_=321. Discussion.=_ Two wires placed si_The Study of Elementary Electricity and Magnetism by Experiment Containing Two Hundred Experiments Performed with Simple, Home-made Apparatus

really form a conductor having twice the size (area of cross section)

of one of the branches. The more paths a current has in going from one

place to another, the less the resistance. (See Exp. 135.) The wires

are said to be in "parallel" or in "multiple arc."

[Illustration: Fig. 100.]

[Illustration: Fig. 101.]

=EXPERIMENT 124. To study the effect of decreasing the

resistance in one branch of a divided circuit.=

_Apparatus._ Galvanoscope, G V (No. 58); resistance coil, R

C (No. 79); two-fluid cell, 2-F C (§ 281), or a dry cell; 6

connecting wires; metal plates, M P.

_Arrange_ as in Fig. 100, so that the current divides into two

branches at M P 1. The branches unite at M P 2.

=322. Directions.= (A) Take the reading of G V with 2 ohms

resistance in the lower branch; that is, with the whole of R C

in circuit.

(B) Take the reading of G V with one ohm in circuit; that is,

with the end of wire, 5, connected to M instead of to R.

metal plate, thus joining wires 3 and 5. Compare the results

from (A), (B), and (C), and explain.

_=323. Current in Divided Circuits.=_ Let us consider a circuit like

that shown in Fig. 101. If the points, C and Z, were at the same

potential, no current would pass from C to Z. As the current does pass,

Z must be at a lower potential than C; there is a _fall of potential_

from C to Z. If the branch, A B, has the same resistance as R X, the

same amount of current will pass through each. Exp. 124 has shown that

when the branches have unequal resistances, most of the current passes

through the one of small resistance. If R X has a greater resistance

than A B, most of the current will pass through A B.

CHAPTER XIX.

MEASUREMENT OF RESISTANCE.

[Illustration: Fig. 102.]

=EXPERIMENT 125. To study the construction and use of a simple

"Wheatstone's Bridge."=

_Apparatus._ Fig. 102. A Wheatstone's bridge, W B (No. 80), (§

324); astatic galvanoscope, A G (No. 59); dry cell, D C (No.

51); key, K (No. 55); 7 wires with spring connectors, two of

which, R and X, are equal in length; metal plate, M P, for

connecting wires.

_Arrange_ as in Fig. 102. The carbon of D C is joined to K, and

this to the point, C, of the bridge. The zinc of D C connects

with the point Z on W B. The A G is placed between the branches

for clearness. Wire 3 is joined to the left-hand binding-post

of A G, and wire 4 joins M P with the right-hand one. When the

end of wire 3 does not touch G-s W, it is evident that as soon

as K is pressed, the current divides at C on its way to Z,

where the branches unite again. K is used so that D C will not

be polarized by steady use.

[Illustration: Fig. 103.]

=324. The Simple Wheatstone's Bridge= (Fig. 103) consists of a

wooden base, W, at the ends of which are fastened two aluminum

conductors, 1 and 3. At one side of W is fastened another

conductor, 2. In Fig. 104 are side views of the conductors.

These are used merely for convenience in making connections,

and take the place of the metal plates used in previous

experiments. A German-silver wire, G-s W, is stretched between

1 and 3, and under this is a scale, S, divided into 100 small

parts, these being tenths of the larger divisions. The ends of

G-s W are held between eyelets, as shown at E, Fig. 104.

[Illustration: Fig. 104.]

=Reading the Scale.= The value of part A can be read

directly from the scale, using the lower row of figures.

The point marked P, for example, would be read 3.7 (three

and seven-tenths large divisions); B would be 6.3, found by

subtracting 3.7 from 10. The sum of A and B must always equal

10. The 6.3 may also be read directly by using the upper row of

figures for the whole numbers, counting the tenths to the left.

Try to divide the smallest divisions into halves, at least;

that is, if A = 3.75, B = 6.25. Take the readings carefully.

=325. Directions.= (A) Touch the free end of wire, 3, to the

point, C, which has a higher potential than M P. Press down K

for an instant only. Some current should pass through A G, as

a shunt. Should it pass from C to M P or the reverse? Note in

which direction the right-hand end of the astatic needle is

deflected.

(B) Swing the end of 3 around and touch it to the point, Z,

which has a lower potential than M P. Press K for an instant,

watch the needle, and compare with the results in (A).

intervals, until a point is found at which the needle of A G

is not deflected. How does the potential of this point compare

with that of M P?

_=326. Discussion; Equipotential Points.=_ Since one end of the G-s W

has a higher, and its other end has a lower potential than M P, there

must be, somewhere on it, a point at which the potential is the same as

at M P. This place is quickly found by sliding the free end of wire, 3,

along, pressing K occasionally, until A G shows that no current tends

to pass through it in either direction, when the current passes from C

to Z through the two branches of the divided circuit. This point and M

P are called _equipotential points_.

If the resistance of the part, X, be increased, it should be evident

that the part of the bridge-wire, B, should be also increased to find a

point having the same potential as M P; that is, the end of 3 should be

moved towards C.

We have, in the bridge-wire, a simple means of varying the resistance

of its parts, A and B.

=327. Use of Wheatstone's Bridge.= It will be found, upon

trial, if we put a resistance of 2 ohms in place of R, Fig.

102, and 2 ohms in place of X, that the free end of wire 3

will have to be at the center of the bridge-wire in order

to get a "balance"; that is, to find the place where A G is

not affected. No matter what the resistance of R and X are,

provided they are equal, this will be true. The value of both

A and B, on the scale, will be 5 whole spaces, no tenths. From

this we see that A: B:: R: X, which reads A _is to_ B _as_ R

_is to_ X; this means that A × X = B × R. Supplying the values

of the letters, we have 5 × 2 = 5 × 2. If we did not know the

value of X, that is, if we were measuring the resistance of

a coil of wire, using a 2-ohm coil as the standard, or R, we

could find the value of X, knowing the other 3 parts of the

proportion. 5 × X = 5 × 2, which means that 5 times the value

of X is 10; hence the value of X is 10 ÷ 5 = 2 ohms.

Suppose that we have R = 2 ohms, which is the standard

resistance coil (No. 79), and are trying to find the resistance

of a coil, X. We slide the end of wire, 3, along on the

bridge-wire until the correct place is found. (See Exp. 125,

126, for details.) Take the values of A and B (§ 324), supply

them in the equation given, and work out the value of X.

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

of X in ohms.

A: B:: R: X, which means that A × X = B × R, or 3.7 × X = 6.3 ×

2. X must equal, then (6.3 × 2) ÷ 3.7 = 3.405 ohms.

=Note.= In practice it is most convenient to make connections

as shown in Fig. 105 when measuring resistances (Exp. 126). The

arrangement given in Fig. 102 is simply for explanation. It

will be seen that the smaller A is, compared with B, the larger

the unknown resistance compared with your standard.

[Illustration: Fig. 105.]

=EXPERIMENT 126. To measure the resistance of a wire by means

of Wheatstone's Bridge; the "bridge method."=

_Apparatus._ Same as in Exp. 125; the two-ohm resistance coil,

R C (No. 79); a coil of wire, X, as, for example, the 15-turn

coil on the galvanoscope, G V (No. 58).

_Arrange_ as in Fig. 105. You will observe that the central

conductor of the bridge (2, Fig. 104) takes the place of M P in

previous explanations. We still have the same kind of a divided

circuit as explained in Exp. 125, A G being connected with

points of equal potential. It will be found convenient to have

D C at the right, and A G facing you at the left, the key being

in front. (See Exp. 107 in regard to adjusting A G.)

Notice that you have a standard resistance (2 ohms) in place of

R, Fig. 102, and an unknown resistance (galvanoscope coil) in

place of X. (See § 330.)

=329. Directions.= (A) Touch the free end of wire, 3, to

the left-hand side of the bridge-wire, press the key for an

instant, only, and note the direction taken by the right-hand

end of the needle. Move the end of wire, 3, to the right-hand

side of the bridge-wire, touch key, watching needle. Does the

needle move more or less than before? In the same or opposite

direction? If the deflections are opposite, the point that has

the same potential as binding-post, 2, must be _between_ the

two points touched.

(B) Be sure that all connections are good. Find the point on

G-s W, at which there is no deflection, as directed in Exp. 125

(C). Note the readings on the scale, as explained in § 324.

resistance of the coil of G V, the resistances of the wires

joining R C and G V to the bridge being neglected.

(D) Make proper allowances for the resistances of the wires

just mentioned (see § 330), and compare them with the results

found in part (C).

=330. Allowances for connections.= It should be remembered that

the wires joining R C and G V to the bridge also have some

resistance. Such connections, in regular instruments, are made

by heavy copper straps or by thick, short wires, so that their

resistances can be neglected. In case you use the ordinary No.

24 copper wire, as directed, the resistances of the pieces

can be measured by means of the bridge, or you can calculate

their resistances from the wire tables. The resistances should

be allowed for. It is evident that your standard resistance

is 2 ohms _plus_ the resistance of the connecting wires, and

that the resistance of the coil, X, is found by deducting the

resistance of its connecting wires from that found from the

proportion previously used.

_Example._ We see from the table that the resistance of about

39 ft. 1 in. of No. 24, B and S copper wire is 1 ohm. This

equals 469 in. If 469 in. have a R (resistance) of 1 ohm, 1 in.

will have a R of one-469th of an ohm; that is 1 divided by 469,

which equals a little over .002 ohm. For every inch of No. 24

wire used, then, for connections, we may allow .002 ohm. This

will be near enough for our purposes.

Suppose that each connection is 18 in. long, the regular wires

with connectors being used. The R of the 36 in. joined to R

C will then be 36 times .002 = .072 ohm. Our standard R must

then be considered as 2.072 ohm. If we substitute this in the

example, as stated in § 328, we have 3.7 × X = 6.3 × 2.072. X

must equal (6.3 × 2.072)/3.7 = 3.528 ohm, which includes the

unknown resistance and 36 in. of connections, the R of which

is .002 ohm; 3.528 - .072 = 3.456, the resistance of X alone.

Compare this with the answer to example, § 328. Make allowances

according to length of connectors used.

_=Note.=_--Carefully keep all the results of these experiments in a

note book for future reference. Be sure that connections are good.

=EXPERIMENTS 127-137. To measure the resistances of various

wires, coils, etc., by the "bridge method."=

_Apparatus._ The coils of wire, etc., as stated in the

"Directions" of each experiment. The details of each piece of

apparatus may be found by referring, from the numbers given,

to the "Apparatus List," and to descriptions in the paragraphs

mentioned. Also all the apparatus of Exp. 126.

=Note.= Make proper allowances for connections (§ 330) in all

experiments in measuring resistances.

=EXPERIMENT 127.=

=331. Directions.= (A) As explained in Exp. 126, measure the

resistance of the 10-turn coil of G V, allowing for connections

(§ 330). Read the bridge-scale carefully.

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

=EXPERIMENT 128.=

=332. Directions.= (A) Measure the resistance of the 5-turn

coil of G V (see Exp. 126, etc.), using 2 ohms as standard.

