CHAPTER XXVIII. WIRE TABLES _The Study of Elementary Electricity and Magnetism by Experiment Containing Two Hundred Experiments Performed with Simple, Home-made Apparatus

APPARATUS LIST 210

INDEX 215

MAGNETISM

A Few Dont's for Young Students.

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.

PART I.--MAGNETISM.

CHAPTER I.

IRON AND STEEL.

_=1. Introduction.=_ We should know something about iron and steel

at the start, because we are to use them in nearly every experiment.

The success with some of the experiments will depend largely upon the

quality of the iron and steel used.

When we buy a piece of iron from the blacksmith, we get more than

iron for our money. Hidden in this iron are other substances (carbon,

phosphorus, silicon, etc.), which are called "impurities" by the

chemist. If all the impurities were taken out of the iron, however,

we should have nothing but a powder left; this the chemist would call

"chemically pure iron," but it would be of no value whatever to the

blacksmith or mechanic. The impurities in iron and steel are just

what are needed to hold the particles of iron together, and to make

them valuable. By regulating the amount of carbon, phosphorus, etc.,

manufacturers can make different grades and qualities of iron or steel.

When carbon is united with the _pure_ iron, we get what is commonly

called iron.

_=2. Kinds of Iron and Steel.=_ _Cast iron_ is the most impure form of

iron. Stoves, large kettles, flatirons, etc., are made of cast iron.

_Wrought iron_ is the purest form of commercial iron. It usually comes

in bars or rods. Blacksmiths hammer these into shapes to use on wagons,

machinery, etc. _Steel_ contains more carbon than wrought iron, and

less than cast iron.

_Soft steel_ is very much like wrought iron in appearance, and it is

used like wrought iron.

_Hard steel_ has more carbon in it than soft steel. Tools, needles,

etc., are made of this.

=EXPERIMENT 1. To study steel.=

_Apparatus._ A steel sewing-needle (No. 1).[A]

[Footnote 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.

_=4. Discussion.=_ "Needle steel" is usually of good quality. It will

be very useful in many experiments. Do you know how to make the needle

softer?

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

made softer.=

[Illustration: Fig. 1.]

_Apparatus._ Fig. 1. A needle; a cork, Ck (No. 2); lighted

candle (No. 3). The bottom of the candle should be warmed and

stuck to a pasteboard base.

=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.

hardness with a file (Exp. 1).

_=6. Annealing.=_ This process of softening steel by first heating it

and then allowing it to cool slowly, is called _annealing_. All pieces

of iron and steel are, of course, hard; but you have learned that some

pieces are much harder than others.

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

hardened.=

_Apparatus._ The needle just annealed and bent; cork, etc., of

Exp. 2; a glass of cold water.

=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.

_=8. Hardening; Tempering.=_ Good steel is a very valuable material;

the same piece may be made hard or soft at will. By sudden cooling, the

steel becomes very hard. This process is called _hardening_, but it

makes the steel too brittle for many purposes. By _tempering_ is meant

the "letting down" of the steel from the very hard state to any desired

degree of hardness. This may be done by suddenly cooling the steel when

at the right temperature, it not being hot enough to produce extreme

hardness. (The approximate temperature of hot steel can be told by the

colors which form on a clean surface. These are due to oxides which

form as the steel gradually rises in temperature.)

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

_Apparatus._ A piece of soft iron wire about 3 in. (7.5 cm.)

long (No. 4); the candle, water, etc., of Exp. 3.

=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.

_=10. Discussion.=_ Soft iron contains much less carbon than steel. The

hardening quality which steel has is due to the proper amount of carbon

in it. If you have performed the experiments so far, you will be much

more able to understand later ones, and you will see why we are obliged

to use soft iron for some parts of electrical apparatus, and hard steel

for other parts.

CHAPTER II.

MAGNETS.

_=11. Kinds of Magnets.=_ Among the varieties of magnets which we

shall discuss, are the natural, artificial, temporary, permanent, bar,

horseshoe, compound, and electro-magnet.

[Illustration: Fig. 2.]

_The Horseshoe Magnet_, H M (Fig. 2), is the most popular form of

small magnets. The red paint has nothing to do with the magnetism. The

piece, A, is called its _armature_, and is made of soft iron, while the

magnet itself should be made of the best steel, properly hardened. The

armature should always be in place when the magnet is not in use, and

care should be taken to thoroughly clean the ends of the magnet before

replacing the armature. The horseshoe magnet is _artificial_, and it is

called a _permanent_ magnet, because it retains its strength for a long

time, if properly cared for.

=EXPERIMENT 5. To study the horseshoe magnet.=

_Apparatus._ Fig. 2. The horseshoe magnet, H M (No. 16).

=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.

_=13. Poles; Equator.=_ The ends of a magnet are called its _poles_.

The end marked with a line, or an N, should be the _north_ pole. The

unmarked end is the _south_ pole. N and S are abbreviations for north

and south. The central part, at which there _seems_ to be no magnetism,

is called the _neutral point_ or _equator_.

=EXPERIMENT 6. To ascertain the nature of substances attracted

by a magnet.=

_Apparatus._ The horseshoe magnet, H M (Fig. 2); silver,

copper, and nickel coins; iron filings (No. 17), nails, tacks,

pins, needles; pieces of brass, lead, copper, tin, etc.

(Ordinary tin is really sheet iron covered with tin.) Use the

various battery plates for the different metals.

=14. Directions.= (A) Try the effect of H M upon the above

substances, and upon any other substances thought of.

_=15. Magnetic Bodies; Diamagnetic Bodies.=_ Substances which are

attracted by a magnet are said to be _magnetic_. A piece of soft iron

wire is magnetic, although not a magnet. Very strong magnets show

that nickel, oxygen, and a few other substances not containing iron,

are also magnetic. Some elements are actually repelled by a powerful

magnet; these are called _diamagnetic_ bodies. It is thought that all

bodies are more or less affected by a magnet.

_=16. Practical Uses of Magnets.=_ Many practical uses are made of

magnets, such as the automatic picking out of small pieces of iron

from grain before it is ground into flour, and the separation of iron

from other metals, etc. The most important uses of magnets are in the

compass and in connection with the electric current, as in machines

like dynamos and motors. (See experiments with electro-magnets.)

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

substances.=

_Apparatus._ Horseshoe magnet, H M; a sheet of stiff paper;

pieces of sheet glass, iron, zinc, copper, lead, thin wood,

etc.; sewing-needle. (A tin box may be used for the iron, and

battery plates for the other metals.)

=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.

through your hand.

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

through water.

_=18. Magnetic Transparency; Magnetic Screens.=_ Substances, like

paper, are said to be _transparent_ to magnetism. Iron does not allow

magnetism to pass through it as readily as paper and glass; in fact,

thick iron may act as a _magnetic screen_.

=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?

_Apparatus._ The horseshoe magnet, H M; two sewing-needles that

have never been near a magnet; iron filings.

=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.

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.

_=21. Discussion; Bar Magnets.=_ A piece of good steel will attract

iron after merely touching a magnet. To thoroughly magnetize it,

however, a mere touch is not sufficient. There are several ways of

making magnets, depending upon the size, shape, and strength desired.

