt the coating. This will, no doubt, make the two pieces of paper stick together, and permanently fix the particles of filings in place. Do not heat the paper too much—just enough to melt the paraffine. If you have no gas, hold a fire-shovel, containing hot coals, over the paper. As soon as the paraffine cools, the figures will stand considerable handling.

Blue print figures are very pretty, and last indefinitely. Get some blue-print paper at a photographer's, who will give you directions about "developing" it with water. Keep this in the dark, and take out but one sheet at a time for experiments. To make the figures, take your apparatus near a window where bright sunlight comes in. Pull down the curtain so that you have but a dim light when you make the magnetic figure, as directed before. After the lines of force show plainly, raise the curtain, and let the bright sunlight shine on it for 5 or 6 minutes, or until the surface of the paper has a rich, bronze color. The paper cannot be acted upon by the light under the particles of filings. Quickly shake the filings from the paper, and wash it in 3 changes of water to "develop" it, then pin the paper up to dry.

EXPERIMENT 172. Lifting power.

418. Directions. (A) Arrange the apparatus as in Exp. 164. Hold an iron strip (No. 76), a screw-driver, or other iron bar directly over and near the poles of the experimental electromagnet. Close the circuit at the key, then lift the magnets by the "armature," as the iron strip may be called, the circuit being kept closed for a few seconds. If your cell is good there should be no trouble in lifting the magnets by the armature. Open the circuit, and see whether the magnets drop.

(B) Hold the magnets upside down directly over nails, tin[174] boxes, iron filings, or other pieces of iron. Close the circuit, move the attracted iron to another place on the table, and open the circuit. Can this principle be used for practical purposes?

Note.—Some experiments illustrating practical uses of electromagnets will be given in a future chapter.

EXPERIMENT 173. Residual magnetism when magnetic circuit is closed.

419. Directions. (A) Arrange as in Exp. 164. You have already seen that each core retains some magnetism after the circuit is closed. Place the iron strip firmly across the poles, close the circuit for an instant, open the circuit, then see whether the armature still clings to the cores with some strength. The armature should fit well upon the cores for this experiment.

(B) Again press the armature upon the cores, no current being used; then lift it as in (A). Compare the attraction with that found in (A).

EXPERIMENT 174. To find whether electricity can be produced by heat.

421. Home-made Thermopile. (Fig. 134.) For this you need 3 hairpins, copper wire, a piece of wood about 3 in. long and 1 in. square on the ends, 2 pieces of tin, and some small nails.

Straighten the hairpins and scrape the coating off with sandpaper or a file. Scrape the insulation from 4 pieces of copper wire, each about 8 in. long. Twist the ends of the copper wire about the ends of the hairpins (Fig. 134), and then fasten the hairpins to the block. They may be held firmly by small nails which should be driven partly into the block and bent over. The hairpins at the right-hand side of the Fig. are shown to be near but not touching each other. This allows all to be heated at the same time.

The tin binding-posts may be nailed or screwed to the block, and if the bare copper wires 1 and 4 be placed under X and Y before they are screwed down they will be electrically connected. The ends of 1 and 4 may be held under the screw-heads. The[176] block may be supported upon other blocks to raise it to the proper height, which will depend upon the length of the candle.

Fig. 135.

A thermopile in the form of a circle with several pairs of metals, can easily be made by fastening the hairpins to a piece of cardboard (Fig. 135) with a hole at the center. This may be supported by blocks, the heat being applied under the center.

422. Directions. (A) Arrange the apparatus as in Fig. 134. See that the astatic needle is properly adjusted, no magnets being near it.

(B) Heat the joints as shown, and watch the needle. Can a current be produced by heat?

(C) Remove the connector on wire 6 from Y to M, thus cutting one pair out of the circuit. Heat the joints again and compare the strength of the current with that produced in (B).

(D) See whether much current is produced by one pair. From results obtained do you see any relation between the strength of the current and the number of pairs?

EXPERIMENTS 175–182. To study induced currents.

EXPERIMENT 175. To find whether a current can be generated with a bar magnet and a hollowed coil of wire.

427. Directions. (A) Arrange as in Fig. 136. The coil (No. 90) of fine wire is joined to A G (No. 59) as shown. Small pieces[179] of tin or copper, 1 and 2, are used to make connections between the coil ends and wires, 3 and 4, which are attached to the galvanoscope. It is best to use the wires, 3 and 4, so that the coil will be 2 feet at least, from A G; otherwise the needle of A G might be affected by the magnet, M (No. 97).