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

the sum with the resistance of the 15-turn coil, as found in

Exp. 126, D. The difference should be but a few hundredths of

an ohm.

=EXPERIMENT 129.=

=333. Directions.= (A) Measure the resistance of the coil

of No. 24 copper wire (No. 89). This coil is used for later

experiments. Spring connectors are fastened to the ends of this

coil, allowing it to be directly connected to the conductor on

the bridge, so no allowance should be made for its connecting

wires. (See Exp. 126 for details.) Mark the resistance upon the

coil for future use. (See Note.)

=Note.= The student will be surprised, perhaps, to find that

different results are obtained for the resistance of a given

wire in case he uses different standard resistances in the

various tests; that is, he will probably get a different result

in Exp. 127 (A) from the result of Exp. 127 (B). The difference

here, however, may not be large. The best results are obtained

by making the standard resistance as nearly equal as possible

to the resistance to be measured, so that a balance can be

found when the end of wire 3 (Fig. 105) is near the center of

the bridge-wire. If R, Fig. 105, is much larger or smaller than

X, the point desired on G-s W will be near one of its ends, and

large errors thereby produced. The approximate resistance of

X can be found by trial, then more or less resistance can be

used for R to suit. The student should make several coils as

explained in Apparatus Book, Chapter XVII. The resistance of

the different coils furnished should be measured and marked.

These can be used to vary the value of R.

=EXPERIMENT 130.=

=334. Directions.= (A) Measure the resistance of the coil of

No. 25 copper wire (No. 90). (See Exp. 126 for details and the

Note, Exp. 129.)

=EXPERIMENT 131.=

=335. Directions.= (A) Measure the combined resistance of the

two coils used in Exps. 129 and 130, when they are joined in

"series"; that is, when one end of one coil is joined to one

end of the other by means of a metal plate, the free ends being

connected to the bridge (Exp. 126). The current has to travel

through the entire length of both coils.

(B) Compare this result with the sum of their separate

resistances found in Exps. 129 and 130. (See Exp. 129, Note.)

=EXPERIMENT 132.=

=336. Directions.= (A) Measure the resistance of the two coils

(Exp. 131) when they are joined "in parallel." (See § 293.)

They may be joined in parallel by connecting them both to the

bridge at the same time, one end of each being slipped onto 2

(Fig. 103), the other end of each being joined to 3. In this

way the current has two paths, side-by-side, to get from 2 to

3. (See Exp. 129, Note.)

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

=EXPERIMENT 133.=

=337. Directions.= (A) Measure the resistance of 1 meter of No.

28 German-silver wire. Use the wire as arranged on a board,

Exp. 122 (Figs. 96 and 98), making the connections with the

bridge from binding-posts, X and Z. (See Exp. 129, Note.) The

wires connecting the bridge with the ends of the G-s wire will

each have to be about 2 ft. long. In making deductions (§ 330)

figure according to the length used.

(B) Divide the total resistance by 100 to get the resistance of

1 cm. of the wire, and carefully mark off the board into cm.

This will give 100 parts between X and Z.

=EXPERIMENT 134.=

=338. Directions.= (A) Using the No. 28 G-s wire on the board,

as arranged for Exp. 122, measure the resistance of the 2

meters in series, the connections being made with the bridge

from X and Y, Fig. 98.

(B) Compare the result with that of Exp. 133. What is the

relation between the length of a wire and its resistance? See

Summary of Laws. (See Exp. 129, Note.)

=EXPERIMENT 135.=

=339. Directions.= (A) Measure the resistance of the above two

meters of No. 28 G-s wire when joined in parallel. (§ 293.)

The binding-posts, X and Y, can be joined by a short wire with

connectors on its ends, or by clamping a thin strip across by

means of spring connectors. Use the 2-ohm coil as the standard,

and make proper allowances. (§ 330.)

(B) From the results of Exps. 132 and 135 what can be said

about the resistances of parallel circuits as compared with the

resistances of the separate branches?

=EXPERIMENT 136.=

=340. Directions.= (A) Arrange the 2 meters of No. 30 G-s wire

on the table or board, again (Exp. 121, Fig. 96).

(B) Measure the resistance of one meter. Find the value of X

approximately, and use a resistance for R that will suit. (See

Exp. 129, Note.)

the wire.

(D) Compare the resistance of one meter of No. 28 G-s wire,

found in Exp. 133, with the resistance of 1 meter of No. 30 G-s

wire. What is the relation, then, between the size (area of

cross-section) of a wire and its resistance? (See the results

of Exp. 122, and § 319, also Summary of Laws.)

=EXPERIMENT 137.=

=341. Directions.= (A) Measure the resistance of 2 meters of

No. 30 copper wire, arranged on a board as in Fig. 96. (See

Exp. 129, Note.) Get the resistance of 1 meter.

(B) Compare the conductivities of copper and German silver

by studying the results of Exps. 136 and 137. Which has the

greater resistance? To find out how many times greater one

resistance is than the other, divide the larger by the smaller.

=EXPERIMENT 138. To study the effect of heat upon the

resistance of metals.=

_Apparatus._ Same as for Exp. 126; the coil of No. 24 wire (No.

89); a lamp or other source of heat. Arrange as in Fig. 105.

=342. Directions.= (A) Measure the resistance of the coil as

before, Exp. 129. The result should nearly agree with that of

Exp. 129, provided connections, etc., are the same.

(B) Remove the coil from the bridge, hold it about a foot above

a lamp or stove, to warm it thoroughly, but do not heat it

enough to injure the covering. It will take a minute or so to

warm it so that the heat will get to the inside also.

result with its resistance when cold. Does heat increase or

decrease the resistance of a copper wire?

_=343. Effect of Heat upon Resistance.=_ Although there was but the

fraction of an ohm difference in the resistances of the hot and cold

coil, it was evident that changes of temperature affect the conducting

power of copper. This is true of all metals; but German silver and

other alloys are much less affected than pure metals, so they are used

in making standard resistance coils. The resistance of liquids that

can be decomposed by the electric current decreases as the temperature

rises. Carbon acts like the liquids, while the resistance of metals

_increases_ as their temperature rises.

=EXPERIMENT 139. To measure the resistance of a wire by the

method of "substitution."=

_Apparatus._ The coil of No. 24 wire (No. 89), the resistance

of which has been measured, but which will be considered an

unknown resistance, X; G V, 2-F C, M P, connecting wires, etc.,

previously used; rheostat (§ 344). Arrange as in Fig. 106

first, then as in Fig. 107.

_=344. Simple Rheostat.=_ The No. 28 and No. 30 G-s wires stretched

upon the board (Fig. 96), make a convenient form of rheostat. The

resistance per cm. being known from the results of Exp. 133 and 136,

the resistance for any number of cm. is easily found. The 10-cm.

divisions should be divided into centimeters. These spaces may be

marked off from the rule (No. 88).

[Illustration: Fig. 106.]

=345. Directions.= (A) Be sure that 2-F C gives a constant

current, shown by the uniform deflection at G V, when arranged

as in Fig. 106. Do not use a cell that quickly polarizes. The

coil, X, forms a part of the circuit; it is joined to wires,

1 and 2, by means of metal plates, so that it may be quickly

removed without disturbing either G V or 2-F C. Carefully read

the deflection at G V.

(B) Remove X from the circuit, and join the free end of

wire, 2, to binding-post, X, and the free end of wire, 1, to

a small piece of sheet copper, which can be firmly pressed

upon the G-s wire to make a contact. Move this along on the

G-s wire until the deflection produced equals that of part

(A), remembering that the longer the G-s wire in the circuit

the less the deflection. Make two or three trials, as one or

two cm. difference in length make but a little difference in

the deflection. Note the number of cm. of G-s wire used, the

resistance of which must equal that of the coil, X.

found by the resistance of each cm., and compare the result

with the value found by using the bridge method directly.

[Illustration: Fig. 107.]

=EXPERIMENT 140. To measure the E. M. F. of a cell by

comparison with the two-fluid cell.=

_Apparatus._ Rheostat (§ 344); the two-fluid cell, 2-F C (Exp.

113), the E. M. F. of which may be taken as 1 volt; dry cell, D

C; galvanoscope, G V. Arrange first as in Fig. 107.

=346. Directions.= (A) Be sure that 2-F C gives a constant

current. Take the reading of G V without the rheostat in the

circuit; that is, with wires, 2 and 1, joined directly. The

deflection should be 50 or 60 degrees at least, and be constant.

(B) Attach a small piece of copper to the end of 1, and firmly

rub it along upon the G-s wire, thus introducing resistance

into the circuit, until the deflection is, say, 60° (50 or 55

degrees will do). Note the length of G-s wire used and call it

(B).

until the deflection is 50°, 10 degrees less than before. (If

the original was 50° make the new 40°). Call the number of cm.

of wire used (C).

(D) Replace 2-F C with the dry cell D C. Add resistance, as

before, until G V indicates a deflection of 60°, being careful

not to keep the circuit closed long enough to partially

polarize D C. Make 2 or 3 trials, allowing D C to rest a few

minutes between each. Call the number of cm. of G-s wire used

(D).

(E) Again add more resistance, as in (C), until the deflection

is reduced to 50°. Call the length used (E).

=347. Calculation.= It is known that resistances that are able

to reduce the strength of the currents equally are proportional

to the electromotive forces; that is, the electromotive forces

of the two cells are to each other as the two resistances

necessary to produce equal changes in the deflections, which,

of course, indicate equal changes in the strength of the

currents. Since the resistances used in the two cases are

directly proportional to the lengths used, we have:

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

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

=EXPERIMENT 141. To measure the internal resistance of a cell

by the "method of opposition."=

_Apparatus._ All the apparatus of Exp. 126. Two simple cells

(§ 275), the plates of which should be of the same size, the

same distance apart, and immersed in acid to the same extent in

both. The acid in both should be of the same strength.

=348. Directions.= (A) Connect the two cells in opposition, so

that no current will be generated by them, and so that the two

can be treated as a dead resistance. Do this by joining the

two zinc plates by a wire with connectors, and use wires to

connect the copper plates to the bridge like any other unknown

resistance.

(B) Measure the resistance of the two by the regular bridge

method, allowing for wires used for connections. One-half of

the resistance found will give the internal resistance of one

cell. (See Note.)

=Note.=--The standard resistance will have to be arranged

to suit each particular case to make the calculations even

approximately correct. (See Exp. 129, Note.) The standard

resistance may be increased by adding the various coils and

rheostat wires, their values being known.

_=349. Summary of Laws of Resistance.=_ 1. _The resistance of a wire

is directly proportional to its length, provided its cross-section,

material, etc., are uniform._

=EXAMPLE.= If 39.1 ft. of No. 24 copper wire has a resistance

of 1 ohm, 78.2 ft. will have a resistance of 2 ohms, because

78.2 is twice 39.1; 70.38 ft. will have a resistance of 1.8

ohms, as (70.38 ÷ 39.1 = 1.8) it is 1.8 times 39.1.

2. _The resistance of a wire is inversely proportional to its area

of cross-section._ The areas of cross-section of round wires vary as

the squares of their diameters; so _the resistance of a wire is also

inversely proportional to the square of its diameter, other things

being equal_.