For these experiments, the student needs only a good horseshoe magnet,

or, better still, the electro-magnets described later; with these any

number of small magnets may be made. Straight magnets are called _bar

magnets_.

=EXPERIMENT 9. To make small magnets.=

_Apparatus._ Fig. 3. The horseshoe magnet, H M; sewing-needles;

iron filings. (See Apparatus Book, Pg. 140, for various kinds

of steel suitable for 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

(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.

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

(D) Test the needle for magnetism with iron filings, and save

it for the next experiment.

[Illustration: Fig. 3.]

[Illustration: Fig. 4.]

=EXPERIMENT 10. To find whether a freely-swinging bar magnet

tends to point in any particular direction.=

_Apparatus._ Fig. 4. A magnetized sewing-needle (Exp. 9); the

flat cork, Ck (No. 2); a dish of water. (You can use a tumbler,

but a larger dish is better.)

=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.

_=25. North-seeking Poles; South-seeking Poles; Pointing Power.=_ It

should be noted that the _point_ swings to the north, provided the

needle is magnetized as directed in Exp. 9. This is called the north,

or north-seeking pole. The N-seeking pole is sometimes called the

marked pole. For convenience, we shall hereafter speak of the N-seeking

pole as the N pole, and of the S-seeking pole as the S pole. We shall

hereafter speak of the tendency which a bar magnet has to point N and

S, as its _pointing power_. An unmagnetized needle has no pointing

power.

_=26. The Magnetic Needle; The Compass.=_ A small bar magnet, supported

upon a pivot, or suspended so that it may freely turn, is called

a _magnetic needle_. When balanced upon a pivot having under it a

graduated circle marked N, E, S, W, etc., it is called a _compass_.

These have been used for centuries. (See Apparatus Book for Home-made

Magnetic Needles.)

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

_Apparatus._ Two magnetized sewing-needles (magnetized as in

Exp. 9); the cork, etc., of Exp. 10.

=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.

(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.=

_Apparatus._ A magnetized sewing-needle; cork, etc., of Exp.

10; a piece of soft iron wire, 3 in. long; iron filings.

=28. Directions.= (A) Test the wire for magnetism with

filings. 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.

and experiment.

_=29. Laws of Attraction and Repulsion.=_ From experiments 11 and 12

are derived these laws:

(_=1=_) _=Like poles repel each other=_; (_=2=_) _=Unlike poles attract

each other=_; (_=3=_) _=Either pole attracts and is attracted by

unmagnetized iron or steel.=_

The attraction between a magnet and a piece of iron or steel is mutual.

Attraction, alone, simply indicates that at least one of the bodies is

magnetized; repulsion proves that both are magnetized.

=EXPERIMENT 13. To learn how to produce a desired pole at a

given end of a piece of steel.=

_Apparatus._ Same as in Exp. 9.

=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.

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?

_=31. Rule for Poles.=_ The end of a piece of steel which last touches

a N pole of a magnet, for example, becomes a S pole.

_=32. Our Compass=_ (No. 18). While the floating magnetic needle

described in Exp. 10, and shown in Fig. 4, does very well, it will

be found more convenient to use a compass whenever poles of pieces

of steel are to be tested. Fig. 5 shows merely the cover of the box

which serves as a base for the magnetic needle furnished. We shall

hereafter speak of this apparatus as _our compass_, O C. (See Apparatus

Book, Chap. VII, for various forms of home-made magnetic needles and

compasses.)

=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.

[Illustration: Fig. 5.]

[Illustration: Fig. 6.]

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

reversed.=

_Apparatus._ Fig. 6. The horseshoe magnet, H M; a thin wire

nail, W N, 2 in. (5 cm.) long; a piece of stiff paper, cut as

shown, to hold W N; thread with which to suspend the paper;

compass, O C (No. 18).

=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.

still repelled? Has its S pole been reversed?

_=35. Discussion; Reversal of Poles.=_ The poles of weak magnets may

be easily reversed. This often occurs when the apparatus is mixed

together. It is always best, before beginning an experiment, to

remagnetize the pieces of steel which have already served as magnets.

The same may be shown by magnetizing a needle, rubbing it first in one

direction, and then in another upon the magnet, testing, in each case,

the poles produced.

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

poles.=

_Apparatus._ The horseshoe magnet, H M; an unmagnetized

sewing-needle; compass, O C (No. 18).

=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.

slide it in either direction.

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

it for the next experiment.

[Illustration: Fig. 7.]

[Illustration: Fig. 8.]

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

_Apparatus._ The strange magnet just made (Exp. 15); iron

filings; compass, O C (No. 18).

=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 new ends produced at that point. Remember that repulsion

is the test for polarity.

_=39. Discussion; Consequent Poles.=_ Iron filings cling to a magnet

where poles are located. In this case, two small magnets were made

in one piece of steel; they had a common S pole at the center. The

pointing power (§ 25) of such a magnet is very slight; would it have

_any_ pointing power if we could make the end poles of equal strength?

Intermediate poles, like those in the needle just discussed, are called

_consequent poles_. Practical uses are made of consequent poles in the

construction of motors and dynamos.

=EXPERIMENT 17. To study consequent poles.=

_Apparatus._ An unmagnetized sewing-needle; horseshoe magnet, H

M (No. 16); iron filings (No. 17); compass (No. 18).

=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.

=EXPERIMENT 18. To study the theory of magnetism.=

_Apparatus._ A thin bar magnet, B M (No. 21); iron filings; a

sheet of paper. Fig. 9 shows simply the edge of B M and the

paper. B M should be magnetized as directed in Exp. 9.

[Illustration: 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?

produced with B M.

_=42. Theory of Magnetism; Magnetic Saturation.=_ This bringing into

line the particles of iron indicates that each particle became a

magnet. This experiment should aid in understanding what is thought to

take place when steel is magnetized. The pile of filings represents the

body to be magnetized, and each little filing stands for a particle of

that body. A bar of steel is composed of extremely small particles,

called _molecules_. They are very close together and do not move from

place to place as easily as the pieces of filings. A magnet, however,

when properly rubbed upon the steel, seems to have power to make the

molecules point in the same direction. This produces an effect upon the

whole bar.

Each molecule of the steel is supposed to be a magnet. When these

little magnets pull together, the whole bar becomes a strong magnet.

When a magnet is jarred, and the little magnetized molecules are mixed

again, they pull in all sorts of directions upon each other. This

lessens the attraction for outside bodies.

Steel is said to be _saturated_, when it contains as much magnetism as

possible. A piece of steel becomes slightly longer when magnetized.

It is thought, by many, that there is a current of electricity around

each molecule, making a little magnet of it. (See electro-magnets.)

=EXPERIMENT 19. To find whether soft iron will permanently

retain magnetism.=

_Apparatus._ A piece of soft iron wire, 3 or 4 in. (7.5 to 10

cm.) long (No. 4); the horseshoe magnet, H M; iron filings;

flat cork, F C (No. 2), and the dish of water used in Exp. 10

(Fig. 4).