(B) Get clearly in mind in which direction the right-hand end of the needle is deflected when a current enters A G at L, the left-hand binding-post. If you have forgotten the results of previous experiments, use the cell for an instant, touching the wire from the carbon to L and that from the zinc to R. If any currents come from the coil, later, you should be able to tell in which direction they flow, the coil and A G forming a closed circuit.

(C) Hold the magnet, M, as shown, and quickly push it into the coil until it has the place of a core, at the same time watching the needle. If a current is produced, in which direction does it flow from the coil? Does the needle remain deflected? Is the current constant or temporary?

(D) After the magnet, M, has been placed in the coil, as in (C), and the needle has come to rest, quickly pull M from the coil, watching the needle. If a current is produced, does it pass from the coil in the same direction as before, in (C)?

(E) Turn M end for end, repeat (C) and (D), and study the results. Are lines of force made to cut the turns of the coil?

(F) Repeat (C) and (D), moving M slowly.

EXPERIMENT 176. To find whether a current can be generated with a bar magnet and a coil of wire having an iron core.

430. Directions. (A) Arrange as in Exp. 175, Fig. 136, and, in addition, place an iron core (No. 92) inside of the coil (No. 90).

(B) Hold the bar magnet (No. 97) as in Fig. 136, and quickly lower it until it touches the core, at the same time watching the needle. Study results, direction of current, etc., as before.

(C) Suddenly withdraw M from the core. Is the current produced in the same direction as that from (B)?

(D) Turn M end for end and repeat (B) and (C).

(E) Repeat (C) and (D), moving magnet slowly.

How does the strength of the current compare with that of Exp. 175? Are lines of force made to cut the turns of the coil?

EXPERIMENT 177. To find whether a current can be generated with a horseshoe magnet and a coil of wire having an iron core.

431. Directions. (A) Arrange the apparatus as in Exp. 176, but use the horseshoe magnet, H M, instead of the bar magnet. Fig. 137 shows the coil (No. 90) with one pole of H M held over the core.

(B) Study the effect of quickly lowering and raising first one pole and then the other over the core, as with the bar magnet. Get clearly in mind the direction in which the induced current flows in each case.

EXPERIMENT 178. To find whether a current can be generated with an electromagnet and a hollow coil of wire.

433. Directions. (A) The hollow coil (No. 90) should be joined to the astatic galvanoscope, as shown in Fig. 136. Instead of the bar magnet in Fig. 136, an electromagnet is to be used, and this should be joined in series with a cell and key, as shown in Fig. 138. The current from the cell will pass only when K is pressed.

(B) Note from the winding which way the current must pass around the coil when the circuit is closed at K, and determine whether the lower end of the long iron core, L I C (No. 96) should be N or S. With the compass test the poles of the core to be sure you are right.

(C) Quickly lower the end of L I C into the hollow coil (H, Fig. 136), the circuit being kept closed long enough to allow the needle[182] to partially come to rest again. Withdraw L I C before you open the circuit. Explain action of needle.

(D) Reverse the direction of the current through the electromagnet, by changing the connections, and repeat (C). Does any induced current pass through A G when the core is held still in the coil H, even though a current passes through coil E?

EXPERIMENT 179. To find whether a current can be generated with an electromagnet and a coil of wire having an iron core.

434. Directions. (A) Fig. 139 shows simply the arrangement of coils. Coil H (No. 90) with core, is joined to the galvanoscope as in Fig. 136. Coil E, with short core, should be joined to key and cell as shown in Fig. 138.

(B) Keeping in mind the polarity of the lower end of core E, quickly lower it to the core of H, the circuit being kept closed for a few seconds. Does the needle remain deflected after the motion ceases?

(C) Quickly raise E, the circuit being still closed, then open the circuit. Compare the directions taken by the induced currents in (B) and (C).

EXPERIMENT 180. To study the effect of starting or stopping a current near a coil of wire or other closed circuit.

436. Directions. (A) Arrange as in Fig. 140. Place the two[183] coils, H and E, on the same core, L I C. Connect E with the key and cell as before (Fig. 138). Connect H with the astatic galvanoscope, A G, as in Fig. 136. Keep the coils 2 or 3 feet from A G, so that the needle will not be affected by them.

(B) Close the circuit at the key, watching the needle, then as soon as the needle regains its former position, open the circuit again. Compare the direction of the induced current in H with that of the current in E, (1) when the main circuit is closed, and (2) when it is opened. Is any current induced in H by a steady current in E? (See Transformers.)

EXPERIMENT 181. To study the effect of starting or stopping a current in a coil placed inside of another coil.