=EXAMPLE.= A No. 30 wire has a diameter of about .01 inch,

while the diameter of a No. 24 wire is about .02 in.; that is,

the No. 24 has _twice_ the diam. that the No. 30 has. The area

of cross-section of the No. 24, however, is four times that of

the No. 30, so its resistance is but 1/4 that of the No. 30,

the lengths, etc., being the same. (See Wire Tables.)

3. _The resistance of a wire depends upon its material, as well as upon

its length, size, etc._

4. _The resistance of a wire depends upon its temperature._ (See

Elementary Electrical Examples.)

CHAPTER XX.

CURRENT STRENGTH.

_=350. Strength of Current.=_ The water in a certain tank may be under

great pressure, but if it is obliged to pass through long tubes before

it can turn a water-wheel, for example, it is evident that the work

done will depend not only upon the pressure in the tank, but upon the

resistance to be overcome before the water gets to the wheel. The work

that the water can do depends upon its _rate of flow_, and may be used

to measure the _strength_ of the current.

The strength of a current of electricity is measured also by the _work_

that it can do, and it depends upon its _rate of flow_ at the point

measured. The strength may be determined from its magnetic, heating, or

chemical effects.

_=351. Unit of Current Strength; The Ampere.=_ A current having the

strength of 1 ampere, when passed through a solution of silver nitrate

under proper conditions, will deposit 0.001118 gramme of silver in _one

second_; if passed through a solution of copper sulphate, copper plates

being used for the electrodes, in the solution, 0.0003277 gramme of

copper will be deposited in _one second_. (See Chemical Effects of the

Current.) The thousandth of an ampere is called the milliampere. The

strength of a current is proportional to the amount of chemical work

that it can do per second. (See § 357.)

_=352. Measurement of Current Strength.=_ The _galvanoscope_ previously

described simply shows the presence of a current, or whether one

current is larger or smaller than another. When the degree-card is

used to get the relative deflections, the instrument may be called a

_galvanometer_.

_=The Tangent Galvanometer=_ is made on the same general idea as our

galvanoscope, the diameter of the coil being twenty times, or more,

the length of the needle. In these the strengths of the two currents

compared are proportional to the tangents of the angles of deflection

produced. (See Elementary Electrical Examples.) There are several

varieties of galvanometers, each designed for its special work. They

are often calibrated or standardized so that the amperes of current

passing through them can be read off directly from the scale.

_=353. The Ammeter=_ is really a galvanometer from which may be read

directly the strength of a current. The coil has a low resistance so

that it will not greatly reduce the strength of the current to be

tested.

_=The Voltameter=_ measures the strength of a current by chemical means.

_=354. Unit of Quantity; The Coulomb.=_ A current having a strength of

1 ampere will do more chemical work by flowing one hour than it can do

in 1 second. In speaking of the _quantity_ of electricity we introduce

the element of _time_. The unit of quantity is called the _coulomb_,

just as a cubic foot of water may be taken as a unit of quantity for

water. A coulomb is the quantity of electricity given, in one second,

by a current having a strength of 1 ampere. Coulombs are found by

multiplying amperes by seconds; thus, a current of 5 amperes will give

20 coulombs in 4 seconds.

_=355. Electrical Horse-power; The Watt.=_ The electric current has

power to do work, and we speak of the horse-power of an electric motor

in the same way as for a steam-engine. A current with the strength of 1

ampere and an E. M. F. of 1 volt has a unit of power called the watt.

746 watts make an electrical horse-power.

Watts = amperes × volts.

Watts ÷ 746 = the number of horse-power.

(See Transformers, also Elementary Electrical Examples.)

_=356. Ohm's Law.=_ It was first shown by Ohm that the strength of

a current is equal to its E. M. F. divided by the resistance in the

circuit; that is,

Strength of current (amperes) = E. M. F. (volts). / resistance (ohms).

If we let C stand for the strength in amperes, E for the E. M. F. in

volts, and R for the resistance in ohms, we have the short formula,

easily remembered,

C = E/R

_=357. An Ampere=_ would be produced by a current of 1 volt pushing

its way through a resistance of 1 ohm. Knowing any two of the three,

C, E, or, R, the other may be found. The resistance, R, it must be

remembered, is the total resistance in the circuit, and is composed of

the total internal and external resistances.

(See Elementary Electrical Examples.)

_=358. Internal Resistance and Current Strength.=_ It is evident that

the internal resistance of a cell varies with the position and size of

the plates. We shall now study the effects of these changes upon the

strength of the current.

[Illustration: Fig. 108.]

=EXPERIMENT 142. Having a cell with LARGE PLATES, to find how

the strength of the current is affected by changes in the

position of the plates, the external resistance being small.=

_Apparatus._ Galvanoscope, G V; materials for simple cell (Exp.

110); connecting wires. Arrange as in figure 108, omitting the

wooden cross-piece.

=359. Directions.= (A) Connect the wires with the 5-turn coil

of G V, which has but little resistance. Have the tumbler

nearly full of dilute acid to get the effect of large plates;

that is, the current has a large liquid conductor to pass

through in the cell, and the _internal_ resistance will be

small. G V should be properly placed N and S.

(B) Place the copper and zinc plates as far apart as possible

in the acid, and press them against the bottom of the tumbler.

Note the reading of G V. It is not necessary to take readings

with reversed current.

keep the same amount of surface under acid, slowly bring them

near each other and watch the needle.

(D) Hold the plates about an inch apart, and against the

bottom, and note the reading of G V. Slowly raise the plates,

keeping them the same distance apart until they are out of the

acid. Watch the action of the needle.

Make a note of your readings in degrees and write your

conclusions. Does a change in internal resistance affect the

strength of the current?

=EXPERIMENT 143. Having a cell with SMALL PLATES to find how

the strength of the current is affected by changes in the

position of the plates, the external resistance being small.=

_Apparatus._ Same as in Exp. 142, the acid, however, being but

1 in. deep in the tumbler; that is, we have the effect of a

cell with small plates, each being about 1 in. by 1/2 in.

=360. Directions.= (A) Repeat (B) and (C) of Exp. 142,

recording the reading of G V in each case.

(B) Compare the results with those of Exp. 142, remembering

that the _internal_ resistance is larger than before. Is the

current as strong with small plates as with large plates when

the external resistance is small? When the external resistance

is small (the 5-turn coil, for example), should the cell have a

high or low internal resistance to produce the greatest effect

upon the needle?

[Illustration: Fig. 109.]

=EXPERIMENT 144. To find whether the changes in current

strength, due to changes in internal resistance, are as great

when the external resistance is large, as they are when the

external resistance is small.=

_Apparatus._ Same as for Exp. 142, 143, also the rheostat

containing the two meters of G-s wire (Exp. 121).

=361. Directions.= (A) Arrange as in Fig. 109, the external

resistance being 2 meters of No. 30 G-s wire in series with G

V. The 2-F C in the Fig. is replaced, however, by the simple

cell as in Exp. 143.

(B) Find the effect upon the strength of the current of moving

the plates about when but 1 in. of acid is in the tumbler.

readings with plates near each other and as far apart as

possible. Lift them nearly out of the acid and take the reading.

(D) Still increase the external resistance of the circuit

by adding coils of wire or the meter of No. 28 G-s wire and

repeat. Is the strength of the current greatly affected by

_slight_ changes in the internal resistance when the external

resistance is large?

_=362. Discussion.=_ We shall study, by means of figures, how changes

in internal resistance affect the strength of the current.

Let R stand for the total external resistance of a circuit, and r for

the total internal resistance of the cell or cells; ohm's law, then,

will be expressed by

C = E / (R + r)

=EXAMPLE.= Let us take a circuit (A) when the external

resistance, R, is small, and (B) when R is large compared with

r, E being taken as 1 volt in both cases.

(A) Let R = 1, and r = 2; substituting these values in the

formula above, we have:

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

Now let the internal resistance, r, be slightly increased from

2 to 3 ohms; the value of C then becomes 1/4 ampere, as R + r

= 4. The change in C, then, is the difference between 1/3 and

1/4; and this expressed in decimals becomes .33 - .25 = .08

ampere.

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

these values we have,

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

Increasing r from 2 to 3, as before, etc., we find that C = 1

divided by 203 = .00492 ampere.

The above shows clearly (A) that the value of C is changed

considerably by changes in r when R is _small_, and (B) that

changes in r produce very slight changes in C when R is

_large_. Review your results of Exps. 142-144. (See Elementary

Electrical Examples.)

_=363. Arrangement of Cells and Current Strength.=_ We have seen that

internal resistance affects current strength. In joining cells, then,

attention must be given to the internal resistance as well as to the E.

M. F. of the combination.

_=364. Cells in Series.=_ It has been shown by careful experiments that

the E. M. F. of two cells joined in series (Fig. 110) is equal to the

sum of the E. M. F. of each. Ten cells, joined in series, have ten

times the E. M. F. of one cell, provided they have the same E. M. F. As

the Zn of one is joined to the Cu of the other, the current is obliged

to pass through one solution after the other; that is, the internal

resistance of the two in series is equal to the sum of their internal

resistances. Ten cells, joined in series, have ten times the internal

resistance of one cell, provided they have equal internal resistances.

[Illustration: Fig. 110.]

[Illustration: Fig. 111.]

_=365. Cells Abreast.=_ When the positive plates are joined together

and the negative plates are also joined together (Fig. 111), the cells

are said to be _abreast_, _in parallel_, or in _multiple arc_. It has

been shown that two cells of equal strength, joined abreast, have the

same E. M. F. as one cell. The two Cu plates, being joined, must have

the same potential; all the Zn plates have the same potential, so the

difference of potential at the terminals of the combination is the same

as that at the terminals of a single cell.

In two cells abreast (Fig. 111) the current has two liquid paths, side

by side, to get from Cu to Zn; this makes the internal resistance

one-half that of one cell, provided their internal resistances are

equal. Ten cells, of equal internal resistance, when joined abreast,

have one-tenth the internal resistance of one cell.

=EXPERIMENT 145. To find the best way to join two similar cells

when the external resistance is small.=

_Apparatus._ Two simple cells using dilute sulphuric acid, with

copper and zinc elements, as in Exp. 112; galvanoscope, G V;

connecting wires, etc. Have the zincs well amalgamated. Remove

them from the acid as soon as readings are taken.

=366. Directions.= (A) Partly fill the tumblers with the acid.

Join the cells in series (Fig. 110), then connect wire 1 (Fig.

110) with the left-hand binding-post of G V, and wire 2 with

the middle one, thus putting the 5-turn coil into the circuit.

Take the reading of G V.

(B) Join the cells in multiple arc (Fig. 111), connecting them

as in (A) with G V. Write down the reading, and compare it with

that found in (A).

=EXPERIMENT 146. To find the best way to join two similar cells

when the external resistance is large.=

_Apparatus._ Same as for Exp. 145, also the rheostat containing

2 metres of No. 28 or 30 G-s wire. Arrange the G-s wire in

series with the 15-turn coil of G V, as shown in Fig. 109, two

simple cells being used, however, instead of 2-F C as shown.