=43. Directions.= (A) Magnetize the wire (Exp. 9). Notice that

the wire clings strongly to H M.

(B) Test the lifting power of the little wire magnet by seeing

about how many iron filings its poles will raise.

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.

_=44. Retentivity or Coercive Force; Residual Magnetism.=_ Soft iron

loses _most_ of its magnetism when simply removed beyond the action of

a magnet. We say that it does not retain magnetism; that is, it has

very little _retentivity or coercive force_. This is an important fact,

the action of many electric machines and instruments depending upon it.

A slight amount of magnetism remains, however, in the softest iron,

after removing it from a magnet. This is called _residual magnetism_. A

piece of iron may show poles, when tested with the compass, although it

may have almost no pointing power.

=EXPERIMENT 20. To test the retentivity of soft steel.=

_Apparatus._ A wire nail, W N (No. 19); horseshoe magnet, H M;

iron filings; flat cork, F C; the dish of water (Exp. 10, Fig.

4).

=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).

(D) Again test its lifting and pointing powers.

_=46. Discussion.=_ Soft steel has a greater retentivity than soft

iron. It contains less carbon than cast or tool steel, and is called

mild steel or machinery steel. You do not want soft steel for permanent

magnets.

=EXPERIMENT 21. To test the retentivity of hard steel.=

_Apparatus._ A hard steel sewing-needle (No. 1); other articles

used in Exp. 20.

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

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

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

_Apparatus._ A magnetized sewing-needle; the candle, cork,

etc., of Exp. 2. (See Fig. 1.)

=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.

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

_=49. Discussion.=_ Heating a body is supposed to thoroughly stir up

its molecules. Jarring or twisting a magnet tends to weaken it. (See

Exp. 19.)

The molecules of steel do not move about or change their relative

positions as readily as those of soft iron. When the molecules of

hard steel are once arranged, by magnetizing them, for example, they

strongly resist any outside influences which tend to mix them up again.

A magnet does not attract a piece of red-hot iron. The particles of the

hot iron are supposed to vibrate too rapidly to be brought into line;

that is, the iron cannot become polarized by induction. (See Exp. 24.)

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

_Apparatus._ A magnetized sewing-needle; iron filings; compass,

O C (No. 18).

[Illustration: 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.

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

_=51. Discussion.=_ The above results agree with the theory that each

molecule is a magnet (Exp. 18). No matter into how many pieces a magnet

is broken, each part becomes a magnet. (Fig. 10). This shows that those

molecules near the equator of the magnet really have magnetism. Their

energy, however, is all used upon the adjoining molecules; hence no

external bodies are attracted at that point.

CHAPTER III.

INDUCED MAGNETISM.

[Illustration: Fig. 11.]

=EXPERIMENT 24. To find whether we can magnetize a piece of

iron without touching it with a magnet.=

_Apparatus._ Horseshoe magnet, H M; iron filings, I F (Fig. 11).

=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.

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.

_=53. Temporary Magnetism; Induced Magnetism.=_ The armature, A, was

induced to become a magnet without even touching H M. Its magnetism was

_temporary_, however, as the filings dropped as soon as the _inductive

action_ of H M was removed. A small amount of residual magnetism (44)

remained in A. Soft iron is exceedingly valuable, because it has very

little retentivity (44), and because it can be easily _magnetized

by induction_. The armature was made of soft iron. It had _induced

magnetism_. It was a _temporary magnet_.

=EXPERIMENT 25. To find whether a piece of steel can be

permanently magnetized by induction.=

_Apparatus._ An unmagnetized sewing-needle; horseshoe magnet, H

M; iron filings; sheet of stiff paper.

=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.

[Illustration: Fig. 12.]

=EXPERIMENT 26. To study the inductive action of a magnet upon

a piece of soft iron.=

_Apparatus._ Horseshoe magnet, H M; iron filings, I F; a piece

of soft iron wire about an inch long, I W (Fig. 12), placed

upon the N pole of H M; compass, O C (No. 18), (§ 32).

=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.

and again test the polarity of the lower end.

_=56. Polarization; Pole Pieces.=_ The wire, I W (Fig. 12), was acted

upon by induction (Exp. 24) and behaved like a magnet. Poles were

produced in it, so we say that the wire was _polarized_. Pieces of

iron, placed upon the poles of a magnet, are called _pole pieces_. It

should be noted that the lower end of the wire has a pole _like_ the

pole of H M, to which it is attached.

=EXPERIMENTS 27-30. To study pole pieces.=

_Apparatus for Experiments 27-30._ Horseshoe magnet, H M; soft

iron wires; iron filings, I F.

=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?

[Illustration: Fig. 13.]

[Illustration: Fig. 14.]

=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).

(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.

[Illustration: Fig. 15.]

[Illustration: Fig. 16.]

=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.

CHAPTER IV.

THE MAGNETIC FIELD.

=EXPERIMENT 31. To study the space around a magnet, in which

pieces of iron become temporary magnets by induction.=

_Apparatus._ A bar magnet, B M (No. 21); a compass (No. 18); a

sheet of stiff paper about 1 ft. (30 cm.) square, with a center

line, C L, drawn parallel to one of its sides (Fig. 16-1/2),

and with another line, E W, drawn perpendicular to C L. (See

Apparatus Book, Chap. VI., for various ways of making home-made

permanent magnets.)

=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.

pole to the north, and its center at the cross line, E W.

[Illustration: Fig. 16-1/2.]

(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.

(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.

16-1/2 by dots, and study the resulting diagram.

_=62. Discussion; The Magnetic Field.=_ The compass-needle was

decidedly affected all around B M (Fig. 17), showing that induction

can take place in a considerable space around a magnet; this space

is called the _magnetic field_ of the magnet. Let us consider _one_

position taken by the compass-needle in the field of B M (Fig. 17),

as, for example, the one in which the needle has been made black. The

S pole of the black needle is attracted by the N pole of B M, and is

repelled by the S pole of B M. The N pole of the compass-needle is

attracted by the S pole of B M, and is repelled by B M's N pole. The

position which it takes, therefore, is due to the action of these 4

forces, together with its tendency to point N and S.

[Illustration: Fig. 17.]

Every magnet has a certain magnetic field, with its lines of force

passing through the surrounding air in certain definite positions. As

soon, however, as a piece of iron or another magnet is brought within

the field, the original position of the lines of force is changed. This

has to be considered in the construction of electrical machinery.

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

_Apparatus._ A sheet of stiff paper; iron filings, I F; bar

magnet, B M (No. 21); a sifter for the filings (No. 24); (See

Apparatus Book, §48, 49, 50, for home-made sifters.)

=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 over B M, then tap the paper gently, to assist the

particles to take final positions. Study the results.

_=64. Magnetic Figures; Lines of Magnetic Force.=_ The filings clearly

indicated the extent and nature of the magnetic field of B M. You

should notice how the filings radiate from the poles, and how they

form curves from one pole to the other. They make upon the paper a

_magnetic figure_. Each particle of the filings becomes a little

magnet, by induction (Exp. 24), and takes a position which depends upon

attractions and repulsions, as discussed in Exp. 31.

