=EXPERIMENT 80. To study the action of t_The Study of Elementary Electricity and Magnetism by Experiment Containing Two Hundred Experiments Performed with Simple, Home-made Apparatus

_Apparatus._ The electrophorus (Exp. 68); small bits of moist

tissue-paper, T P.

=170. Directions.= (A) Thoroughly electrify E S, Fig. 34, and

place E C upon it by its handle, E R.

(B) Touch E C, as directed in Exp. 68, and listen for a small

spark which should pass from E C to your finger.

it upon E S. Do not touch E C, but bring your finger near T P.

What does T P do? Now, touch E C and see, when you bring your

finger near it, if T P acts as it did before.

(D) Again, place several pieces of T P upon E C (E S being

thoroughly charged); touch E C, then lift it by its handle.

Note action of T P, which should be slightly moist.

_=171. Discussion.=_ The electrification upon the ebonite is negative

(Exp. 60). Although E S and E C (Fig. 34) seem quite smooth, there are

many little hills, valleys, and air-spaces between them, which keep

them from touching each other perfectly. The ebonite has the electric

field at the start, and it really acts across these minute air-spaces

_by induction_ (Exp. 75), and polarizes E C. The air-spaces form the

dielectric (Exp. 78). The - electrification of E C being repelled by

the - of E S, it is driven to the top of E C, while the + is drawn to

the bottom. This + is kept from rushing to the - of E S by the air

dielectric, and because E S is a non-conductor. By touching E C the

free - escapes to the earth, leaving E C _positively_ charged when it

is lifted.

[Illustration: Fig. 47.]

[Illustration: Fig. 48.]

[Illustration: Fig. 49.]

[Illustration: Fig. 50.]

[Illustration: Fig. 51.]

=172. Details of Action.= The different steps in the action of the

electrophorus are shown graphically in Figs. 47 to 51. Fig. 47 shows

E S negatively charged. E C is neutral at first, Fig. 48; that is, it

is supposed to contain both + and -, as shown by the "double sign" (§

160). Fig. 49 shows that E C has been polarized by the inductive action

of E S. The repelled - escapes to the finger (this escaping is what

gave the small spark to the finger and charged the T P in the last

experiment), leaving the top uncharged, while the + is _bound_ (Fig.

50). As soon as E C is lifted (Fig. 51) the + spreads all over E C,

which is then charged. The +, upon going to the top, charged the pieces

of T P (Exp. 80, D), causing them to be repelled. The charge of - upon

E S has not been removed, so the operation may be repeated many times

before E S must be again electrified.

The - electrification on the ebonite acts inductively through E S,

drawing up + electrification from the earth. To make this action easier

a "sole," or metal conductor, is often placed under the ebonite.

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

influence and polarization.=

_Apparatus._ Ebonite sheet, E S (No. 26); insulating table, I

T; flannel cloth, F C.

=173. Directions.= (A) Thoroughly charge E S with F C. With the

right hand bring E S near and parallel to the top surface of I

T, but do not let them touch each other.

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

their flat surfaces being parallel, touch I T. Watch for any

sparks, and note any peculiar actions of E S.

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

touch I T with your knuckle.

_=174. Discussion.=_ This apparatus is really the electrophorus upside

down. It shows very clearly (1) the escape of the - electrification

from I T, by the spark; (2) that the attraction between I T and E S is

much greater than before, when this - is removed; and (3) it shows the

different steps of the inducing and charging process, as described in

Exp. 75, and as shown in Figs. 43 and 44.

CHAPTER X.

CONDENSATION OF ELECTRIFICATION.

=EXPERIMENT 82. To find whether a large surface will hold more

electrification than a small one.=

_Apparatus._ The insulating table (for details, see Exp. 64); a

large tin basin or pan (not furnished); the electrophorus (Exp.

68).

=175. Directions.= (A) Test the electrophorus and be sure that

it is working properly.

(B) As in Exp. 70, see how many good sparks I T will take

from E C (which should be recharged at each trial) before the

potential of I T is raised so that it equals the potential of E

number of good sparks you can pass to it from E C (recharged at

each trial). Compare the number of sparks necessary to raise

the potential of the large surface until it equals that of E C,

with the number found in part (B).

_=176. Electrical Capacity.=_ It takes more heat to raise the

temperature of a gallon of ice-water to the boiling point, than it

takes for a quart of ice-water. You have just seen that a large

insulated surface will take more sparks from a charged body than a

small one, before its potential is raised to that of the small one, and

to that of the charging body. We say that a large surface has a greater

_capacity_ than a small one, the shape and other conditions being the

same.