=367. Directions.= (A) Take the reading of G V when the two

cells are in series (Exp. 145), the external resistance being

the 15-turn coil and G-s wire.

(B) Join the cells in parallel and take the reading, using the

same external resistance as in (A).

2 metres of No. 28 G-s wire and repeat (A) and (B). What does

the experiment show?

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

_=368. Best Arrangement of Cells.=_ It will be seen by experiments that

with a given number of cells the strongest current is produced when

they are arranged so that the internal resistance of the battery nearly

equals the external resistance of the circuit.

When the external resistance is small, the internal resistance may be

kept down by joining the cells in parallel; and, although the E. M. F.

is also kept small, the value of C will be larger than it would be with

a larger internal resistance and a larger E. M. F.

When the external resistance is large, the internal resistance can

be made large by joining the cells in series. The advantage comes,

however, from having a large value of E. A large resistance can not

hold back a current of large E. M. F. By joining the cells in series

the value of E is made large, and the value of C becomes large even

though there is an increased internal resistance. (See Elementary

Electrical Examples.)

CHAPTER XXI.

CHEMICAL EFFECTS OF THE ELECTRIC CURRENT.

_=369. Chemical Action and Electricity.=_ We have learned that the

electric current is produced, in the cell, by chemical action. There

is a definite relation between the chemical action and the current

produced. We are now to study the changing of electrical energy back,

again, to chemical energy.

[Illustration: Fig. 112.]

_=370. Electrolysis=_ is the name given to the process of decomposing

chemical compounds by passing the electric current through them. The

compound decomposed is the _electrolyte_. Fig. 112 shows a tumbler

of liquid (electrolyte) through which the current is to pass in the

direction of the arrow. Two carbon plates, A and C, are in the liquid,

and are joined to the source of electricity. The current enters at A

(_anode_) and leaves at C (_cathode_).

[Illustration: Fig. 113.]

=EXPERIMENT 147. To study the electrolysis of water.=

_Apparatus._ The two simple cells (§ 275) joined in series (§

364), although two Daniell or two dry cells will be better. A

tumbler of water containing a few drops of sulphuric acid to

make the water a conductor. Two pieces of sheet copper will

serve as the electrodes. The galvanoscope may also be put into

the circuit as in Fig. 113.

=371. Directions.= (A) Allow the current to pass, and note (1)

whether gas is set free at both electrodes, A and C, and (2)

at which the quantity of gas is the greater. If very little

gas is produced use more cells.

(B) Remove A and C from the liquid, to remove the gas, then

watch the action of the needle of G V as the water is again

decomposed.

_=372. Composition of Water.=_ The two gases liberated in Exp. 147 were

hydrogen (H) and oxygen (O). The chemical formula for water is H_{2}O,

which means that it is composed of two parts, by volume, of H and one

part of O. With proper apparatus these gases may be collected, tested,

and the amounts measured.

_=373. Electromotive Force of Polarization.=_

We know that H and O have a strong chemical attraction, or affinity,

for each other. In order, then, for the current to decompose water,

this attraction between the gases must be overcome; and as soon as the

current ceases, these gases try to rush together again to form water.

This sets up an electromotive force of almost 1.5 volts; in fact, a

current is produced if the H and O be allowed to form water again (See

Storage Cells). To decompose water the current must have an E. M. F. of

over 1.5 volts to overcome this E. M. F. of polarization. It was seen

in the study of simple cells that the current became rapidly weaker

as hydrogen was deposited upon the copper plate, on account of this

opposing electromotive force.

In decomposing other compounds, the anode is made of the metal which

is to be deposited at the cathode. If copper is to be deposited from a

solution of copper sulphate the anode should be a copper plate; this

keeps the solution at same strength, and avoids the opposing E. M. F.

of polarization; that is, a very weak current will do the work (See

Exp. 149), because the electrodes are of the same metal.

=EXPERIMENT 148. To coat iron with copper.=

_Apparatus._ Iron nail, solution of copper sulphate (§ 283).

=374. Directions.= (A) Clean the nail with sandpaper, then hold

it in the copper solution for a few seconds. Machinists often

cover iron or steel tools with a thin coating of copper in this

way.

[Illustration: Fig. 114.]

=EXPERIMENT 149. To study the electrolysis of a solution of

copper sulphate.=

_Apparatus._ Galvanoscope, G V; two-fluid cell, 2-F C; a

tumbler, T, containing about an inch of copper sulphate

solution (§ 283); a wooden cross-piece to which is fastened a

copper strip; carbon rod, C; wire 2 is held to C by a rubber

band. _Arrange_ as in Fig. 114, so that Cu will be the _anode_

(§ 370), the current passing as shown by arrow. A dry cell may

be used for short experiments instead of the 2-F C.

=375. Directions.= (A) The carbon being clean, allow the

current to pass, C and Cu being kept about 1/2 in. apart. Watch

the surface of C, and note the beautiful color of the deposited

copper. Save the coated rod for the next experiment. Has the Cu

plate been acted upon?

_=376. Electroplating=_ is the name given to the process of coating

substances with metal with the aid of the electric current. The copper

sulphate, CuSO_{4}, is broken up into Cu and SO_{4} by the current. The

Cu goes to the cathode, and the SO_{4} attacks the anode, gradually

dissolving it if it be copper; that is, the _metal_ part of CuSO_{4} is

carried in the direction of the current.

Most metals are coated with copper before they are silver or gold

plated. A solution of silver is used for silver plating, silver being

used as the anode.

=EXPERIMENT 150. To study the chemistry of electroplating.=

_Apparatus._ Same as in last experiment, but use two carbon

rods for the electrodes. Arrange as in Fig. 114, with the Cu

replaced by another carbon. Two simple cells (§ 275) are also

needed.

=377. Directions.= (A) Allow the current to pass as before. Is

copper still deposited? Does anything occur now at the surface

of the anode? Is the copper deposited as rapidly as before?

(B) Try the effect of the two simple cells joined in series,

Instead of the two-fluid cell.

carbon cathode, reverse the direction of the current through

the copper solution; that is, use the coated rod for the anode.

Allow the current to pass until a change takes place in the

anode.

_=378. Discussion.=_ Ions are the names given to the parts into which

an electrolyte is decomposed by the electric current. In the case of

CuSO_{4}, the ions are Cu and SO_{4}, which is called an acid radical.

This SO_{4} can not dissolve carbon or platinum, so these are used

when water is to be electrolyzed. Where copper is used as the anode

for copper plating, the SO_{4} attacks it, forming CuSO_{4} again,

and this keeps the solution strong. If carbon were used instead, the

SO_{4} would take H_{2} from the water around the anode and H_{2}SO_{4}

(sulphuric acid) would be formed, the oxygen of the water being set

free at the anode. The amount of Cu dissolved from the copper anode

equals nearly the amount deposited upon the cathode. Exp. 150 shows

that the metal is carried in the direction of the current. As hydrogen

is produced at the cathode it is chemically considered a metal.

_=379. Electrotyping=_ consists in making a copy in metal, of a

woodcut, page of type, etc. A mould or impression of the type is first

made in wax, or other suitable material (the pages of this book, for

example, as set up by the printer). These moulds are, of course, the

reverse of the type. They are coated with graphite to make them

conduct electricity, and hung as the cathode, in a bath of copper

sulphate. After a thin coat of copper has been deposited by an electric

current, the wax is removed and the thin copper backed with soft metal.

The metal surface next to the wax will be just like the type, only made

of copper. These plates or _electrotypes_ can be printed from, the

original type being used to set up another page. (See "Things a Boy

Should Know About Electricity.")

_=380. Voltameters=_ are cells used to measure the strength of an

electric current. In the _Water Voltameter_ the hydrogen and oxygen

produced are measured. The H acts like a metal and goes to the cathode,

two parts of H being formed to one of O.

_Copper Voltameter._ This cell measures the amount of copper deposited

in a given time by a current. The copper cathode is weighed before and

after the current flows. The weight of Cu deposited is then divided by

the number of seconds during which the current passed, and this result,

in turn, by .000328, which will give the average strength of the

current in amperes. (See § 351.) Other forms of voltameters are also

used.

In all voltameters the quantity of metal deposited is proportional to

the time that the current flows, and to its strength.

[Illustration: Fig. 115.]

=EXPERIMENT 151. To study the construction and action of a

simple "storage" cell.=

_Apparatus._ Two lead plates, L P, (Nos. 77, 78) fastened to a

wooden cross-piece (§ 275). The spring-connectors should not be

forced upon the thick lead. Fasten one end of the wire under

the screw-head. A tumbler two-thirds full of dilute sulphuric

acid (§ 258); the astatic galvanoscope, A G; wires to form

connections; the two simple cells joined in series. _Arrange_

as in Fig. 115. One L P is joined to binding-post, L, of A

G by the wire marked 1; wire 2 connects the other L P to the

copper Cu. Wire 3 joins the zinc to any thin metal plate, M P,

which is used for convenience, so that the spring connectors

can be quickly slipped on or off. Wire 4 joins M P with

binding-post R of A G.

=381. Directions.= (A) Get clearly in mind the direction in

which the right-hand end of the astatic needle is deflected

when the current passes, remembering that it passes into A G

at L and leaves at R. Allow the current to flow for 10 or 15

minutes through the circuit, at the same time watching the

needle to see whether the strength of the current remains

constant.

(B) Remove the connector from Cu, swing it over into the

position of the dotted line (Fig. 115), slip the connector

upon M P and watch the needle. This cuts the cells out of the

circuit; but, if you desire, also remove wire 3 from M P.

Does the storage cell, S C, produce any current? Does it pass

through A G in the same direction as that which came directly

from the two cells?

other cells in series if you have them.

_=382. Secondary or Storage Cells=_ must be charged by a current

before they can give out a current. _Electricity_ is not really

stored. Chemical changes are produced in the storage cell by the

charging current, as in the voltameter or electroplating bath; and

it is, then, potential chemical energy that is stored. When the new

compounds are allowed to go back to their original condition by joining

the electrodes of the charged cell a current is produced. In other

words, an electric current produces chemical changes in the cell by

electrolysis, and these new compounds have an E. M. F. of polarization

because they are constantly willing and anxious to get back to their

old state. The plates are lead and are usually coated with compounds of

lead. Hydrogen and oxygen are given out at the electrodes. The current

from a dynamo is used to charge secondary batteries. (See "Things a Boy

Should Know About Electricity.")

CHAPTER XXII.

ELECTROMAGNETISM.

_=383. Electromagnetism=_ is the name given to magnetism that is

developed by electricity. You have already seen that if a magnetic

needle be placed in the magnetic field of a _magnet_, its N pole will

point in the direction in which the lines of force pass on their way

from the N to the S pole of the magnet. You have also seen that in the

galvanoscope, etc., a coil of wire acts like a magnet when a current

passes through it. Can we not, then, use the needle to study the lines

of force about wires and coils?

[Illustration: Fig. 116.]

[Illustration: Fig. 117.]