Magnetism seems to reach out in lines from the poles of a magnet. The

position and direction of some of the lines are shown by the lines of

filings. They are very distinct near the poles, and are considered, for

convenience, to start from the N pole of a magnet, where they separate.

They then pass through the air on all sides of the magnet, and finally

enter it again at the S pole. These lines are called _lines of force_

or _lines of magnetic induction_.

The poles must not be considered mere points at the ends of a magnet.

As shown by magnetic figures, the lines of magnetic induction flow from

a considerable portion of the magnet's ends.

=EXPERIMENTS 33-37. To study the magnetic fields of various

combinations of bar magnets.=

_Apparatus for Exps. 33-37._ Two bar magnets, B M (Nos. 21,

22); an iron ring, I R (No. 23); iron filings, I F; a sheet of

stiff paper; the sifter (No. 24).

=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

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.

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.

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.)

[Illustration: Fig. 18.]

[Illustration: Fig. 19.]

=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.=

_Apparatus for Exps. 38-39._ Two bar magnets, B M (No. 21, 22),

of about equal strength; iron filings, I F.

=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.

[Illustration: Fig. 20.]

=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).

_=73. Discussion; Compound Magnets.=_ Many lines of force pass into

the air from two like poles. Such a combination is called a _compound

magnet_. A piece of thin steel can be magnetized more strongly in

proportion to its weight than a thick piece, because the magnetism

does not seem to penetrate beyond a certain distance into the steel.

Thin steel may be magnetized practically through and through. A thick

magnet has but a crust of magnetized molecules; in fact, a thick magnet

may be greatly weakened by eating the outside crust away with acid.

By riveting several thin bar or horseshoe magnets together, thick

permanent magnets of considerable strength are made.

_=74.=_ Lines of force, in passing from the N to the S pole of a magnet,

meet a resistance in the air, which does not carry or conduct them as

easily as iron or steel. In the arrangement of Exp. 39 the lines of

force are not obliged to push their way through the air, as each magnet

serves as a return conductor for the lines of force of the other.

Either magnet may be considered an armature for the other.

To show in another way that few lines of force pass into the air,

the student may lay the above combination upon the table and make a

magnetic figure. (See Apparatus Book, p. 38, for method of making

home-made compound magnets.)

In the case where a ring was placed between the poles of two bar

magnets (Exp. 34), the lines of force from the N pole jumped across the

first air-space. They then disappeared in the body of the ring, until

they were obliged to jump across the second air-space, to get to the S

pole. The weakness of the field in the central space was clearly shown

by the filings. There were no stray lines of force passing through the

air, because it was easier for them to go through the iron ring. This

will be discussed again under "Dynamos and Motors." (See also § 78.)

=EXPERIMENTS 40-42. To study the magnetic field of the

horseshoe magnet.=

_Apparatus for Exps. 40-42._ Horseshoe magnet, H M; iron

filings, I F; sheet of stiff paper.

=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?

_=78. Discussion; Resistance to lines of Force.=_ It is evident, from

the last 3 experiments, that lines of force will pass through iron

whenever possible, on their way from the N to the S pole of a magnet.

When the armature of a horseshoe magnet is in place, most of the lines

of magnetic induction crowd together and pass through it rather than

push their way through the air. Air is not a good conductor of lines of

force; and the magnet has to do work to overcome the resistance of the

air, when the armature is removed, in order to complete the magnetic

circuit. This work causes a magnet to become gradually weaker. The soft

iron armature is an excellent conductor of lines of force; it completes

the magnetic circuit so perfectly that very little work is left for the

magnet to do.

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

a magnet.=

_Apparatus._ Our compass, O C (No. 18); horseshoe magnet, H

M; glass tumbler, G T; sheet of stiff paper; iron filings, I

F. Arrange as in Fig. 21. H M may be supported in a vertical

position by placing paper, or a handkerchief, under it. The

poles should just touch the stiff paper placed over the tumbler.

[Illustration: 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?

_=80. Discussion.=_ The student should keep in mind the fact that the

filings in the magnetic figure show the approximate extent and form of

the magnetic field simply in one plane. If the paper were held in some

other position near the magnet (in a tilted position, for example,)

the lines of filings would not be the same as those produced in Exp.

40-42. The lines of force come out of every side of the N pole. When

a magnetic needle is placed in any magnetic field, its N pole points

in the direction in which the lines of force are passing; that is, it

points towards the S pole of the magnet producing the field.

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

_Apparatus._ A narrow strip of spring steel, S S (No. 25); iron

filings, I F.

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

(B) Bend S S until its poles are about 1/4 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.

again test its lifting power with filings.

[Illustration: Fig. 22.]

_=82. Discussion; Advantages of Horseshoe Magnets.=_ When the opposite

poles of the flexible magnet are pressed together, the lines of force

do not have to pass through the air; there is very little attraction

for outside bodies. The same effect is produced with the armature (Exp.

41). A horseshoe magnet has a strong attraction for its armature,

because it has a _double power to induce and to attract_. Suppose the N

pole of a bar magnet, B M (Fig. 22), be placed near one end of a piece

of iron, as, for example, the armature, A. A will become a temporary

magnet by induction (Exp. 24). The S pole of A, polarized by induction,

will be attracted by B M, while its N pole will be repelled by B M; so,

you see, that a bar magnet does not pull to advantage.

CHAPTER V.

TERRESTRIAL MAGNETISM.

_=83. The Magnetism of the Earth.=_ The student must have guessed,

before this, that the earth acts like a magnet. It causes the magnetic

needle to take a certain position at every place upon its surface, and

this position depends upon the earth's attractions and repulsions for

it. The earth has lines of force which flow from its N magnetic pole,

and these lines, before they can get to the earth's S magnetic pole,

must spread out through the air on all sides of the earth.

As the magnetic needle points to the earth's N magnetic pole (which

is more than 1,000 miles from its _real_ N pole), it is evident that

the compass-needle does not show the _true_ north for all places upon

the earth's surface. In fact, the N pole of the needle may point E, W,

or even S. This effect would be seen by carrying a compass around the

earth's N magnetic pole.

[Illustration: Fig. 23.]

_=84. Declination.=_ For convenience, we shall represent the true N

and S, at the place where you are experimenting, by the full line,

N S, in Fig. 23. The dotted line shows the direction taken by the

compass-needle. The angle, A, between them, is called the _angle of

variation_ or the _declination_. This angle is not the same for all

places; and, in fact, it changes slowly at any given place; so it

becomes necessary to construct _magnetic maps_ for the use of mariners

and others.

=EXPERIMENT 45. To study the lines of force above and below a

bar magnet placed horizontally.=

_Apparatus._ A bar magnet, B M (No. 21); compass, O C (No. 18).

=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?

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?

[Illustration: Fig. 24.]

_=86. The Dip or Inclination of the Magnetic Needle.=_ The needle

is said to dip when it takes positions like those in Fig. 24.

Compass-needles should be horizontal, when properly balanced, and

entirely free from all effects other than those of the earth. The

excessive dip shown (Fig. 24) is due, of course, to the efforts of the

magnetic needle to place itself in the direction in which the lines of

force of B M pass.