=EXPERIMENT 83. To find whether the capacity of a given

conductor can be increased without increasing its size.=

_Apparatus._ Fig. 52. Insulating table. I T (Exp. 64); the

extra ebonite sheet, E S (No. 27); the complete flat box, F B

(No. 40, 41); the charged electrophorus cover, E C (Exp. 68).

Arrange, as shown, I T being insulated from the earth by E S. F

B should rest upon a wooden table or other large conductor.

=177. Directions.= (A) See how many good sparks I T will take

from E C. Re-charge E C at each count, and note the relative

sizes of the sparks.

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

[Illustration: Fig. 52.]

_=178. Condensation; Condensers.=_ As I T easily held more sparks

than it would take before (Exp. 70), we say that its _capacity_ has

been increased. Its potential didn't increase, because that could not

get greater than the potential of E C, the charging body. To describe

this state of affairs, we say that the electrification was denser than

before, and that it was _condensed_. The _capacity of I T was greatly

increased by the presence of another conductor, F B, insulated from I

T, but "grounded_." Such a combination, 2 conductors, with a dielectric

between them, is called a condenser.

A condenser can hold much more electrification at a certain potential

than an equal amount of surface can hold when not properly arranged.

We might call a condenser a storage battery for static electricity.

The capacity of a condenser depends, among other things, upon the area

of the conducting surfaces, and upon the thickness and nature of the

dielectric. Among the various forms of condensers may be mentioned the

Leyden jar, and the fulminating pane.

=179. The Leyden Jar= consists of a wide-mouthed glass jar,

with tin-foil pasted upon the inside and outside to within

2 or 3 inches of the top. The inner coat or conductor is

connected to a knob or ball at the top by means of a chain. To

charge the jar, the outer coat is connected with the earth by

holding it in the hand, or by resting it upon a table while

the electrification is passed to the knob. A _Leyden Battery_

consists of 2 or more connected jars, the object being to

increase the area of the surface. The jar is discharged by

touching one end of a _discharger_ (§ 188) to the outer coat,

and swinging its other end over to the knob, when a bright

spark will pass between the knob and discharger. (See Exp. 86.)

=180. Fulminating Panes=, or Franklin's Plates, are practically

the same as a Leyden Jar. The tin-foil, however, is pasted upon

the opposite sides of a pane of glass, a margin of about an

inch being left all around. One side of the pane is charged,

and takes the place of the inside coat of the jar. The other

side is grounded. The pane is discharged by connecting the two

sheets of foil.

=181. Induction Coil Condensers= consist of sheets of tin-foil

separated by sheets of paraffined paper, which act as the

dielectric. (See Induction Coils.)

=182. Submarine Cables=, with the surrounding water, act like

condensers, the result being that the condensing effect slows

up the electric current and retards the signals. These make

a condenser of enormous capacity. The wires inside form one

conductor, and the water the other, while the insulation around

the wires forms the dielectric.

=EXPERIMENT 84. To study the condensation of electrification.=

_Apparatus._ Same as in last experiment, but arrange so that

F B and I T shall be near each other at one side; that is, so

that the edge of E S shall be even with the edges of the two

tins.

=183. Directions.= (A) Pass good sparks to I T from the charged

E C until something happens. Watch the side where I T and F B

are near each other.

_=184. Discussion.=_ We may say that the electrification was condensed,

in this experiment, until the charge became so great that the

_condenser_ suddenly discharged itself. Condensers may be made in many

ways, but they all consist of 2 conductors, with a dielectric between

them. One conductor is insulated, and receives the charge; the other

conductor is grounded.

[Illustration: Fig. 53.]

=EXPERIMENT 85. To study the action of the condenser.=

_Apparatus._ Fig. 53. The insulating table, I T; ebonite

sheet, E S (No. 27); flat box, F B, complete (Nos. 40, 41);

the electrophorus (Exp. 68). Note that this is really the same

apparatus as that just used; both conductors of this condenser,

however, are insulated and reversed in position.

=185. Directions.= (A) See that your electrophorus works

properly, then find out how many good sparks you can pass from

E C to F B, recharging E C each time. Note the relative sizes

of the sparks, and compare the result with the number taken by

the condenser in the last experiment.

(B) When F B seems to be fully charged, touch I T with your

knuckle. (From your study of induction what should be the

result?)

E C. Pass sparks to F B until it seems fully charged.

(D) Again touch I T, then repeat (A) and (B) several times,

until a bright spark passes from F B over the edge of E S to I

_=186. Discussion.=_ The action of the condenser, as clearly shown,

depends upon induction. You should now be able to explain and show by

diagram the different steps.