=EXPERIMENT 152. To study the lines of magnetic force about a

straight wire carrying a current.=

_Apparatus._ The compass, O C; key, K; dry cell, D C. Arrange

as in Fig. 116.

=384. Directions.= (A) Arrange the wire so that the current

will flow through it from N to S over the compass-needle as

soon as the circuit is closed (Fig. 117, A). Press K for an

instant only, and note the direction in which the N pole is

deflected. Repeat two or three times until you get clearly in

mind the direction taken by the needle. Sketch the result in

your note-book, and compare with Fig. 118, A. The arrow shows

the direction of the current.

(B) Let the current pass for an instant from N to S and _under_

the needle, as shown in Fig. 117, B. Sketch result.

needle (Fig. 117, C). Sketch result.

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

Sketch result.

(E) Let it pass through the wire from east to west (Fig. 117,

F) above the needle, then under it, and note result. Compare

the results with those indicated in Fig. 118.

[Illustration: Fig. 118.]

[Illustration: Fig. 119.]

_=385. Lines of Force About a Wire.=_ When a current passes through a

wire, the needle, over or under it, tends to take a position at right

angles to the wire. This shows that the lines of force pass _around_

the wire and not in the direction of its length. The needle does not

swing entirely perpendicular to the wire; that is, to the E and W line,

because the earth is at the same time pulling its N pole towards the N.

If the needle had no pointing power, and at the same time retained its

magnetic field, it would point exactly at right angles to the wire as

soon as the current passed.

If you look along the wire, Fig. 119, from the point, C, towards the

positions, A and B, you will see (A) that _under_ the wire the lines

of force pass to the left, and that _above_ the wire (B) they pass

towards the right. This is because the N pole points in the directions

mentioned. (See Fig. 118.) Looking along the wire from Z towards

position, D and C, you will see just the opposite to the above, as the

current comes _towards_ you.

_Rule._--When the current goes from you, the lines of force pass around

the wire in a clockwise direction, and when the current comes toward

you they pass around it in an anti-clockwise direction.

_=386. Ampere's Rule=_ may be used to remember what has been learned in

Exp. 152.

_If you imagine yourself swimming in the wire with the current, always

facing the needle, the N-seeking pole of the needle will always be

deflected towards your left hand._

When the needle is above the wire you must imagine that you swim upon

your back, in order to _face_ the needle.

_Another Rule._--Hold the right hand with the thumb extended and with

the fingers pointing in the direction of the current, the palm being

towards the needle and on the opposite side of the wire from the

needle. The N-seeking pole will then be deflected in the direction in

which the thumb points.

_=387.=_ If a wire carrying a strong current be dipped in iron filings,

the magnetic field about the wire acts by induction upon the particles

of filings, making magnets of them. These cling to each other simply

because they are little magnets.

_=388. Lines of Force about Parallel Wires.=_ When a current passes

in the same direction in two parallel wires the lines of force pass

around the wires in the same direction in both, and the magnetic fields

attract each other. When the currents flow in opposite directions the

magnetic fields repel each other.

=EXPERIMENT 153. To study the lines of force about a coil of

wire like that upon the galvanoscope.=

_Apparatus._ Galvanoscope, G V; dry cell; key; compass. Arrange

as in Fig. 116, using G V instead of the compass shown. The

coil of G V should be placed in the E and W line. The current

can pass only when the key is pressed. Connect the wires with G

V, so that the current will pass through the 15-turn coil from

W to E on top of the coil; that is, so that the current will

have a "clockwise" motion. Fig. 120 represents a front view of

the coil.

[Illustration: Fig. 120.]

[Illustration: Fig. 121.]

=389. Directions.= (A) Hold the compass in the various places

marked with a dot (Fig. 120) and note the directions taken by

its N pole. Make a circle similar to the one shown to represent

the coil, and sketch upon it the way in which the lines of

force pass around it according to your observations.

(B) Make a diagram like Fig. 121, which represents a

cross-section of the coil through the center. Imagine that you

have removed the top half of the coil and that you are looking

down upon the ends of the wire of the lower half. Draw curved

arrows about the coil at W and E to show which way the lines of

force are passing. Compare your results with those in Fig. 119,

remembering that at E, Fig. 121, the current is going away from

you.

N and S through the coil, and note the positions of the

compass-needle. Does the coil seem to have poles?

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

observations.

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

of wire.=

_Apparatus._ A coil of wire (No. 89), described in § 390;

current reverser, C R (No. 57); dry cell; connecting wires, etc.

=390. Coils= of wire for some of the following experiments

should be wound upon wooden spools that have been turned down

thin, so that the wire will be as near the central hole as

possible. They should be wound with a winder. (See Apparatus

Book, Chapter X.)

For convenience we shall call the starting end of the coil,

that is, the end that comes from the wire that is near the

center, the _inside end_, I E. The end of the last layer of the

coil we shall call the _outside end_, O E. These letters should

be noted in the diagrams. See Apparatus List for details of the

special coils used in these experiments.

[Illustration: Fig. 122.]

=391. Directions.= (A) Arrange as in Fig. 122, so that the axis

of the coil will lie in the E and W line. Place O C about 2 in.

from the E end of the coil. Press one lever of C R so that the

current will pass around the coil for an instant in a clockwise

direction; that is, so that it will enter the coil at O E. Note

the action of the needle. If the needle is not affected move it

nearer the coil and press the lever again. Get clearly in mind

the connections, the direction in which the N end of the needle

is deflected, etc. Is the E end of the coil a N or a S pole?

(B) Reverse the current through the coil. What effect has it

upon the polarity of the E end of the coil?

(B).

(D) Place O C in various positions about the coil and note the

action of the needle when the current passes. Does this coil

act like a magnet, having poles, magnetic field, etc.?

[Illustration: Fig. 123.]

_=392. Polarity of Coils.=_ It is evident from Exps. 153 and 154 that

a coiled conductor has poles, magnetic field, etc., when a current

passes, and that it strongly resembles a magnet, even though no

iron enters into its construction. We may say that the coil becomes

magnetized by the electric current. Fig. 123 shows a right handed coil

or helix of wire, the current passing as shown by the small arrows.

The left-hand end is a S pole because the current passes around it in

a clockwise direction. When you face the right-hand end of the coil the

current is seen to pass around it in an anti-clockwise direction; this

produces a N pole. As the N pole of the magnetic needle is attracted

toward the S pole of the coil, it is clear that the lines of force pass

through the inside of the coil as shown by the large arrows. They then

curve through the air and return to the S pole as with magnets.

=EXPERIMENT 155. To test the attracting and "sucking" power of

a magnetized coil or helix.=

_Apparatus._ The coil, battery, etc., used in Exp. 154, Fig.

122; a sewing-needle.

[Illustration: Fig. 124.]

=393. Directions.= (A) Arrange the coil, etc., as described in

Exp. 154. The coil need not lie in the E and W line, however,

and a key may be used instead of the current reverser.

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

hold the thread in the hand so that the S pole of the needle

will swing freely at the hole at the right-hand end of the coil

(Fig. 124). If the current passes as directed, the right-hand

end of the coil will be a N pole. What happens to the needle

when the key is pressed for an instant.

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

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

=EXPERIMENT 156. To find whether a piece of steel can be

permanently magnetized by an electric current.=

_Apparatus._ Same as for last experiment; an unmagnetized

sewing-needle; the compass.

=394. Directions.= (A) Be sure that the needle is not

magnetized. It should attract both ends of the compass-needle.

How can any magnetism in the needle be removed?

(B) Place the needle inside of the coil with its _point_ to the

east; that is, with its point at the N pole of the coil, and

its head at the S pole. Close the circuit for an instant. Test

the needle again for poles. Is the point a N or a S pole?

its polarity can be reversed.

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

etc.

[Illustration: Fig. 125.]

=EXPERIMENT 157. To study the effect of a piece of iron placed

inside of a magnetized coil of wire.=

_Apparatus._ Same as in Exp. 154; a short rod or iron _core_, I

C, of soft iron (No. 92) that will fit inside of the coil. This

combination is called an electromagnet.

=395. Directions.= (A) Arrange first as for Exp. 154, Fig. 122,

with the coil in the E and W line, no core being used, and

place O C about 6 in. from the right-hand end of the coil.

(B) Press the lever for an instant to see whether the field of

the coil is strong enough to move the compass-needle at that

distance. Move O C a little nearer or farther from the coil

until the needle _just_ moves, when the circuit is closed.

to see whether the magnetic field of the coil is stronger or

weaker than before.

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

CHAPTER XXIII.

ELECTROMAGNETS.

_=396. Electromagnets=_ are important to the student of electricity.

They form the principal part of nearly every electrical instrument.

You have seen that a wire has a magnetic field about it the instant a

current passes through it. A coil, or helix of wire, has a stronger

field than a straight wire carrying the same current, because each

turn, or convolution, adds its field to the fields of the other

turns. By having a _core_ of soft iron instead of air, wood, or

other non-magnetic material, the strength of the magnet is greatly

increased. The central core may be permanently fixed in the coil, or

it may be removable. (See Apparatus Book, Chapter IX, for Home-made

Electromagnets.)

_=397. Cores of Electromagnets.=_ A strong magnet has more lines of

force passing from its N pole through the air to its S pole than a weak

magnet. By increasing the number of lines of force we increase the

strength of a magnet. It has been seen, in experiments with permanent

magnets, that lines of force do not pass as readily through air as

through soft iron, and that lines of force will go out of their way to

pass through iron. It was learned in Exp. 154 that inside of a helix

(Fig. 123) the lines of force pass from the S to the N pole; they then

spread out through the air and pass back on all sides of the coil to

its S pole, as in the case of permanent magnets. The air around and

inside of a helix offers a great resistance to the lines of force, and

tends to weaken the magnetic field. When part of the circuit consists

of an iron core, which is a splendid conductor of lines of force, the

magnetic field is greatly increased in strength.

=EXPERIMENTS 158-163. To study straight electromagnets.=

_Apparatus._ A good dry cell or other source of a fairly strong

current; coil with soft iron core; key; wires with connectors,

etc.; small nails; iron-filings; compass; large wire nail; tin

box (No. 94) to act as a base for the electromagnets.

=EXPERIMENT 158. Lifting power.=

=398. Directions.= (A) Join the cell, key, and coil, as

explained in Exp. 154, so that the current will pass only when

the key is pressed. Place the core inside of the coil (Fig.

125). Two good cells in series can be used to advantage.

(B) Hold the coil in a vertical position near small nails, iron

filings, tin boxes, etc.; then press the key and raise coil;

carry the clinging iron to another place, break the circuit

at the key, and explain the result. Why do nails cling more

strongly to the core than filings after the circuit is broken?

=EXPERIMENT 159. Residual magnetism of core.=

=399. Directions.= (A) After the current has passed through

the coil with the core in place, remove the core and test it

for magnetism with the compass. Will the small end of the core

attract both poles of the compass-needle, or is it slightly

magnetized?

(B) If there is any residual magnetism, strike the core with a

hammer and test again.

and (B). Why does soft iron make a better core than steel for

electromagnets? Which should be the more easily magnetized?