=EXPERIMENT 46. To study the dip or inclination of the magnetic

needle, due to the action of the earth.=

_Apparatus._ Fig. 25. Our compass, O C (No. 18); horseshoe

magnet, H M (No. 16); piece of paper.

=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.

[Illustration: 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.

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.

[Illustration: Fig. 26.]

_=88. Discussion; Balancing Magnetic Needles.=_ If a piece of

unmagnetized steel be balanced and then magnetized, it will no longer

remain horizontal; it will dip. Try this. Compass-needles are balanced

after they are magnetized. Can you now see why the needle did not

remain horizontal after its poles were changed? A piece of steel first

balanced and then magnetized, has to have its S pole slightly weighted,

as suggested by the line at S (Fig. 26 x), to make it horizontal.

The magnetic needle does not tend to dip at the earth's equator,

because the lines of force of the earth are nearly horizontal at the

equator. As we pass toward the north or south on the earth, the lines

of force slant more and more as they come from or enter the earth's

magnetic poles. What position would the needle take if we should hold

it directly over the earth's N magnetic pole? Fig. 24 shows what the

needle does when held near the poles of a bar magnet.

=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. 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.

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.

magnet?

_=92. Discussion.=_ Dipping the poker places it nearly in the same

direction as that taken by the earth's lines of force. The magnetic

influence of the earth acts to advantage upon the poker, by induction,

only when the poker is properly held.

It no doubt occurs to the student that the end of a magnetic needle

which points to the north is really opposite in nature to the north

magnetic pole of the earth. The N pole of a needle, then, must be in

reality a S pole to be attracted by the earth's N pole. It has been

agreed, for convenience, to call the N-seeking pole of a magnet its N

pole.

_=93. Natural Magnets.=_ Nearly all pieces of iron become more or less

magnetized by the inductive action of the earth's magnetism. Your poker

was slightly magnetized at the start, perhaps, from standing in a

dipping position.

Induction takes place along lines of force. In northern latitudes the

earth's lines of force have a dip to the north. You should now see why

the greatest effect was produced upon the poker when it, also, was made

to dip.

Parts of machinery, steel frames of bridges and buildings, tools in the

shop, and even certain iron ores, become polarized by this inductive

action. These might all be called natural magnets. Magnetic iron ore,

called lodestone, is referred to, however, when speaking of _natural

magnets_. Lodestone was used thousands of years ago to indicate N and

S, and it was discovered, later, that it could impart its power to

pieces of steel when the two were rubbed together.

=EXPERIMENT 49. To test the effect of twisting a wire held

north and south in the earth's magnetic field.=

_Apparatus._ Compass, O C (No. 18); a piece of soft iron wire,

6 in. (15 cm.) long (No. 15). Bend up about an inch of the wire

at each end so that it may be firmly held when twisting it.

=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.

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.=

_Apparatus._ Steel drills; files; chisels; bars or rods of

iron that have been standing in an upright position; stove-lid

lifters; stove pokers, etc., etc.; a compass.

=95. Directions.= (A) With the compass test the ends of the

above for magnetism, and note which ends are S.

Notes.

STATIC ELECTRICITY

PART II.--STATIC ELECTRICITY

CHAPTER VI.

ELECTRIFICATION.

_=100. Some Varieties of Electricity.=_ _Static electricity_ does not

seem to "flow in currents" as readily as some other varieties; its

tendency is to stand still, hence the name, static. The simplest way to

produce it is by friction. _Thermo electricity_ is produced by changes

in temperature. When certain combinations of metals become hotter or

colder, a current is produced. _Voltaic_ or _Galvanic electricity_ is

produced by chemical action. Batteries give this variety. _Induced

electricity_ is produced by other currents, and by combinations of

magnets and moving coils of wire, as in the dynamo. This is, by far,

the most important variety of electricity, and the dynamo is the most

important producer of it.

Each of the above varieties of electricity will be studied

experimentally with simple apparatus.

=EXPERIMENTS 51-52.= To study electrification by friction.

_Apparatus._ Ebonite sheet, E S (No. 26); flannel cloth, F C

(No. 30). See what is said in preface about static electricity.

=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?

slide it about. Can you easily pick it up?

(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.

_=102. Discussion; Electrified and Neutral Bodies.=_ The ebonite

sheet became _electrified_ or _charged_; and as the _electrification_

was produced by friction, we may say that the action of the ebonite

indicated the presence of _frictional electricity_. No one can tell

_just_ why the ebonite acted so queerly, but we can learn a great

deal by experimenting. Bodies which are not charged are said to be

_neutral_. The table, chairs, earth, etc., are neutral. We may consider

that a neutral body has been _discharged_.

_=103. Force; Resistance; Work; Potential Energy; Electrification.=_ It

takes _force_ to raise water into a tank placed on the roof. In raising

the water, _work_ has to be done, and _we_ have to do the work; but

when we once have the water in the tank we have accomplished something.

The water has _potential energy_; that is, on account of its high

_position_, we can make it do some work by simply turning a stop-cock

so that the water can run out and turn a water-wheel, for example.

It takes _force_ to vigorously rub a piece of ebonite with a flannel

cloth, for _resistance_ has to be _overcome_; that is, _work_ has to be

done. Several things are accomplished by this work; heat is produced,

for we can _feel_ that the ebonite gets warm; we can _hear_ sounds and

_see_ sparks. The simple muscular exertion on our part has been changed

to heat, light, and sound. The most wonderful part of it all, however,

is that we have electrified or charged the ebonite. _We_ did the work

at first, and now the ebonite has the power to do something, as you

will soon see. _Electrification_ is, then, a sort of potential energy.

_=104. Heat and Electrification.=_ We say that heat passes to or from a

body to make it hot or cold. Heat _produces_ the sensation of warmth,

but heat isn't warmth. We can force a cold body to become hot; in

other words, we can get it into a hot condition in various ways, such

as rubbing it, hammering it, or by placing it near or in contact with

another hot body.

Electrification is, also, a condition or state into which we can force

a body; but electrification isn't electricity. We know whether a body

is hot or cold by its effects upon us, upon thermometers, and upon

other bodies. We can tell, also, whether a body is electrified or not

by the way it acts, and, in certain cases, by the sound, heat, and

light which accompany the electrification.

Do not get the idea that an electrified body is covered with a layer of

electricity just as a board is covered with a layer of paint.

[Illustration: Fig. 28.]

=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.=

_Apparatus._ The ebonite sheet, E S (No. 26); flannel cloth,

F C (No. 30); small pieces of dry tissue-paper, T P (No. 31);

thread (No. 32).

=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.

_=107. Discussion.=_ Exp. 53 shows that _an electrified body attracts

neutral ones_. This much was known about electricity over 2,000 years

ago. They didn't have ebonite then, but some of the educated men of

Greece knew that amber would attract light bodies after being rubbed.

The Greek word for amber is _elektron_, and from this has been made the

word _electricity_.