E C was positively charged (Exp. 80). This also charged F

B positively by contact. F B acted inductively through the

dielectric, E S, drawing up _some_ of the - in I T, and

repelling _some_ of the +. As I T was insulated, this free +

electrification could not escape. Before we touched I T, its

+ and - electrifications, although partially separated, were

struggling against this inductive action; and, on account of

their strong attraction for each other, our efforts to charge

the condenser were retarded. Upon touching I T the free +

escaped to the earth. (This was the cause of the spark.) This

left _some_ - electrification bound on the underside of E S,

and some + bound on the upperside of E S. The capacity of F

B was increased by this process, as the + already put into

it was very much occupied by the attractions of the induced -

just under E S. As more + was given to F B, more - was drawn up

under E S and more + was pushed out of I T. This action went on

until the two conductors were strongly and oppositely charged.

This action goes on continuously when the lower conductor is

grounded. The spark between the tins was due to the rushing

together of the + and - electrifications; it showed that there

was a _momentary current of electricity_.

=EXPERIMENT 86. To study the effect of electrical discharges

upon the human body.=

_Apparatus._ The condenser (Fig. 52), with E S centrally placed

so that the apparatus cannot discharge itself; the hairpin

discharger, H P D (No. 48); the electrophorus.

=187. Directions.= (A) Charge the condenser (Exp. 83) with 10

good sparks from E C, then touch I T (Fig. 52).

(B) Recharge the condenser with 10 sparks, then touch F B.

Discharge it by again touching I T as in (A).

and quickly swing the first finger of the same hand over to I

T, and get a slight shock.

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

(E) Instead of using your hand to discharge the condenser, try

the bent hairpin. Keeping one end against F B, swing the other

end over near I T.

_=188. Shocks; Dischargers.=_ The two conductors being oppositely

charged in the condenser (Exp. 85), it is only necessary to place

some conductor between them to allow the charges to rush together.

Any conductor so used is called a _discharger_. The hand carried the

whole current which caused the _shock_. When I T was touched first,

the current was obliged to pass through your body, through the floor,

and up the table-legs into F B. Always touch the "grounded" conductor

first with the discharger, so that you will get a good spark and _not_

a shock.

=EXPERIMENTS 87-88. To show the strong attraction between the

opposite electrifications in the condenser.=

_Apparatus._ Flat box, F B (Nos. 40, 41); sheet of glass, G

(No. 38); electrophorus (Exp. 68). The two parts of F B are

used for the conductors of the condenser (Fig. 54) for the sake

of lightness. The bottoms should be next to the glass, which is

used for the dielectric on account of its stiffness. The lower

tin should rest upon the table. The glass should be perfectly

clean and dry (hot).

=189. Directions.= (A) Charge the condenser with 15 or 20 good

sparks from E C.

(B) Lift the condenser by one corner of G (Fig. 54), being

careful not to discharge it. Explain why the lower conductor

follows the glass.

[Illustration: Fig. 54.]

=EXPERIMENT 88.=

=190. Directions.= (A) Charge and lift the condenser as just

explained (Exp. 87). Fig. 54.

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

lower tin alone.

the lower one.

_=191. Discussion.=_ This clearly shows how strongly the two

electrifications are _bound_ in the condenser. Each refuses to

escape to the earth, but they instantly rush together at the first

opportunity. The dielectric may be shattered in a very heavily-charged

condenser by this strong attraction.

[Illustration: Fig. 55.]

=EXPERIMENT 89. To show how the condenser maybe slowly

discharged.=

_Apparatus._ Fig. 55. The condenser (Exp. 83); the carbon

electroscope with support (Exp. 58); the electrophorus (Exp.

68).

=192. Directions.= (A) Charge the condenser by means of the

electrophorus; then hang the carbon so that it can swing

between the upper conductor and E C placed as shown.

_=193. The Electric Chime.=_ The charging and discharging of the carbon

being rapid, it acts like a _chime_ as it taps against the tins.

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

condenser.=

_Apparatus._ The condenser, consisting of flat box, F B (Nos.

40, 41); ebonite sheet, E S (No. 27); insulating table, I

T (Exp. 64); (when charging, arrange as in Fig. 52.); the

electrophorus; hairpin discharger, H P D (No. 48).

=194. Directions.= (A) Charge the condenser with 15 or 20 good

sparks from E C.

(B) Lift I T away from E S by its insulating handle, and set it

upon the table. (It may be necessary to hold E S down.)

do not scrape E S along on F B; do not allow E S to touch your

clothing.)

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

condenser complete again.

(E) With H P D see if the condenser still holds a charge. Touch

F B first (Exp. 86).

_=195. Discussion.=_ As the _conductors_ were completely discharged,

being left for a few moments upon the table, it is evident that the

opposite electrifications must reside in and upon the _dielectric_.