=EXPERIMENT 160. Magnetic tick.=

=400. Directions.= (A) Join the electromagnet with the cell

and key as before (Exp. 154). Hold one end of the core firmly

against the top of a tin box which should stand upon the table

and which should act as a sounding-board. The flat boxes used

in the experiments on static electricity are good for this, or

use the tin box, No. 94, for a base. Rapidly open and close

the circuit by means of the key and listen for any clicks made

by the core.

(B) Listen for this sound in telegraph sounders, electric

bells, etc., if you have them. The armature should be held, of

course, so that slight sounds can be heard.

_=401. Discussion.=_ A bar of iron becomes slightly longer when

it is magnetized, the particles of iron being made to point

in the same direction. As soon as the current ceases to flow

through the coil the particles of the soft core nearly all

resume their mixed positions. The click heard is supposed to be

due to the changes in the molecules of iron. The core becomes

gradually warmer when it is rapidly magnetized and demagnetized

by a strong current.

[Illustration: Fig. 126.]

=EXPERIMENT 161. Magnetic figures.=

=402. Directions.= (A) Arrange as in Fig. 126. The key should

be used in case a dry cell acts as the source of the current.

Two good cells joined in series can be used to advantage. Lay

the coil flat upon the table and place on it a piece of stiff,

smooth paper, or a sheet of glass.

(B) Sprinkle a few iron filings upon the glass, which may be

held in place by books. Gently tap the glass with a pencil

while you close the circuit at the key. Do the filings arrange

themselves as in the case of permanent magnets? Make a sketch

of the field, remembering that you have both N and S poles, and

compare it with previous results.

[Illustration: Fig. 127.]

=EXPERIMENT 162. Magnetic figures.=

=403. Directions.= (A) Arrange as in Fig. 126, but stand the

coil on end, using the base as directed in § 407, to hold it

firmly in position. Join the ends, O E and I E, to the key as

before. Fig. 127 shows a top view of the coil and base.

(B) With books, etc., fix a piece of stiff, smooth paper, or

glass just over the top of the core, and proceed as in Exp. 161

to study the field. See § 417 for making permanent pictures of

magnetic fields.

=EXPERIMENT 163. Magnetic field.=

=404. Directions.= (A) Use same arrangement as for Exp. 162,

except filings and glass, which are replaced by the compass.

(B) Hold the compass about 2 in. from the top pole of the

electromagnet, close the circuit for a second or two and note

action of needle. Is the top N or S, when the current enters

the coil at O E? Compare result with § 392.

being closed, and note action of needle. Compare result with

directions taken by particles of iron filings in Exp. 163.

(D) Reverse the direction of the current through the coil and

test the nature of the pole at the top.

[Illustration: Fig. 128.]

[Illustration: Fig. 129.]

_=405. Horseshoe Electromagnets.=_ Fig. 128 shows a simple form of

electromagnet with two coils which have a bent piece of iron as a core

for both. The coils have to be wound on by hand in this form. As this

is troublesome, the coils are generally wound on two separate cores

which are joined by a _yoke_ (§ 406), which takes the place of the

curved part in Fig. 128. The separate coils can be quickly made with

a "winder" and joined to suit. (See Apparatus Book, Chapter IX, for

Home-made Electromagnets.) Fig. 129 shows a top view of a home-made

experimental horseshoe electromagnet. The coils are joined by an iron

strap, called the _yoke_, which is screwed to a wooden base. A strip of

iron placed above the magnets to be attracted by them, when the current

passes, is called the _armature_. (See Telegraph Sounders.)

_=406. Use of Yoke.=_ It has been explained (§ 82) why horseshoe

magnets are, in general, better than straight ones. The same is true

of electromagnets; there are two poles to attract, and two to induce.

The lines of force pass through the yoke on their way from one core to

the other, and this reduces the resistance to them. The strength of

the horseshoe magnet would be greatly reduced if the lines of force

were obliged to pass through two air spaces instead of one; in fact, if

there were no yoke we should have simply two straight magnets. The yoke

should be made of soft iron.

[Illustration: Fig. 130.]

[Illustration: Fig. 131.]

=407. Experimental Magnets= are quickly joined to a tin base

(No. 94), which has 3 holes punched in, through which screws

can be put to hold the cores in place. Fig. 127 shows plan of

tin. Fig. 130 shows how removable cores are fastened to the

base, the coils being on the spools, and Fig. 131 shows how

home-made coils on bolts can be used. The coils on bolts should

be wound as directed in Apparatus Book, Chapter X. The tin base

also serves as the yoke.

_Removable Cores._ Fig. 130. These are of soft iron (No. 92,

93). In one end of each is a hole for the screws, S. Part of

the tin has been cut away in the Fig. The copper washer, C W,

should be used. (See § 408.) Connectors are fastened to the

ends of the coils (§ 226-230).

_Bolt Cores._ Fig. 131. After winding on the coils, as directed

in Apparatus Book, remove the nut and put on an extra washer,

E W, so that the ends of the coils will not be pressed against

the tin, but come out between the two washers. Push the

screw-end of the bolt through holes (about 2 in. apart) punched

in the tin, then put on the nut, as shown. Do not force the nut

on too far,--just far enough to hold the cores in place. The

ends of the wires are not shown in Figs. 130, 131. Connectors

are fastened to them (§ 408).

[Illustration: Fig. 132.]

=408. Method of Joining Coils.= To produce the best results

the poles of the horseshoe electromagnet should be unlike. As

the coils are wound alike, their ends must be joined in such

a manner that the current will pass around them in opposite

directions; that is, if the current enters one coil at the

outside end, O E, it must enter the other coil at the inside

end, I E. Fig. 132 shows a plan of the connections, spring

connectors being fastened to the coil-ends, to allow rapid

and easy changes in the arrangement. L, M, and R are pieces

of metal fastened to a strip of wood (No. 95), used to make

connections from cells or other apparatus. They are turned up

at each end as in Fig. 104, 3. Care should be taken not to get

short circuits by allowing two wires to touch the tin base.

By changing the ends of the coils upon L, M, and R (left,

middle, and right), and by changing the direction in which the

current enters the "combination connecting plates" (No. 95),

it is evident that the nature of the poles can be regulated to

suit.

=EXPERIMENTS 164-173. To study horseshoe electromagnets.=

_Apparatus._ Coils of wire with cores and yoke like those

explained in this chapter. Coils fastened to tin base or yoke

with wires leading from them to the combination connecting

plates (No. 95, Fig. 132), are very handy. Cells; iron filings;

compass; iron strip (No. 76).

=EXPERIMENT 164. To test the poles.=

=409. Directions.= (A) Arrange as in Fig. 126, but use the

experimental magnets and combination connections (Fig. 132) in

place of the single coil shown in Fig. 126. Join O of the key

with L, and Zn of the cell with R of Fig. 132. When the key is

pressed the current will enter the magnets from L and leave at

(B) With the compass test the polarity of the cores as in Exp.

163, B, C. Make a sketch of the arrangement, and note which

pole is N and which S.

the way it is wound, and compare the results of (B) with Exp.

154, Fig. 123.

=EXPERIMENT 165. To test the poles.=

=410. Directions.= (A) Arrange as in Exp. 164, but reverse the

direction of the current through the coils. Do this by joining

O of the key (Fig. 126) with R of Fig. 132, and Zn of the cell

with L.

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

[Illustration: Fig. 133.]

=EXPERIMENT 166. To test the poles.=

=411. Directions.= (A) Arrange all connections as in Exp. 164,

then reverse the positions of O E and I E of coil A; that is,

join O E to M, and I E to L, Fig. 132. This will make unlike

ends come together at M; in other words, when the current

enters at L and leaves at R it will pass around both coils in

the same direction.

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

note results.

_Note._--Fig. 133 shows simply the two cores of a horseshoe

electromagnet with arrows to indicate in which direction the

current is passing in each coil to produce N and S poles.

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

the other.=

=412. Directions.= (A) Arrange as for Exp. 164, but join the

wire from Zn of the cell to M (Fig. 132). In this way coil B

will be cut out of the circuit. Place the coils in the E and W

line.

(B) Find about how far the residual magnetism of the core of

B can act upon the compass-needle, holding the compass on the

side away from coil A, no current passing.

magnetism of coil B has been made stronger or weaker. Explain

the action of core A on core B.

=EXPERIMENT 168. Magnetic figures.=

=413. Directions.= (A) Arrange as in Exp. 164. With books,

etc., fix a piece of smooth, stiff paper, or a sheet of glass,

just above the poles of the electromagnets.

(B) Sprinkle iron filings upon the glass, and gently tap it

while the circuit is closed at the key for a few seconds. Make

a sketch of the magnetic figure produced. Do the lines of force

from the opposite poles attract or repel each other? See § 417

for making permanent figures. (See "Things a Boy Should Know

About Electricity" for drawings of magnetic figures.)

_Note._--If possible, use two or three good cells in series for

making magnetic figures, as a fairly strong field is best.

=EXPERIMENT 169. Magnetic figures.=

=414. Directions.= (A) Arrange apparatus as for Exp. 165, and

make the magnetic figure for this combination, as directed in

Exp. 168. Sketch and study the results.

=EXPERIMENT 170. Magnetic figures.=

=415. Directions.= (A) Arrange the apparatus and connections as

in Exp. 166, and make the magnetic figure of this combination

as directed in Exp. 168. In this case the poles are alike.

Sketch and study the results.

=EXPERIMENT 171. Magnetic figures.=

=416. Directions.= (A) Arrange apparatus and connections as in

Exp. 167, and make the magnetic figure of the combination as

directed in Exp. 168. Compare the figure produced with that of

Exp. 168. In this case the current passes through but one coil.

=417. Permanent Magnetic Figures= can be made in several ways

for future study and comparison.

(A) _Paraffine paper figures._ Make paraffine paper as directed

in Apparatus Book, page 135. For this purpose smooth, stiff,

_white_ paper is best, so that the filings will show plainly,

and but a thin coating of paraffine should be given. Place the

magnets upon the table, lay over them a piece of unparaffined

paper, and fix the paraffine paper directly over this. This

is necessary, as the coated paper sticks when heated. For

electromagnets it will be necessary to support the edges of

the paper with books, etc. Sprinkle on the filings and tap

the paper to make them arrange themselves while the circuit

is closed. After the lines of force show plainly, the current

need not be used again, provided the paper be kept perfectly

still. Pass the flame of a Bunsen burner over the paper to melt

the coating. This will, no doubt, make the two pieces of paper

stick together, and permanently fix the particles of filings in

place. Do not heat the paper too much--just enough to melt the

paraffine. If you have no gas, hold a fire-shovel, containing

hot coals, over the paper. As soon as the paraffine cools, the

figures will stand considerable handling.

_Blue print figures_ are very pretty, and last indefinitely.

Get some blue-print paper at a photographer's, who will give

you directions about "developing" it with water. Keep this in

the dark, and take out but one sheet at a time for experiments.