=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.=

_Apparatus._ The support and its attachments (See § 109);

support wire, S W (No. 36); silk thread, S T (No. 33), or a

rubber band, R B (No. 45); ebonite rod, E R (No. 28); flannel

cloth, F C (No. 30); wire swing, W S (No. 37).

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).

[Illustration: Fig. 29.]

(B) Bring your finger near one side of the rubbed end of E R,

then near the unrubbed end, and compare the results.

=111. Mutual Attractions.= _A neutral body_, like the hand, for

example, _attracts electrified ones_. From Exp. 53, 54, 55, it is

seen that the attraction between a neutral and an electrified body is

mutual; each attracts the other.

=EXPERIMENT 56. To study electrical repulsions.=

_Apparatus._ Same as for Exp. 55; ebonite sheet, E S (No. 26).

=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.=

_Apparatus._ A sheet of tissue-paper, T P (No. 31); shears or a

knife. Cut T P, as in Fig. 30. Each leg should be about 1/4 in.

wide and 3 or 4 in. long.

=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.

[Illustration: Fig. 30.]

[Illustration: Fig. 31.]

=EXPERIMENT 58. To study electrical repulsions.=

_Apparatus._ Ebonite rod, E R (No. 28); a carbon electroscope,

C E, Fig. 31 (see § 114); the support complete (see § 109);

small piece of damp tissue-paper.

_=114. The Carbon Electroscope.=_ Light an ordinary match, and let

it burn until it is charred through and through. The black substance

remaining is _carbon_. This is very light; it has, also, another

important property which you will soon understand. Tie a small piece

of the carbon to one end of a dry _silk_ thread, and fasten the other

end of the thread to the support wire, S W, which is fastened to the

support (Fig. 31). We shall call this piece of apparatus the _carbon

e-lec-tro-scope_. (See Electroscopes, Chapter XVIII., Apparatus Book.)

=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.

_=116. Discussion of Experiments 56, 57, 58.=_ In 56 the two pieces

of ebonite were made of the same material, and both were rubbed with

flannel. They must have been similarly electrified. In 57, different

parts of the same piece of paper were similarly electrified. In 58,

the little piece of carbon took some of the electrification from the

charged rod, just as it would take molasses from your finger should

your sticky finger touch it. The electrification on the carbon must

have been of the same kind as that on the rod. The carbon was _charged

by contact_. We learn, then, that _two bodies repel each other when

they have the same kind of electrification_. Do two charged bodies

_always_ repel each other? Is it possible that there are different

kinds of electrifications?

=EXPERIMENT 59. To study the electrification of glass.=

_Apparatus._ The sheet of glass, G (No. 38), heated (a hot

bottle or lamp chimney will do); a piece of silk large enough

to rub G. (A silk handkerchief is just the thing, but in case

you have no silk, use the flannel cloth, F C, No. 30.)

=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.

_=118. Questions.=_ Will two pieces of electrified glass repel each

other? Arrange an experiment to show whether you are right or not. Is

the charge on the glass exactly like that on the ebonite? Do you know

how to find out?

=EXPERIMENT 60. To compare the electrification produced by

ebonite and flannel with that produced by glass and silk.=

_Apparatus._ The support (see § 109); wire swing, W S (No. 37);

ebonite rod, etc., of Exp. 55 (Fig. 29); the glass, G, and silk

of Exp. 59.

=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

whether the attraction between them is stronger or weaker than

before, or whether they repel each other.

_=120. Discussion.=_ We know that the glass was electrified, because it

lifted tissue-paper; hence, its charge was not of the same kind as that

on the ebonite. Had the electrifications been exactly alike, we should

have had a repulsion (Exps. 56, 57, 58).

The exact difference between these two kinds of electrifications is not

known. It has been agreed, for convenience, to call that produced by

glass and silk a _positive_ electrification. With ebonite and flannel a

_negative_ electrification is produced. The sign + is generally written

for the word positive, and - for negative. These signs indicate _kind_,

and not more or less, as in arithmetic.

_=121. Laws.=_ We have learned from the experiments these facts, which

are called _laws_:

(1) Charges of the same kind repel each other; (2) charges of unlike

kinds attract each other; (3) either kind of a charge attracts, and is

attracted by a neutral body.

CHAPTER VII.

INSULATORS AND CONDUCTORS.

=EXPERIMENT 61. To study insulators.=

_Apparatus._ Ebonite rod, E R (No. 28); flannel cloth, F C (No.

30); tissue-paper, T P (No. 31).

=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.=

_Apparatus._ The ebonite sheet, E S (No. 26); flannel cloth, F

C (No. 30).

=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.=

_Apparatus._ A hard-rubber comb (not furnished); flannel cloth,

F C (No. 30); dull pointed nail (No. 19).

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

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

carefully.

_=125. Discussion of Experiments 61, 62, 63; Insulators.=_ In 61 the

electrification remained at one end of the rod. In 62 and 63 the sparks

showed that all parts of the ebonite were not discharged at the same

time. A substance, like ebonite, which will not allow electrification

to pass from one part of it to another, is called an _insulator_. Silk

and glass are also insulators. Do you now see why a silk thread was

used to make the carbon electroscope?

Why do they fasten telegraph wires to glass insulators?

_=126. Conductors.=_ It has already been stated that water in an

elevated tank has potential energy. We can allow the water to flow

through a conducting pipe to another tank a little lower than the

first, and it will still retain much of the potential energy, but not

all.

Can we conduct from one place to another this peculiar state of things,

this queer form of potential energy which we call electrification? It

is clear, from the last experiments, that in order to do it we need

something besides ebonite, which really acts like a closed stop-cock to

the flow of electrification.

To keep electrification in one place we need an insulator; to get it

from one place to another we need a _conductor_. Insulators are as

important as conductors.

You saw that sparks went to the finger from the ebonite, so we call the

finger a conductor. You have learned that attractions and repulsions

show the presence of electrification. Can we have our charged body in

one place and get attractions or repulsions at some other place?

[Illustration: Fig. 32.]

=EXPERIMENT 64. To study conduction.=

_Apparatus._ Fig. 32; the support (see § 109); a bent hairpin,

H P (No. 39); ebonite sheet, E S; flannel cloth, F C; tin disk,

B F B (No. 40), which is the bottom of the flat-box, F B; the

insulating table, I T (see § 127).

=127. The Insulating Table= consists of a tin box (exactly like

that used for the electrophorus cover), and an ebonite rod

about 1-3/4 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.

_=129. Discussion.=_ Ebonite being an insulator (§ 125), we say that I

T, H P and B F B were _insulated_. You can see that the electrification

must have passed through I T and H P to get to the disk, B F B. H P was

the _conductor_, allowing the disk, also, to become charged. The wood,

S R, is a conductor, and, as it was not insulated from the earth, S R

was neutral. Account for the attraction. (See § 121.)

[Illustration: Fig. 33.]

=EXPERIMENT 65. To study conduction.=

_Apparatus._ A copper wire, C W (No. 44); insulating rubber

band, R B (No. 45, Fig. 33); wire swing, W S (No. 37); the

other half of the flat box, T F B (No. 41); apparatus of Exp.

69.