The conductors allow an even and _rapid_ discharge from all parts

of the dielectric at the same time. The dielectric is considerably

strained when a condenser is heavily charged. This strain, caused by

the attraction of the opposite electrifications, may be great enough to

break or puncture the dielectric.

=EXPERIMENT 91. To find whether any electrification remains in

the condenser after it has once been discharged.=

_Apparatus._ The condenser (Fig. 52); the electrophorus (Exp.

68); hairpin discharger, H P D.

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

(B) Discharge it with H P D, being sure to touch F B first, and

to touch I T for an instant while H P D is against F B.

slight spark.

_=197. Residual Charge.=_ The two electrifications on the opposite

sides of the dielectric have such an attraction for each other,

when the condenser is charged, that they seem to penetrate, or soak

into, the dielectric. These do not completely soak out again at the

discharge. The small amount left is called a _residual charge_.

[Illustration: Fig. 56.]

=EXPERIMENT 92. To study successive condensation; the chime

cascade.=

_Apparatus._ Fig. 56. This really consists of two condensers,

joined by a wire. The upper condenser consists of T F B (No.

41), E S (No. 27), and the insulating table, I T. (See Exp.

64.) The lower condenser consists of the cover of the tin box,

C T B (No. 47), the sheet of glass G (No. 38), and B F B (No.

40). The tin box, T B (No. 47), is placed under this to raise

it, simply. A wire or hairpin, H P, is hung upon the edge of T

F B, its lower end being inside of C T B and not quite touching

it. This acts like a pendulum, which is to swing to C T B at

the proper time. The source of electrification is E C.

=Note.= You have learned that in charging the condenser with

the positively charged E C, + electrification is driven from F

B into the earth. Can we use this to charge a second condenser?

=198. Directions.= (A) Pass 15 or 20 good sparks from E C to

the under side of I T (Fig. 56), noting the action of H P.

(B) Hold E C in the hand, and, with its insulating handle, poke

H P away from the condensers. Do not discharge them.

T B first.

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

upper condenser.

_=199. Discussion.=_ A long row of condensers may be charged in this

way. There is no advantage in it, as the electrification is merely

divided between them. How can two condensers be joined to get the

advantages of a large surface?

CHAPTER XI.

ELECTROSCOPES.

_=200. Electroscopes=_ are instruments to show the presence, relative

amount, or kind of electrification on a body. (See Apparatus Book,

Chap. XVIII, for Home-Made Electroscopes.) The _carbon electroscope_

has been described (Exp. 58). The _pith-ball electroscope_ is made

by using pith from elder, corn-stalk, or milk-weed, in place of the

carbon. The _gold-leaf electroscope_ is a very delicate instrument. The

gold-leaf is supported, as suggested in Fig. 57, at the lower end of a

wire conductor which sticks through and hangs from the cork of a glass

jar or flask. To the top end of the wire is soldered a ball or disk.

The glass jar insulates the gold-leaf, and keeps it dry and free from

dust.

[Illustration: Fig. 57.]

=201. Our Leaf Electroscope= (Fig. 57) is made with

aluminum-leaf. Gold-leaf is too delicate for unskilful

handling, and aluminum will do for all ordinary experiments. To

cut it into any desired shape, place it between two sheets of

paper, then cut through paper and all.

=202. Construction.= Bend one leg of a hairpin, H P, as in Fig.

57, and slide it onto I T. Hang a wire, W, or another hair pin

straightened, then bent, from the horizontal leg of H P. This

is to support the "leaves," L, which are made from a strip of

aluminum-leaf about 4 in. long and 3/4 in. wide. Moisten the

under side of the horizontal part of W with paste or mucilage;

press it upon the middle of the strip laid flat upon the table,

and then lift W. The leaves should cling to W. Each leaf should

be, then, 2 in. long. They should hang close together when not

in use. A large chimney, or fruit-jar, may be used to surround

the leaves, and to keep currents of air from them. The leaves

should not touch the side of the jar when spread.

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

conduction.=

_Apparatus._ The leaf electroscope (Fig. 57, § 201, 202);

ebonite rod, E R (No. 28); flannel cloth, F C (No. 30).

=203. Directions.= (A) Thoroughly charge E R, then scrape it

along upon I T, noting the action of the leaves, L.

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

_=204. Discussion.=_ No explanation should be necessary for this. Are

the leaves charged alike? As they were charged by contact, is the

electrification on them + or -?

=EXPERIMENT 94. To charge the leaf electroscope by induction.=

_Apparatus._ Our electroscope (Fig. 57, § 202); ebonite sheet,

E S (No. 27); flannel cloth, F C (No. 30).