To make the figures, take your apparatus near a window where

bright sunlight comes in. Pull down the curtain so that you

have but a dim light when you make the magnetic figure, as

directed before. After the lines of force show plainly, raise

the curtain, and let the bright sunlight shine on it for 5 or 6

minutes, or until the surface of the paper has a rich, bronze

color. The paper cannot be acted upon by the light under the

particles of filings. Quickly shake the filings from the paper,

and wash it in 3 changes of water to "develop" it, then pin the

paper up to dry.

=EXPERIMENT 172. Lifting power.=

=418. Directions.= (A) Arrange the apparatus as in Exp. 164.

Hold an iron strip (No. 76), a screw-driver, or other iron

bar directly over and near the poles of the experimental

electromagnet. Close the circuit at the key, then lift the

magnets by the "armature," as the iron strip may be called, the

circuit being kept closed for a few seconds. If your cell is

good there should be no trouble in lifting the magnets by the

armature. Open the circuit, and see whether the magnets drop.

(B) Hold the magnets upside down directly over nails, tin

boxes, iron filings, or other pieces of iron. Close the

circuit, move the attracted iron to another place on the table,

and open the circuit. Can this principle be used for practical

purposes?

_Note._--Some experiments illustrating practical uses of

electromagnets will be given in a future chapter.

=EXPERIMENT 173. Residual magnetism when magnetic circuit is

closed.=

=419. Directions.= (A) Arrange as in Exp. 164. You have already

seen that each core retains some magnetism after the circuit

is closed. Place the iron strip firmly across the poles, close

the circuit for an instant, open the circuit, then see whether

the armature still clings to the cores with some strength. The

armature should fit well upon the cores for this experiment.

(B) Again press the armature upon the cores, no current being

used; then lift it as in (A). Compare the attraction with that

found in (A).

_=420. Closed Magnetic Circuits.=_ It was seen in the study of the

permanent horseshoe magnet, that the armature clung strongly to the

magnet. The armature closed the magnetic circuit, the lines of force

having almost no resistance. In the case of electromagnets the magnetic

circuit becomes closed when the armature touches both poles at the same

time. The armature clings strongly to the poles even after the current

ceases to flow. As soon as the magnetic circuit is broken, however, but

little residual magnetism remains. The armatures of electromagnets are

usually arranged so that they can not quite touch the cores, to avoid

this sticking.

CHAPTER XXIV.

THERMOELECTRICITY.

[Illustration: Fig. 134.]

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

heat.=

_Apparatus._ The home-made thermopile described in §421;

astatic galvanoscope; connecting wires; candle or alcohol lamp.

=421. Home-made Thermopile.= (Fig. 134.) For this you need 3

hairpins, copper wire, a piece of wood about 3 in. long and 1

in. square on the ends, 2 pieces of tin, and some small nails.

Straighten the hairpins and scrape the coating off with

sandpaper or a file. Scrape the insulation from 4 pieces of

copper wire, each about 8 in. long. Twist the ends of the

copper wire about the ends of the hairpins (Fig. 134), and then

fasten the hairpins to the block. They may be held firmly by

small nails which should be driven partly into the block and

bent over. The hairpins at the right-hand side of the Fig. are

shown to be near but not touching each other. This allows all

to be heated at the same time.

The tin binding-posts may be nailed or screwed to the block,

and if the bare copper wires 1 and 4 be placed under X and

Y before they are screwed down they will be electrically

connected. The ends of 1 and 4 may be held under the

screw-heads. The block may be supported upon other blocks

to raise it to the proper height, which will depend upon the

length of the candle.

[Illustration: Fig. 135.]

A thermopile in the form of a circle with several pairs of

metals, can easily be made by fastening the hairpins to a piece

of cardboard (Fig. 135) with a hole at the center. This may be

supported by blocks, the heat being applied under the center.

=422. Directions.= (A) Arrange the apparatus as in Fig. 134.

See that the astatic needle is properly adjusted, no magnets

being near it.

(B) Heat the joints as shown, and watch the needle. Can a

current be produced by heat?

one pair out of the circuit. Heat the joints again and compare

the strength of the current with that produced in (B).

(D) See whether much current is produced by one pair. From

results obtained do you see any relation between the strength

of the current and the number of pairs?

_=423. Thermoelectricity=_ is produced by heating the junction between

two metals. Different pairs of metals produce different results.

Antimony and bismuth are often used. If the end of a strip of bismuth

be soldered to the end of a similar strip of antimony, and the free

ends be connected to a galvanometer of low resistance, the presence

of a current will be shown when the point of contact becomes hotter

than the rest of the circuit. The current will flow from the bismuth

to antimony across the joint. By cooling the junction below the

temperature of the rest of the circuit a current will be produced in

the opposite direction.

Thermoelectric currents have a low potential. The energy of the current

is kept up by the heat absorbed.

_=424. Peltier Effect.=_ The action noted in § 423 can be reversed;

that is, if a current from a battery be sent through the metals, the

parts at the junction become slightly warmer or cooler than before,

depending upon the direction of the current. This is known as the

_Peltier Effect_, the heat not being due to the resistance to the

current.

_=425. Thermopiles.=_ As the E. M. F. of the current produced by a

single pair of metals is small, several pairs are usually joined in

series in such a way that the different currents help each other and

flow in the same direction. Such combinations, usually made of antimony

and bismuth, are called thermoelectric piles, or simply thermopiles.

They are useful in detecting very small differences in temperature. The

heat of a match, or the cold of a piece of ice, will produce a current

even at some distance, the thermopile being connected with a sensitive

short-coil astatic galvanometer. (See "Things a Boy Should Know About

Electricity.")

CHAPTER XXV.

INDUCED CURRENTS.

_=426. Electromagnetic Induction.=_ You have seen, by experiments,

that a magnet has the power to induce another piece of iron or steel

to become a magnet. You have also seen, in the study of static

electricity, that an electrified body has the power to act through

space upon another conductor. A body may be polarized and charged with

static electricity by induction.

Several questions now come up. Can a _current_ of electricity in a

conductor induce a _current_ in another conductor not in any way

connected with the first? Can current electricity produce effects

through space? Is there an electromagnetic induction?

It has been seen that a current-carrying wire has a magnetic field,

and that magnetic fields can act through space. It is evident, then,

that a conductor will be surrounded and cut by lines of force when it

is placed in a magnetic field, or near a wire or coil through which a

current passes. Let us study this by experiments.

=EXPERIMENTS 175-182. To study induced currents.=

_Apparatus._ The two coils of wire (Nos. 89, 90); two short,

soft iron cores (Nos. 92, 93); long iron core (No. 96); bar

magnet (No. 97); astatic galvanoscope (No. 59); dry cell (No.

51); key (No. 55); horseshoe magnet; connecting wires with

spring connectors (No. 54) on the ends (§ 226-230); coil of

wire (No. 98) wound on an iron core; compass.

[Illustration: Fig. 136.]

=EXPERIMENT 175. To find whether a current can be generated

with a bar magnet and a hollowed coil of wire.=

=427. Directions.= (A) Arrange as in Fig. 136. The coil (No.

90) of fine wire is joined to A G (No. 59) as shown. Small

pieces of tin or copper, 1 and 2, are used to make connections

between the coil ends and wires, 3 and 4, which are attached

to the galvanoscope. It is best to use the wires, 3 and 4, so

that the coil will be 2 feet at least, from A G; otherwise the

needle of A G might be affected by the magnet, M (No. 97).

(B) Get clearly in mind in which direction the right-hand end

of the needle is deflected when a current enters A G at L, the

left-hand binding-post. If you have forgotten the results of

previous experiments, use the cell for an instant, touching the

wire from the carbon to L and that from the zinc to R. If any

currents come from the coil, later, you should be able to tell

in which direction they flow, the coil and A G forming a closed

circuit.

the coil until it has the place of a core, at the same time

watching the needle. If a current is produced, in which

direction does it flow from the coil? Does the needle remain

deflected? Is the current constant or temporary?

(D) After the magnet, M, has been placed in the coil, as in

(C), and the needle has come to rest, quickly pull M from the

coil, watching the needle. If a current is produced, does it

pass from the coil in the same direction as before, in (C)?

(E) Turn M end for end, repeat (C) and (D), and study the

results. Are lines of force made to cut the turns of the coil?

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

_=428. Discussion.=_ An induced current, produced as in the above

experiment, is a momentary one. No current passes when the magnet and

coil are still; at least one of them has to be in motion. When the

magnet is inserted, the induced current is said to be an _inverse_ one,

as it passes in a direction opposite to that which would be necessary

to give the magnet its poles, it being considered a core magnetized

by the current. A _direct_ current is produced when the magnet is

withdrawn from the coil. Rapid movements produce stronger currents than

slow ones. (See § 439.)

_=429. Induced Currents and Work.=_ It takes force to move a magnet

through the center of a coil, and it is this work that is the source of

the induced current. When the coil is pushed on to the magnet, or when

it is moved through a magnetic field, force is also required. We have,

in this simple experiment, the key to the action of the dynamo and

other important electrical machines. These will be discussed later.

=EXPERIMENT 176. To find whether a current can be generated

with a bar magnet and a coil of wire having an iron core.=

=430. Directions.= (A) Arrange as in Exp. 175, Fig. 136, and,

in addition, place an iron core (No. 92) inside of the coil

(No. 90).

(B) Hold the bar magnet (No. 97) as in Fig. 136, and quickly

lower it until it touches the core, at the same time watching

the needle. Study results, direction of current, etc., as

before.

in the same direction as that from (B)?

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

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

How does the strength of the current compare with that of Exp.

175? Are lines of force made to cut the turns of the coil?

[Illustration: Fig. 137.]

=EXPERIMENT 177. To find whether a current can be generated

with a horseshoe magnet and a coil of wire having an iron core.=

=431. Directions.= (A) Arrange the apparatus as in Exp. 176,

but use the horseshoe magnet, H M, instead of the bar magnet.

Fig. 137 shows the coil (No. 90) with one pole of H M held over

the core.

(B) Study the effect of quickly lowering and raising first one

pole and then the other over the core, as with the bar magnet.

Get clearly in mind the direction in which the induced current

flows in each case.

_=432. Induced Currents and Lines of Force.=_ In the experiments just

given, it should be remembered that the permanent magnets are sending

out thousands of lines of force from their N poles, and receiving them

again at their S poles. As the magnet is pushed into the coil (Exp.

175), the lines of force not only cut through the turns of the coil,

but the number of lines of force that cut the coil at any instant

varies rapidly as the magnet is moved.

Motion is necessary, with this arrangement, to make a change in the

number of cutting lines of force. The current passes only while the

magnet moves; and the direction of the current at any moment depends

upon whether the number of lines of force is increasing or decreasing

at that moment. (See § 438, 439.)

[Illustration: Fig. 138.]

=EXPERIMENT 178. To find whether a current can be generated

with an electromagnet and a hollow coil of wire.=

=433. Directions.= (A) The hollow coil (No. 90) should be

joined to the astatic galvanoscope, as shown in Fig. 136.

Instead of the bar magnet in Fig. 136, an electromagnet is to

be used, and this should be joined in series with a cell and

key, as shown in Fig. 138. The current from the cell will pass

only when K is pressed.