437. Directions. (A) Arrange as in Fig. 141. Join coil H with the astatic galvanoscope, A G. Place the small coil P (No. 98) with core, inside of H, and connect the ends of P with the key and cell, as shown.

(B) Close the circuit at K; watch the needle, and as soon as it regains its position, open the circuit again.

Compare the direction of the induced current in H with that of the inducing current in P, (1) when the inducing circuit is closed, and (2) when it is broken. (See Induction Coils.)

EXPERIMENT 182. To see what is meant by alternating currents.

442. Directions. (A) Arrange as in Fig. 143. Connect coil H with A G, as before. Place one pole of H M against the end of the core I C, hold H with one hand, and with the other quickly push the other pole of H M onto the core. This should produce a momentary current through A G, first in one direction, and then in the other. Let the needle come to rest.

(B) Move H M back and forth upon the end of I C, changing its polarity rapidly. A minute's practice will enable you to slide the core from one pole of H M to the other and back again rapidly—3 complete vibrations per second being about right. The needle should be parallel to the coil of A G, and if properly done, the needle will be made to vibrate back and forth slightly at each change in the polarity of I C.

EXPERIMENTS 183–190. To study the production of motion by means of the electric current.

EXPERIMENT 183. Motion produced with a hollow coil and a piece of iron.

446. Directions. (A) Arrange as in Fig. 144. Coil H (No. 90) is to be used as a pendulum, and can be supported by fastening a string to it, the upper end of which should be tied to S W.

Connect the ends of H with K and D C. There will be a slight magnetic field about H as soon as the circuit is closed.

(B) Hold I C near the end of the coil. Close the circuit for an instant. Is there any motion produced in H? While the motion will be slight, there should be enough to be noticed if the cell is strong.

(C) Swing the suspended coil back and forth like a pendulum for a minute, until you get in mind the rapidity of its vibrations. Stop it, then repeat (B), closing and opening the circuit at regular intervals, so that the little impulses given by the attraction for[188] I C will gradually cause H to vibrate. The wires leading from H should not drag upon the table.

EXPERIMENT 184. Motion with hollow coil and bar magnet.

447. Directions. (A) Substitute the bar magnet M (No. 97) for the iron of Exp. 183 (Fig. 144). Get clearly in mind the polarity of the coil from the way the current flows through it, then test it with the compass to find whether you are right.

(B) Hold the N pole of M near the left-hand end of the coil, close the circuit for an instant and study results.

(C) Reverse the magnet and repeat (B). Compare the results with those of Exp. 183. Try to make the coil vibrate.

EXPERIMENT 185. Motion with electromagnet and piece of iron.

448. Directions. (A) Arrange as described in Exp. 183, Fig. 144. Place a short core inside of the coil and repeat. (See § 446 for directions.) Why is the motion produced much larger than that given by a hollow coil?

(B) The coil can gradually be made to swing through quite a little space by closing and opening the circuit regularly (§ 446, C). Could any use be made of such a motion, if it were on a large scale? Could it be made to run a machine?

EXPERIMENT 186. Motion with electromagnet and bar magnet.

449. Directions. (A) Arrange as in Fig. 145, the coil being suspended and connected as in Exp. 183 (Fig. 144).

(B) Study the effect of closing the circuit when the N pole of M is held near the core of H. Reverse M, and repeat.

EXPERIMENT 187. Motion with electromagnet and horseshoe magnet.

450. Directions. (A) Arrange as in Fig. 146. The ends of H (No. 89) are joined to X and Y of the current reverser C R (No. 57). It is evident, then, that the direction of the current through H can be easily and rapidly reversed by C R. (See Exp. 103.) Either pole of the horseshoe magnet H M will attract I C when it is not magnetized.

[189]

(B) Place the end of I C near the N pole of H M so that it will be attracted to it. You have learned that like poles repel each other, so press the lever of C R that will produce a N pole at the left-hand end of I C. The core I C should be repelled by the N pole of H M and be instantly attracted by its S pole.

(C) Rapidly reverse the current and make I C jump back and forth from one pole to the other. The results of this experiment should be remembered, as they will aid in understanding motors. A core ¼ in. in diameter can be placed in between the poles and be made to vibrate rapidly as the current is reversed.

EXPERIMENT 188. Motion with two electromagnets.

451. Directions. (A) Arrange as in Fig. 147. Join the two coils, H and E, in parallel. Connect their two outside ends O E to a metal plate A, and their inside ends I E to B. Join wires 1 and 6 to K, D C, A and B, as shown. When the circuit is closed at K, the current will pass along wire 1 and divide at A, entering E and H at the same time by wires 2 and 4 and returning through 3 and 5 to B, and thence to D C.