=205. Directions.= (A) Charge E S with F C, then hold E S above

I T (Fig. 57), their surfaces being kept parallel and about 2

or 3 inches apart. Watch the leaves.

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

(D) Remove your finger from I T, then withdraw E S. Do the

leaves now remain spread?

_=206. Discussion.=_ The permanent divergence of L was due to a charge

given by induction. (Exp. 76.) As E S was -, what was the kind of a

charge in L? Did any electrification go to the electroscope from E S?

In (C) what became of the charge in L? Explain why the leaves again

diverged in (D). The electroscope was charged with + electrification by

taking - out of it.

=EXPERIMENT 95. To learn some uses of the electroscope.=

_Apparatus._ Our electroscope (Fig. 57, § 202); ebonite rod,

E R (No. 28); ebonite sheet, E S (No. 27); glass, G (No. 38);

flannel cloth, F C (No. 30).

=207. Directions.= (A) With the charged E R charge the

electroscope negatively by conduction (Exp. 93). Note the

amount of permanent divergence of the leaves.

(B) Electrify the glass, which will be +, (or use the + E C),

and _slowly_ lower it over I T, noting the effect upon L. Raise

and lower G or E C several times. Does G, which has an opposite

charge to the electroscope, make L diverge more or less?

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

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

=Note.= If the + G is brought too near the -ly charged

electroscope, L will first collapse and then instantly diverge

again with a + charge by contact. The _first_ motions should be

observed.

_=208. Discussion.=_ As a neutral body causes a slight _collapse_ of

the leaves, as well as a body charged positively (when the charge in

the leaves is -), an increase of divergence is really the only sure

test to tell how a body is charged. The - leaves collapse when a + body

is brought near I T, because the - in them is drawn up towards the

body. The leaves diverge more when a - body is brought near, because

the - in I T is repelled into them.

_=209. The Proof-plane.=_ Since charges of static electricity reside

upon the outside of conductors, it is an easy matter to take samples of

the electrification. This may be done with a little instrument called

a carrier, or proof-plane. It consists of a small conductor with an

insulating handle. A ring or coin may be used for the conductor, and

a silk thread for the handle. By touching the carrier to any charged

body, it, also, becomes charged; and the nature of the charge may be

determined by the use of a previously charged leaf electroscope (Exp.

95). A delicate gold-leaf electroscope would be ruined by coming in

contact with a heavily charged body. The carrier allows a small sample

to be tested.

CHAPTER XII.

MISCELLANEOUS EXPERIMENTS.

[Illustration: Fig. 58.]

=EXPERIMENT 96. To show that friction always produces two kinds

of electrifications.=

_Apparatus._ Fig. 58. The carbon electroscope (Exp. 58);

flannel cloth, F C, doubled twice to make 4 thicknesses (see

Fig. 58); ebonite sheet, E S (No. 26); ebonite rod, E R (No.

28); charged electrophorus cover, E C.

=210. Directions.= (A) Vigorously rub E S with F C (folded as

in Fig. 58). See if you can discover any attraction between

them.

(B) Rub E S again, but do not lift F C from it with the hand

alone. Slip E R under the top fold in F C (Fig. 58), and lift F

C straight up from E S. Do not let F C touch the table or your

hand.

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

strongly repelled.

(E) Bring the positively charged E C slowly near the carbon,

and note the result.

(F) Slowly bring the negatively charged E S near the carbon

that has been charged by contact with F C.

_=211. Discussion.=_ This experiment showed that while the ebonite was

negatively charged, the flannel was positively charged. One kind of

electrification is never produced without the other. It can also be

shown that the two kinds are equal in amount.

=EXPERIMENT 97. To show "successive sparks."=

_Apparatus._ Fig. 59. The electrophorus (Exp. 68); the extra

ebonite sheet, E S (No. 27); three coins (marked A, B, C, in

Fig. 59). The coins should nearly touch each other, and rest

upon E S. A part, only, of the electrophorus cover is shown.

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

cover.

(B) Place your finger upon the coin marked A, to "ground" it,

then quickly touch the coin C with the charged cover, at the

same time watching for sparks between the coins. If you cannot

see the sparks, darken the room a little, and look at the

center coin, B, while doing the experiment.

[Illustration: Fig. 59.]

[Illustration: Fig. 60.]

=EXPERIMENT 98. To show to the eye the strong attraction

between a charged and a neutral body.=

_Apparatus._ The flat box, F B (Nos. 40, 41); the electrophorus

(see Exp. 68).

=213. Directions.= (A) Stand F B upon its edge upon a level

table, then bring the charged electrophorus cover near it.

(B) Instead of the above, use light hoops made of paper,

eggshells, feathers, sawdust, etc.