=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.

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.)

_=132. Discussion.=_ The action here was like that in the previous

experiment, the difference being that a longer _conductor_ was used.

Electrification is always looking for some place to get to the earth,

just as water will run from a roof to the ground. You will understand

more about it a little later. In our apparatus just described, the only

way that the earth could be reached was through the wooden rod S R. Do

not get the idea that real messages are sent in any such way, or that

electricity flows through a wire as water flows through a pipe.

_=133. Relation between Conductors and Insulators.=_ The above terms

are merely relative. Static electricity is easily conducted by dry

wood, while Galvanic electricity is practically insulated by it. A

substance may be an insulator for currents of low potential, while at

the same time it will conduct high potential currents. (See Potential §

144.)

_=134. Electrics and Non-electrics.=_ Bodies like glass, sealing-wax,

amber, etc., were called electrics by the first students of

electricity, because it was upon these substances that they could

easily produce electrification. They called iron and other metals

non-electrics, because they could detect no electrification

after rubbing them. Can you explain why they did not detect any

electrification on metals? Can you devise an experiment to prove that

metals may be charged? Do you see any relation between a non-electric

and a conductor?

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

insulator.=

_Apparatus._ Same as for Exp. 65, with the exception of the

copper wire; this is to be replaced by a dry silk thread about

2 feet (60 cm.) long (No. 33).

=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.

_=136. Discussion.=_ Dry silk is an insulator, while wet silk is a good

conductor of _static_ electricity. It is the water, however, which

really does the conducting. Even small amounts of moisture on glass, or

other insulators, will allow the charge to escape. Glass collects much

moisture from the air. Do you now see why it is necessary, to get good

results, to have the paper, glass, etc., hot before electrifying them?

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

be electrified.=

_Apparatus._ Two pieces of newspaper, each about 4 in. (10 cm.)

square.

=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.

CHAPTER VIII.

CHARGING AND DISCHARGING CONDUCTORS.

_=138. The Electrophorus.=_ While the ebonite sheet alone, or a good

hard-rubber comb, may be used for many experiments in frictional

electricity, the sparks produced are small, and the ebonite has to be

electrified as often as it is discharged. To obtain real good sparks,

and to avoid this continual rubbing, the student should be provided

with an _e-lec-troph'-o-rus_. This is, really, a simple, cheap, and

efficient frictional electric machine. An electrophorus consists of 2

insulators and 1 conductor--that is, of 3 parts: (1) insulating handle,

(2) cover, and (3) a plate or base of insulating material.

[Illustration: Fig. 34.]

=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.=

_Apparatus._ Shown in Figs. 34, 35. _Do not fail to read_ § 139.

=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.

(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.

[Illustration: Fig. 35.]

[Illustration: 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 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."=

_Apparatus._ Fig. 37. To one end of a _silk_ thread, S T, is

tied a little bent clamp, B C (No. 46); the other end of S T is

tied to the support wire, S W (No. 36); the bottom of the flat

box, B F B (No. 40), is supported by B C, and thus _insulated_

from the table and earth; the electrophorus (Exp. 68) is also

necessary.

=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?

[Illustration: Fig. 37.]

[Illustration: Fig. 38.]

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

_Apparatus._ The insulating table, I T, Fig. 38. (For details

see Exp. 64; the electrophorus Exp. 68).

=143. Directions.= (A) Pass a spark from the thoroughly charged

E C (Exp. 68) to I T.

(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.

E C to see if it is completely discharged.

(D) Touch I T.

_=144. Pressure; Potential; Electro-motive Force.=_ Water runs down

hill. It always tries to run from a high place to a lower one.

Electrification acts very much like water in this respect. We say that

water has a _pressure_, or a _head_ of so many feet. In speaking of a

charge, we say that it has a _potential, or an electro-motive force_.

Water may have a high or low pressure, and a charge may have a high or

low potential. The greater the pressure of water, the harder it tries

to break away and get somewhere; the greater the potential of a charge,

the farther it will jump to your hand.

_=144a. Current; Spark.=_ Electrification will easily pass from a place

of high potential to one of low potential through a conductor, and

when it _passes_ we say we have an _electric current_, or a _current

of electricity_. Water has no desire to flow on a dead level, and the

electric current does not care to flow between two places of equal

potential. The potential of the earth and of all neutral bodies is

zero; that is, they have no charge, no potential; so it is very easy

for a charge to escape into the earth.

Dry air is a pretty good insulator, but when the attraction between

a charged and a neutral body gets great enough, the spark rips

right through the air. Benjamin Franklin proved by experiment that

lightning is caused by the electrification in the clouds and air. (See

Atmospheric Electricity.)

=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 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.

[Illustration: Fig. 39.]

=EXPERIMENT 71. To study some methods of discharging an

electrified body.=

_Apparatus._ The electrophorus (Exp. 68); an ordinary pin (Fig.

39).

=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?

=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.

toward it. Touch E C to see whether it has been discharged or

not.

_=149. Disruptive, Conductive, and Convective Discharges.=_ Sudden

discharges, accompanied by bright sparks, are said to be _disruptive_.

When the electrification is continuously carried away by a conductor,

there is a _conductive_ discharge. There is a _convective_ discharge

when the electrification escapes from points into the air. (See § 155.)

The nature of the discharge depends upon the potential of the charge,

upon the nature of the charged conductor, and upon the nature of the

surrounding air and objects. Convective discharges are often _silent_,

as in Exp. 71 (C). In this case, electrification passed from the earth

through the pin-point to the cover to neutralize it. (See Induced

Electricity.)

[Illustration: Fig. 40.]

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

discharges.=

_Apparatus._ Electrophorus (Exp. 68); carbon electroscope (§

114), (Exp. 58).

=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?

_=151. Discussion.=_ The carbon and E C were insulated, while the hand

was "grounded"--that is, it was connected with the earth. Carbon is a

good conductor; it may be quickly charged and discharged.

=EXPERIMENT 73. To ascertain the location of the charge upon an

electrified conductor.=

_Apparatus._ The electrophorus (Exp. 68); the insulating table,

I T (Exp. 64); the tin box, T B (No. 47), Fig. 41; a piece of

moist cotton thread, C T, 5 or 6 in. long, bent double, and

hung over the edge of the open box, T B. One-half of C T should

be inside of T B, which, in turn, should stand on I T.

[Illustration: Fig. 41.]

=152. Directions.= (A) Charge E C; pass a spark to T B, and

note the action of both parts of C T.

_=153. Hollow and Solid Conductors.=_ The moist thread, being a

conductor, became charged as well as the box. The electrification

seemed to be entirely on the outside of T B. A hollow conductor will

hold as large a charge as a solid one having the same amount of

surface. This refers to charges of static electricity, not to currents.

An electric current passes through the whole substance of a conductor.

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

conductor.=

_Apparatus._ The electrophorus (Fig. 34); a pin, bent slightly

to keep it from rolling.

=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.

so that its point will project into the air. The point should

stick out about 1/4 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.

_=155. Electric Density; Electric Wind.=_ A charge resides upon the

outside of a conductor (Exp. 73), and it continually tries to escape.