(B) Note from the winding which way the current must pass

around the coil when the circuit is closed at K, and determine

whether the lower end of the long iron core, L I C (No. 96)

should be N or S. With the compass test the poles of the core

to be sure you are right.

Fig. 136), the circuit being kept closed long enough to allow

the needle to partially come to rest again. Withdraw L I C

before you open the circuit. Explain action of needle.

(D) Reverse the direction of the current through the

electromagnet, by changing the connections, and repeat (C).

Does any induced current pass through A G when the core is held

still in the coil H, even though a current passes through coil

[Illustration: Fig. 139.]

=EXPERIMENT 179. To find whether a current can be generated

with an electromagnet and a coil of wire having an iron core.=

=434. Directions.= (A) Fig. 139 shows simply the arrangement of

coils. Coil H (No. 90) with core, is joined to the galvanoscope

as in Fig. 136. Coil E, with short core, should be joined to

key and cell as shown in Fig. 138.

(B) Keeping in mind the polarity of the lower end of core E,

quickly lower it to the core of H, the circuit being kept

closed for a few seconds. Does the needle remain deflected

after the motion ceases?

open the circuit. Compare the directions taken by the induced

currents in (B) and (C).

_=435. Discussion of Exps. 178, 179.=_ This motion in straight lines

is not suitable for producing currents strong enough for commercial

purposes. In order to produce currents of considerable strength, the

coils of wire have to be pushed past magnets with great speed. Special

machines (see Dynamos) are constructed in which the coils are wound

so that they can be given a rapid _rotary motion_ as they fly past

strong electromagnets. In this way the coil can keep on passing the

same magnets, in the same direction, as long as force is applied to the

shaft that carries them.

[Illustration: Fig. 140.]

=EXPERIMENT 180. To study the effect of starting or stopping a

current near a coil of wire or other closed circuit.=

=436. Directions.= (A) Arrange as in Fig. 140. Place the two

coils, H and E, on the same core, L I C. Connect E with the

key and cell as before (Fig. 138). Connect H with the astatic

galvanoscope, A G, as in Fig. 136. Keep the coils 2 or 3 feet

from A G, so that the needle will not be affected by them.

(B) Close the circuit at the key, watching the needle, then

as soon as the needle regains its former position, open the

circuit again. Compare the direction of the induced current in

H with that of the current in E, (1) when the main circuit is

closed, and (2) when it is opened. Is any current induced in H

by a steady current in E? (See Transformers.)

[Illustration: Fig. 141.]

=EXPERIMENT 181. To study the effect of starting or stopping a

current in a coil placed inside of another coil.=

=437. Directions.= (A) Arrange as in Fig. 141. Join coil H with

the astatic galvanoscope, A G. Place the small coil P (No. 98)

with core, inside of H, and connect the ends of P with the key

and cell, as shown.

(B) Close the circuit at K; watch the needle, and as soon as it

regains its position, open the circuit again.

Compare the direction of the induced current in H with that of

the inducing current in P, (1) when the inducing circuit is

closed, and (2) when it is broken. (See Induction Coils.)

_=438. Discussion of Exps. 180, 181.=_ When a current suddenly begins

to flow through a coil, the effect upon a neighboring coil is the same

as that produced by suddenly bringing a magnet near it; and when the

current stops, the opposite effect is produced.

We may consider that when the inducing circuit is closed, the lines of

force shoot out through the turns of the outside coil. Upon opening the

circuit the lines of force cease to exist; that is, we may imagine them

drawn in again.

[Illustration: Fig. 142.]

_=439. Direction of Induced Current.=_ Fig. 142 shows the magnet on its

way into the coil; the number of lines of force is increasing in the

coil, and the induced current passes in an anti-clockwise direction

when looking down into the coil along the lines of force. This produces

an _indirect_ current. If a current from a cell were passed through the

coil in the direction of this indirect current, the lower end of a bar

of iron would become a S pole. (See § 428.)

_=440. Laws of Induction.=_ (1) An increase in the number of lines of

force that pass through a closed circuit produces an indirect induced

current; while a decrease produces a direct one. (See § 428.)

(2) The E. M. F. of the induced current is equal to the rate of

increase or decrease in the number of lines of force that pass through

the circuit.

(3) A constant current produces no induced current, provided there is

no motion.

(4) Closing a circuit produces an indirect current.

(5) Opening a circuit produces a direct current.

(6) _Lenz's Law._ Induced currents have a direction that tends to stop

the motion that produces them.

_=441. Primary and Secondary Currents.=_ In the preceding experiments

in induction, it must be kept in mind that the current from the cell

did not pass through the galvanoscope. There were two entirely separate

circuits, in no way connected. The _primary_ current comes from the

cell, while the _secondary_ current is an induced one.

[Illustration: Fig. 143.]

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

=442. Directions.= (A) Arrange as in Fig. 143. Connect coil H

with A G, as before. Place one pole of H M against the end of

the core I C, hold H with one hand, and with the other quickly

push the other pole of H M onto the core. This should produce a

momentary current through A G, first in one direction, and then

in the other. Let the needle come to rest.

(B) Move H M back and forth upon the end of I C, changing

its polarity rapidly. A minute's practice will enable you to

slide the core from one pole of H M to the other and back

again rapidly--3 complete vibrations per second being about

right. The needle should be parallel to the coil of A G, and

if properly done, the needle will be made to vibrate back and

forth slightly at each change in the polarity of I C.

_=443. Direct and Alternating Currents.=_ A current that flows steadily

in one direction is said to be a _direct_ current. A cell gives a

direct current when the circuit is closed. When the current passes in

one direction for an instant, and then reverses immediately and flows

in the opposite direction, it is said to _alternate_. The induced

current which flowed through the galvanoscope in Exp. 182 was an

alternating one. Currents of this class have great practical uses.

_=444. Self-Induction; Extra Currents.=_ It has been shown that

a magnetized coil can act through space and induce a current in

a neighboring coil. The lines of force which reach out from an

electromagnet will generate a current in any conductor which happens

to be in the field, or which is moved across the lines. It is evident,

then, since the lines of force from each turn of a coil cut all the

other turns of the same coil, that each turn acts as a conductor placed

in the field of every other turn. The instant a current begins to flow

through a coil, there is an inverse current of self-induction started

in the coil, which opposes the current in the cell. When the circuit is

broken, this _extra current_, as it is also called, is a direct one and

adds its strength to that of the current from the cell; as this takes

place at the instant the circuit is broken, a bright spark is seen at

the key, and this shows that the E. M. F. of this extra current is

high. Practical uses are made of it.

CHAPTER XXVI.

THE PRODUCTION OF MOTION BY CURRENTS.

_=445. Currents and Motion.=_ We have seen, in the experiments on

induced currents, that a current of electricity can be generated by

properly moving magnets near coils of wire. (See Dynamo-electric

Machines.) Can we reverse this process? Can motion be produced by the

electric current?

=EXPERIMENTS 183-190. To study the production of motion by

means of the electric current.=

_Apparatus._ The support, including base, rod, and support

wire, S W (Fig. 144.) Coils of wire (No. 89, 90); iron cores

for coils; cell; key; connecting wires; compass; current

reverser; bar magnet; horseshoe magnet.

[Illustration: Fig. 144.]

=EXPERIMENT 183. Motion produced with a hollow coil and a piece

of iron.=

=446. Directions.= (A) Arrange as in Fig. 144. Coil H (No. 90)

is to be used as a pendulum, and can be supported by fastening

a string to it, the upper end of which should be tied to S W.

Connect the ends of H with K and D C. There will be a slight

magnetic field about H as soon as the circuit is closed.

(B) Hold I C near the end of the coil. Close the circuit for an

instant. Is there any motion produced in H? While the motion

will be slight, there should be enough to be noticed if the

cell is strong.

a minute, until you get in mind the rapidity of its vibrations.

Stop it, then repeat (B), closing and opening the circuit at

regular intervals, so that the little impulses given by the

attraction for I C will gradually cause H to vibrate. The

wires leading from H should not drag upon the table.

=EXPERIMENT 184. Motion with hollow coil and bar magnet.=

=447. Directions.= (A) Substitute the bar magnet M (No. 97)

for the iron of Exp. 183 (Fig. 144). Get clearly in mind the

polarity of the coil from the way the current flows through it,

then test it with the compass to find whether you are right.

(B) Hold the N pole of M near the left-hand end of the coil,

close the circuit for an instant and study results.

those of Exp. 183. Try to make the coil vibrate.

=EXPERIMENT 185. Motion with electromagnet and piece of iron.=

=448. Directions.= (A) Arrange as described in Exp. 183, Fig.

144. Place a short core inside of the coil and repeat. (See §

446 for directions.) Why is the motion produced much larger

than that given by a hollow coil?

(B) The coil can gradually be made to swing through quite a

little space by closing and opening the circuit regularly (§

446, C). Could any use be made of such a motion, if it were on

a large scale? Could it be made to run a machine?

[Illustration: Fig. 145.]

=EXPERIMENT 186. Motion with electromagnet and bar magnet.=

=449. Directions.= (A) Arrange as in Fig. 145, the coil being

suspended and connected as in Exp. 183 (Fig. 144).

(B) Study the effect of closing the circuit when the N pole of

M is held near the core of H. Reverse M, and repeat.

[Illustration: Fig. 146.]

=EXPERIMENT 187. Motion with electromagnet and horseshoe

magnet.=

=450. Directions.= (A) Arrange as in Fig. 146. The ends of H

(No. 89) are joined to X and Y of the current reverser C R (No.

57). It is evident, then, that the direction of the current

through H can be easily and rapidly reversed by C R. (See Exp.

103.) Either pole of the horseshoe magnet H M will attract I C

when it is not magnetized.

(B) Place the end of I C near the N pole of H M so that it will

be attracted to it. You have learned that like poles repel each

other, so press the lever of C R that will produce a N pole at

the left-hand end of I C. The core I C should be repelled by

the N pole of H M and be instantly attracted by its S pole.

and forth from one pole to the other. The results of this

experiment should be remembered, as they will aid in

understanding motors. A core 1/4 in. in diameter can be placed

in between the poles and be made to vibrate rapidly as the

current is reversed.

[Illustration: Fig. 147.]

=EXPERIMENT 188. Motion with two electromagnets.=

=451. Directions.= (A) Arrange as in Fig. 147. Join the two

coils, H and E, in parallel. Connect their two outside ends

O E to a metal plate A, and their inside ends I E to B. Join

wires 1 and 6 to K, D C, A and B, as shown. When the circuit

is closed at K, the current will pass along wire 1 and divide

at A, entering E and H at the same time by wires 2 and 4 and

returning through 3 and 5 to B, and thence to D C.

(B) Close the circuit for an instant with wires arranged as in

Fig. 147. Do the electromagnets attract or repel each other?

Study out the direction in which the current passes around the

coils, and see whether they _should_ attract or repel.

only, will be changed when this circuit is closed. Press the

key for an instant and study the results.

The Study of Elementary Electricity and Magnetism by Experiment Containing Two Hundred Experiments Performed with Simple, Home-made Apparatus · 第 18 / 19 章

_=321. Discussion.=_ Two wires placed side by side as in (B), Exp. 123,