=EXPERIMENT 99. To feel the strong attraction between a charged

and a neutral body.=

_Apparatus._ Fig. 60. The flat box F B (Nos. 40, 41); the

electrophorus (Exp. 68).

=214. Directions.= (A) Hold F B in the left hand, as shown,

then _slowly_ bring near it the charged cover, at the same

time looking between them so that you can keep them the same

distance apart all the way round.

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

F B.

=EXPERIMENT 100. The human body a frictional electric machine.=

_Apparatus._ Yourself; a carpet; a room with dry air, easily

had on a cold winter's day.

=215. Directions.= (A) Scrape your feet along upon the carpet,

then quickly touch your finger to some conductor, as, for

example, a friend's nose.

(B) It is possible to light the gas by the above process. Have

a friend turn on the gas just before you bring your finger to

the jet, and be sure that the spark from your finger passes

through the gas on its way to the conductor, the jet.

paper after you have scraped your feet along to charge your

body.

_=216. Static Electric Machines=_ are used to produce large quantities

of static electricity. In the early forms, the electrification was

produced by friction. Modern machines depend upon the principle of

induction. The electrophorus (Exp. 68) is really a very simple form

of induction machine. The potential of these machines is very great,

as the spark may jump many inches. Thousands of Galvanic cells would

be needed to make a spark an inch long. When the spark passes through

the air it meets with an extremely high resistance, as air practically

insulates ordinary electricity. This high resistance in the circuit

reduces the strength of the current. While the potential is very high,

the strength of the current is very low. (See Ohm's Law.)

CHAPTER XIII.

ATMOSPHERIC ELECTRICITY.

_=217. Atmospheric Electricity.=_ The air is generally electrified,

even in clear weather. Its charge is usually +. Clouds are sometimes +,

and sometimes -.

The cause of atmospheric electricity is not thoroughly understood. It

is thought, by some, to be due to the friction of the particles of

vapor upon each other. It is also thought that the evaporation of sea

water, and the friction of winds, produce it.

_=218. Lightning.=_ Benjamin Franklin, in 1752, proved by his famous

kite experiment that atmospheric and frictional electricities were of

the same nature. By means of a kite, the string being wet by the rain,

he succeeded, during a thunder-storm, in drawing sparks, charging

condensers, etc.

Lightning may be produced by the passage of electricity between clouds,

or between the cloud and the earth, which, with the intervening

air, have the effect of a condenser. If one cloud is charged, it

acts inductively upon another, producing in it the opposite kind of

electrification. When the attraction between the two electrifications

becomes great enough, it overcomes the resistance of the air, and

lightning is produced.

The flash is practically instantaneous. The direction taken seems to

depend upon the conditions of the surrounding air. It has generally a

zigzag motion, and is then called _chain lightning_. The air in the

path of the electricity becomes intensely heated; it is this effect,

and not the electricity which we see. In hot weather flashes are

often seen which light up whole clouds, no thunder being heard. This

is called _heat lightning_, and is generally considered to be due to

distant discharges, the light of which is reflected by the clouds.

The _potential_ of the lightning spark is beyond all calculation. The

spark jumps through miles of air, which is, really, an insulator. This

spark represents billions and billions of volts.

_=219. Thunder=_ is caused by the violent disturbances produced in the

atmosphere by lightning. The nature of the sound depends, among other

things, upon the distance of the observer from the discharge, and

upon the length and shape of the path taken. Clouds, hills, and other

objects produce echoes, which also modify the original sound. It takes

nearly five seconds for the sound to travel one mile.

_=220. Lightning-Rods=_, when well constructed, often prevent violent

discharges of atmospheric electricity. They have pointed prongs at the

top, which allow the negative of the earth (which is being attracted by

the cloud when it is positively charged) to pass quietly into the air

above; this neutralizes the cloud. In case of a discharge, the rods aid

in conducting the electricity to the earth.

_=221. Causes of Atmospheric Electricity.=_ There are several theories

about this. Some think that it is due to the rotation of the earth,

different parts being acted upon differently by the heat of the sun.

Some claim that the evaporation of the water in the ocean produces it,

while others say that the electrification is produced in the clouds by

the friction of their particles upon each other. The matter has not

been settled.

_=222. St. Elmo's Fire=_ Electrification from the earth is often drawn

up through the masts of ships to neutralize that in the clouds (see

§ 220), and, as it escapes from the points of the masts, light is

produced. This may be clearly shown by repeating Exp. 71 in the dark;

the head of the pin (Fig. 39) will represent the end of a mast, and the

charged electrophorus cover will be the charged cloud.

_=223. Aurora Borealis=_, also called Northern Lights, are luminous

effects often seen in the north. They often occur at the same time

with magnetic storms, at which times telegraph and telephone work may

be disturbed. The exact cause of this light is not known, but it is

thought by many to be due to disturbances in the earth's magnetism,

caused by the action of the sun.