(B) Close the circuit for an instant with wires arranged as in Fig. 147. Do the electromagnets attract or repel each other?[190] Study out the direction in which the current passes around the coils, and see whether they should attract or repel.

(C) Change wire 4 to B, and wire 5 to A. The polarity of H, only, will be changed when this circuit is closed. Press the key for an instant and study the results.

EXPERIMENT 189. Rotary motion with a hollow coil of wire and a permanent magnet.

453. Directions. (A) Arrange as in Fig. 148. A key can be used instead of the reverser. The coil of the galvanoscope, G V, has a magnetic field about it when the circuit is closed. The needle has a permanent field.

(B) Close the circuit for an instant, let the needle swing back past the zero mark, close the circuit again, etc., until the added impulses give the needle a complete turn.

(C) Keep the needle turning on its axis by opening and closing the circuit at the proper time. With a little practice you can make it turn rapidly.

(D) Reverse the motion of the needle. (See § 455.)

EXPERIMENT 190. Rotary motion with an electromagnet and a permanent magnet.

454. Directions. (A) Arrange as in Fig. 149. Place the compass a short distance from the end of the core of the coil H (No. 89). Close the circuit, and as soon as the needle gets part way around open it again, closing it at the proper time to give the needle a new impulse. The speed can be regulated, somewhat, by changing its distance from the core. A key may be used in place of a reverser.

(B) Reverse the direction of rotation.

EXPERIMENT 191. To study the action of a simple "telegraph sounder."

457. Directions. (A) Arrange as in Fig. 150. The electromagnet is supported upon its base, as directed in § 407. Coil H, K, and D C are joined in series. The iron strip, I, can be held by the left hand, while K is worked with the right.

(B) Press the key, closing the circuit for different lengths of time, and note that the armature, I, responds exactly to the motions at K.

EXPERIMENT 192. To study the action and use of the "relay" on telegraph lines.

461. Directions. (A) Arrange as in Fig. 152. Place K and D C at one end of the table to represent the sending station. At the other end of the table place E, which is the electromagnet of the relay, and H, the electromagnet of the sounder. Connect the ends of E with K and D C, L W being the line wire, and R W the return. In practice, the return is through the earth. The relay armature, R A, should vibrate towards E every time K is pressed. C is a piece of copper against which R A presses each time it is attracted by E, and this closes what is called the local circuit. Connect the poles of another battery, L B, with C and H, and the[195] other end of coil H with R A. The sounder armature, S A, should be arranged as in Exp. 191. Small springs are shown on the two armatures, and these keep them away from the cores when the circuits are open.

(B) Fasten the parts to a board, and study the connections and action of this home-made outfit.

462. The Relay replaces the sounder in the line wire circuit, and its coils are usually wound with many turns of fine wire, so that a feeble current will move its nicely adjusted armature. Owing to the large resistance of long telegraph lines, the current is weak when it reaches a distant station, and not strong enough to work an ordinary sounder. The current passes back from the relay to the sending station through the earth. The relay armature acts as an automatic key to open and close the local circuit, which includes also a battery and sounder. The line current does not enter the sounder. (See "Things A Boy Should Know About Electricity.")

EXPERIMENT 193. To study the action of a two-pole telegraph instrument.

463. Directions. (A) Arrange as in Fig. 153. Connect the two coils to the connecting plates, as described in § 408. Join a strip of copper Cu with wire 2 leading from D C, and join the zinc of D C to M. The ends of wires 1 and 3 should be near Cu but they must not touch it. If Cu be slightly curved so that its ends are raised above the table, the ends of wires 1 and 3 may be put directly under the ends of Cu; each half of Cu can then be used as a key. Two armatures, A and B, should be held as shown. D C can be placed at one side, of course, its terminals being joined to M and Cu.

(B) Press first one end and then the other of Cu, so that the current will pass through H or E at will.

(C) Paste pieces of paper to the armatures, the left one being marked with a dot, and the other with a dash. The one who[196] sends the message can make dots or dashes at the instrument by pressing the proper key. This form of instrument can be easily made by boys, and the messages are more easily read by the eye than by the ear, as in regular sounders.

EXPERIMENT 194. To study the action of a simple "single needle telegraph instrument."

464. Directions. (A) Arrange as in Fig. 154. Stick a pin on each side of the N pole of the galvanoscope-needle through the degree-card, so that the needle can make but part of a turn when the circuit is closed.

(B) Touch one lever of the reverser C R, then the other, to see whether connections are right. The needle should be forced against one pin and then against the other. If motions to the left represent dots, and those to the right dashes, combinations of dots and dashes can be used for letters as in the "sounder" (Exp. 191).