It seems to pile up at points and corners, and we say that it is denser

at such places than at well-rounded parts of a charged conductor. All

points and sharp places should be removed from a conductor, if it is

desired to keep a charge for any length of time.

Electrification may escape from a point so rapidly that currents

are produced in the surrounding air. As the particles of air become

charged, they repel each other. The movement of the air particles may

be so great that a lighted candle will be affected when placed near the

point. This current of air is called _electric wind_.

Electrification easily passes from points, and the electrophorus may be

easily and silently discharged by holding a pointed pin near it (Exp.

71, C). Thorns, leaves with sharp edges, etc., have a great effect upon

atmospheric electricity. They allow a silent escape of electrification

from the earth to neutralize that in the clouds which is opposite in

nature. (See Atmospheric Electricity.)

CHAPTER IX.

INDUCED ELECTRIFICATION.

_=156. Electric Field; Lines of Force.=_ In our study of magnetism

you learned that a magnet can act through the air, and induce a piece

of iron to become a magnet. You saw how the iron filings arranged

themselves around the magnet, showing that the lines of force reached

out from the poles in a very peculiar manner. There is an _electric

field_ all around a charged conductor, just as there is a magnetic

field about a magnet. The lines of force in the electric field pass

from the positively charged body to the negatively charged one, or to

some neutral one, which, you will soon see, is practically the same

thing. When the positively charged electrophorus cover is held above

the negatively charged ebonite sheet, a very strong electric field

exists between them.

=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?

[Illustration: Fig. 42.]

=EXPERIMENT 75. To study electric induction.=

_Apparatus._ Fig. 42. The insulating table, I T (for details

see Exp. 64); tin box, T B (No. 47, Fig. 42); moist cotton

thread, C T; the electrophorus (Exp. 68); tie C T around one

end of the closed T B, and leave the ends of C T long enough to

hang down over the end. Place a match on each side of T B to

keep it from rolling.

=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.

_=159. Electric Polarization; Theory of Induction.=_ This experiment

should remind the student of Exp. 24, in magnetism, in which a piece of

soft iron was magnetized by the inductive action of a magnet. The soft

iron was in a magnetic field; it became polarized. Is it possible that

the box, T B, was polarized, being in the electric field of E C?

We know, by the action of C T (Fig. 42), that the top end of T B was

charged while E C was in place. The charge was not conducted.

You know, from previous experiments, that + and - electrifications rush

together whenever possible. Why can we not suppose that a neutral body,

like the box at the start, contains an equal amount of both kinds, and

that these different electrifications have already rushed together?

If you imagine a small army of positive soldiers struggling, "man to

man," with the same number of equally strong negative soldiers, you can

readily see that one-half of them can hold the other half from running

away. A body remains neutral, then, according to this idea, as long as

it has an equal quantity of the two opposite kinds of electrification.

(See Theories, § 145, 146.)

As soon as the positively charged E C was brought near T B, it

destroyed the neutrality of T B, by pulling at its - electrification,

and by pushing back its + electrification to the top end and into C

T. We say that the charged E C produced a separation of the combined

electrifications of T B by _induction_, and not by contact. As soon as

the inductive action of E C was removed, T B became neutral again.

[Illustration: Figs. 43-44.]

=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.=

_Apparatus._ Fig. 42, same as in Exp. 75.

=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?

_=162. Free and Bound Electrifications.=_ As explained in Exp. 75, and

as shown in Fig. 44, T B became polarized. The - electrification was

drawn towards E C; it was held or _bound_ there as long as E C was

near. The + was actually repelled by E C, and it was _free_ to escape

through your arm as soon as T B was touched, leaving the top end of T B

neutral. As soon as E C was removed, the - electrification, no longer

held by E C, spread all over T B and on to C T. T B was _charged by

induction_. It was charged negatively by driving out + electrification.

=EXPERIMENT 77. To show that a neutral body is polarized before

it is attracted by a charged one.=

_Apparatus._ The electrophorus (Exp. 68); dry tissue-paper, T

P. Cut out 2 pieces of T P, each about 1/4 inch square.

=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.

_=164. Polarization Precedes Attraction.=_ Dry tissue-paper is not a

good conductor; you have seen (Exp. 52) that it can be electrified,

which indicates that it is at least a partial insulator. Insulators

are not easily polarized. (Why?) Even if the pieces of T P were

polarized, the opposite electrifications were so near each other that

the attraction of E C for the - was nearly overcome by the repulsion

for the +; the result being that T P was not strongly attracted by E C

until the + had a chance to escape. The moist tissue-paper allowed its

+ to escape more quickly than the dry piece. A conductor is attracted

by a charged body more strongly than an insulator, because the latter

is not easily polarized. A neutral body, then, is really no longer

neutral when it is in the electric field. _Polarization precedes

attraction._

=EXPERIMENT 78. To find whether electric induction will act

through an insulator.=

_Apparatus._ Small bits of carbon (Exp. 58); bits of moist

tissue-paper, T P; one-half of the flat box, T F B (No. 41);

sheet of glass, G (No. 38); electrophorus (Exp. 68). Place the

carbon and T P into T F B (Fig. 45), and cover with the glass.

=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.

_=166. Dielectrics.=_ The carbon must have been polarized and attracted

_through_ the glass. You saw, Exp. 7, that the lines of magnetic force

could penetrate and act through paper, glass, etc.; it is now evident

that the electric field is not easily fenced in, even by an insulator.

Substances, like the glass, which allow this inductive influence to act

through them, are called _dielectrics_.

[Illustration: Fig. 45.]

[Illustration: Fig. 46.]

=EXPERIMENT 79. To find whether a polarized conductor can act

inductively upon another conductor.=

_Apparatus._ Fig. 46. Insulating table, I T (for details see

Exp. 64); ebonite sheet, E S (No. 27); flat box complete F

B (Nos. 40, 41); sheet of glass, G (No. 38); small piece of

slightly moist tissue-paper, T P; charged electrophorus cover,

E C. Arrange as shown.

=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.

_=168. Successive Induction.=_ The inductive influence of E C first

polarized I T; this acted through the dielectric, E S, and polarized

F B, which, in turn, polarized T P through the second dielectric, G.

This induction after induction is called _successive induction_.

_=169. Inductive Capacity.=_ Dielectrics are insulators. Two substances

may be equally good insulators, that is, they may equally well resist

the _spread_ of electrification _over_ their surfaces, or the _flow_ of

the electric current _through_ them, while one may be, nevertheless,

a better _dielectric_ than the other. The better the dielectric,

the easier it is for the electric field to polarize a conductor

placed beyond the dielectric. A good dielectric is said to have a

high _inductive power or capacity_. Glass is about 3 times as good a

dielectric as dry air; and as the latter (under certain conditions)

is taken as the standard, or as unity, we may say that the _specific

inductive capacity_ of glass is about 3.

The Study of Elementary Electricity and Magnetism by Experiment Containing Two Hundred Experiments Performed with Simple, Home-made Apparatus · 第 16 / 19 章

CHAPTER XXVIII. WIRE TABLES 208