CURRENT ELECTRICITY.

PART III.--CURRENT ELECTRICITY.

CHAPTER XIV.

CONSTRUCTION AND USE OF APPARATUS.

=Note.=--Before taking up the study of cells and the electric

current, let us perform a few experiments in order to

understand the construction and use of some of the apparatus

needed for such study. A dry cell will be used as the source

of the electricity for these first experiments, because it is

convenient. You will understand its action later. Use this cell

only as directed; improper use of it might spoil it.

[Illustration: Fig. 61.]

=EXPERIMENT 101. To study the effect of the electric current

upon the magnetic needle.=

_Apparatus._ A compass (No. 18); a dry cell, D C (No. 51);

wires with spring connectors attached (§ 226) for making

connections. Fig. 66 shows a plan or top view of the

arrangement. Any other form of cell will do in place of D C.

=226. Electrical Connections.= One must constantly join wires,

connect wires with apparatus, or connect one piece of apparatus

to another, to make the proper electrical connections. A very

simple method of connections has been used in all the apparatus

described in these experiments.

A little arrangement which we shall call a spring connector, S

C, (Fig. 61), gives us a means of quickly making connections;

that is, it does away with expensive binding-posts. It is made

of brass, nickel plated, and may be used anywhere without

affecting the magnetic needle.

Six or eight wires, about No. 24 gauge, each about 1-1/2 ft.

long, should be prepared with a connector at each end. You

may use wire furnished (No. 53). Scrape the insulation from

the ends of the wires for about 1-1/2 inches, then twist the

bare ends around the connectors as shown in Fig. 61. The wire

should pass around tightly at least 4 or 5 times and then be

twisted a little, as shown, to help tighten it. Do not put it

on so poorly or in such quantity that the part, B, will spread.

=227.= Fig. 62 shows how the connector should be slipped upon

a thin piece of metal, M, like that on the galvanoscope, for

example. The wire, W, from the apparatus itself is permanently

fastened under the head of the screw, S, while the wire from

any other apparatus is one of those kept on hand as above

mentioned and connected with S C.

[Illustration: Fig. 62.]

[Illustration: Fig. 63.]

[Illustration: Fig. 64.]

=228.= Fig. 63 shows how several wires may be quickly joined,

electrically, by slipping the connectors at their ends

upon a thin metal plate, M P, which may be a piece of tin,

zinc, copper, etc. M P should not be too thick. In case the

connectors become too much spread to pinch the plate, squeeze

the part, A, a little more together.

=229.= Fig. 64 shows the method of connecting with the special

form of binding-post used, for example, upon the resistance

coil, R C. The end, C, of S C, is pressed down into the tube,

T, until you feel, by moving it, that it springs firmly against

the sides of the tube. In case you wish S C to fit very tightly

in T, one of the legs may be slightly bent outwards.

[Illustration: Fig. 65.]

=230.= The connector may be used in still another way; that

is, by pushing the part, A, into the hole of an ordinary

binding-post, (Fig. 65), and using it just the same as a thick

wire.

=231. Directions.= (A) Stand the compass and D C near each

other (Fig. 66). Attach one end of an insulated copper wire, C

W, to the binding-post, C, which is on the carbon plate of the

cell. Do _not_ join the other end to the other binding-post,

Zn, of the zinc plate.

(B) With the left hand hold C W above and near the

compass-needle, and in the N and S line, so that it will extend

over the entire length of the needle.

binding-post, Zn, for an instant only, watching the needle.

Repeat.

_=232. Current Detectors.=_ We know that a magnet can act, by

induction, through the air upon a piece of iron or upon another magnet.

The deflection of the needle in this experiment shows that there must

be a magnetic field around a wire carrying a current. This fact is of

the greatest possible importance. The simple magnetic needle, when used

as above, becomes a detector of electricity.

[Illustration: Fig. 66.]

[Illustration: Fig. 67.]

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

"key."=

_Apparatus._ A key, K (No. 55) (§ 233); a dry cell, D C, (No.

51); a compass, O C (No. 18). _Arrange_ as shown in Fig. 67,

which is a top view or plan. Connect the pieces of apparatus

with wires and spring connectors (§ 226). Binding-post, C,

is joined to I (in) of the key; O (out) of key is joined to

binding-post, Zn, the wire, C W, passing directly over and near

O C, which is to be used as a detector. The current cannot pass

until the lever, L, is pressed. A metal plate, M P, is used to

connect two short wires (§ 228) in case C W is not long enough.

[Illustration: Fig. 68.]