(C) Arrange the apparatus shown in Fig. 122 so that messages can be sent.

EXPERIMENT 195. To study the action of a simple automatic "contact breaker," or "current interrupter."

465. Directions. (A) Arrange as in Fig. 155. Slip a spring connector attached to wire 1 upon the iron strip I, a short distance from its end. Hold the left-hand end of I firmly in one hand, and with the other hold the connector on wire 2 just above that on 1. The right-hand end of I should be just above the core of H.

(B) Allow the current to pass through the circuit by touching the two connectors together gently. Does the armature make one click, as in the telegraph sounder, or does it vibrate rapidly?

(C) Try the connectors in various positions on I.

EXPERIMENT 196. To study the action of a simple "electric bell," or a "buzzer."

467. Directions. (A) Fig. 156 shows the circuit explained in Exp. 195, with a key or push-button put in, so that the circuit can be closed at a distance from the vibrating armature.

(B) Have a friend work the key while you hold I and wires 1 and 2 as directed in Exp. 195. The circuit must not be broken at two places, of course, so begin by holding the two connectors together. The armature should vibrate rapidly each time K is pressed.

EXPERIMENT 197. To study the action of a simple telegraph "recorder."

469. Directions. (A) Cut from a tin box, or can, a piece of tin about 4 in. long and 1½ in. wide. Bend this double to make two thicknesses. This will serve as an armature I (Fig. 157). Nail to one end of I a small spool, S, and into this put a short length of lead-pencil, P, which may be held firmly in S by wrapping a little paper around it. Connect the ends of coil H to a key and cell as in Fig. 156.

(B) Hold or fasten I in place, and have a friend make dots and dashes at the key, while you draw a piece of paper past the end of P. A little adjusting will be necessary to get the pencil to write only while the circuit is closed. In regular machines all the parts are automatic.

EXPERIMENT 198. To study the action of a simple "annunciator."

470. Directions. (A) Arrange as in Fig. 158. Fasten the two electromagnets, H and E, to a board or a piece of stiff cardboard. They may be held in place by passing strings over them and through the board, tying on the other side. The ends of coils H and E should be joined to pieces of tin, A, B, C, by means of connectors. K and K are keys or push-buttons, which in real instruments are in different rooms. Two steel pens may be[199] swung on pins a short distance from the ends of the cores, so that their lower ends will be attracted to the cores the instant the current passes through them. The residual magnetism should hold them against the cores until removed. Hairpins, nails, or needles can be used instead of pens.

(B) Press first one K and then the other to see whether your connections are correct.

EXPERIMENT 199. To study the shocking effects of the "extra current."

472. Directions. (A) Use the two electromagnets joined to the connecting plates (Fig. 132), to generate a self-induced or extra current. Connect R of Fig. 132 with the zinc of a dry cell, and between L and the carbon of the cell place a key; in other words, join the electromagnets, cell, and key in series. Two good cells in series can be used to advantage.

(B) Wet the ends of two fingers of the left hand, press one upon L and the other on R, thus making a shunt with your hand. With the right hand work the key rapidly. If the current is strong enough you should feel a slight shock in the fingers each time the circuit is broken. The extra current (§ 444) causes the shock as it shoots through the fingers.

(C) If you have electric bells or telegraph sounders use them for this experiment.

474. Action of Induction Coils. Fig. 159 shows a top view of one of the home-made induction coils described, in full, in the Apparatus Book. Wires 5 and 6 are the ends of the primary coil, while wires 7 and 8 are the terminals of the secondary coil. The battery wires should be joined to binding-posts W and X, and the handles to Y and Z. Fig. 160 shows the details of the automatic interrupter which is placed in the primary circuit.

Fig. 160.

If the current enters at W, it will pass through the primary coil and out at X, after going through 5, R, F, S I, B, E and C. The instant the current passes, the bolt becomes magnetized; this attracts A, which pulls B away from the end of S I, thus automatically opening the circuit. B at once springs back to its[201] former position against S I, as A is no longer attracted; the circuit being closed, the operation is rapidly repeated. (For commercial forms and uses of induction coils see "Things A Boy Should Know About Electricity.")

EXPERIMENT 200. To study the action of the telephone.

478. Directions. (A) Join the ends of coil H (Fig. 164) to the astatic galvanoscope. Move magnet M back and forth in front of the soft iron core, while H is held in position. Watch the needle. Imagine that vibrations in the air caused by the voice are strong enough to give M a slight motion to and fro, and you can see how a current would be sent through the galvanoscope by speaking against M.