=233. A key= is merely a piece of apparatus by which the

circuit can be conveniently and rapidly opened and closed at

the will of the operator; that is, by it the electricity can

be quickly turned on or off. Fig. 68 shows a simple form of

key. To the base, B, are fastened two metal pieces or straps,

the upper one, L, being the lever or key proper. The front end

of L is raised above O, so that the two do not touch each

other unless L is firmly pressed down. A screw, S, keeps L from

springing too far above O. For convenience we shall suppose

that the wire leading to the key joins it at I (in); the wire

_from_ the key is joined to O (out), by means of connectors (§

226).

The key may be put into any circuit by first cutting a wire and

then joining the ends to I and O. Spring connectors make the

best connections with this form of key. (For Home-Made Keys see

Apparatus Book.)

=234. Directions.= (A) The magnetic needle being directly under

the wire, press L down for an _instant only_ and note the

action of the needle.

(B) Press L again, hold it down for 3 seconds, not over that,

and watch the needle.

_=Discussion.=_ The key allows us to easily regulate the length of time

during which the current passes. This experiment shows, also, that the

magnetic field about the wire disappears as soon as the current ceases

to pass.

[Illustration: Fig. 69.]

=EXPERIMENT 103. To study the construction and use of a simple

"current reverser."=

_Apparatus._ A dry cell, D C (No. 51); a compass, O C (No. 18);

a current reverser, C R (No. 57) (See § 235); an insulated

copper wire, C W, 2 or 3 feet long, with spring connectors

joined to its ends (§ 226).

_Arrange_ as in Fig. 70. The wire, C W, leading from X should

be held by the left hand so that it will be just above (or

below) and parallel to the magnetic needle.

The current cannot pass through C W until one of the straps or

levers on C R is pressed. (See Apparatus Book for Home-Made

Reversers.)

=235. The Current Reverser.= (No. 57.) To the wooden base (Fig.

69) are fastened four metal straps, each turned up at the end

so that spring connectors (§ 227) can be slipped on to make

electric connections with other pieces of apparatus.

Suppose that at C and Z connections are made with the carbon

and zinc of the cell, by means of wires and spring connectors

(§ 226). The current comes from the cell to C. As the two

straps, 2 and 3, press firmly up against strap 4, and do not

touch 1, it is evident that no current can pass from 1 to 2 or

to 3 until they are pressed down upon 1. Two wires are joined

by spring connectors to 2 and 3 at their turned up ends, X and

Y, and these wires lead to any desired instrument.

[Illustration: Fig. 70.]

=236. Directions.= (A) Press down lever 2 (Fig. 70), for an

instant only, at the same time noting carefully in which

direction the N pole of the needle is deflected.

(B) After allowing lever 2 to spring up again, and after the

needle comes to rest, press down lever 3 for an instant,

watching the needle. Is the N pole of the needle deflected in

the same direction as it was in (A)?

_=237. Discussion.=_ The reverser gives us a quick and easy means of

reversing the current which is to pass through any desired instrument,

first in one direction and then in the opposite direction. Suppose

(Fig. 69) that the current enters C R at C, as it does when C is joined

to the carbon of the cell; the current can go no farther until one

lever is lowered. If lever 2 (Fig. 69) be now pressed down, as in part

(A), the current will pass along 2, which does not now touch 4, out

through X to a coil of wire or any instrument, and back to the reverser

by the wire joined to Y. It will then pass from 3 onto 4, to Z, and

back to the Cell; that is, the current enters C W at X. When lever

3 is pressed, the current still entering C R at C, the electricity

will pass onto 3 and out at Y, and back through X, 4 and Z to the

cell. The current, then, can be made to pass out of X or Y at will by

pressing the proper lever. This experiment also teaches something about

currents, but these will be discussed later.

[Illustration: Fig. 71.]

=EXPERIMENT 104. To study the simple current detector.=

_Apparatus._ The compass (No. 18); dry cell, D C (No. 51);

current reverser, C R (No. 57); copper wire, C W, a few feet

long, with spring connectors on its ends. (See Apparatus Book,

Chapter XIII, for Home-Made Detectors.)

=238. Directions.= (A) Join the ends of the wire to X and Y of

the reverser, C R, as in the last experiment. Coil up C W so

that you can hold the coil with your left hand, as shown in

Fig. 71, the magnetic needle being inside of it and parallel to

it.

(B) Press lever 2 of the reverser for an instant only. Is the

needle deflected more or less than it was when the wire simply

passed over or under it once?

note the result.

(D) Get clearly in mind which way the N pole of the needle is

deflected when the current enters C W at X, also when it enters

at Y.

_=239. Discussion.=_ The current passed _over_ the needle in one

direction, and _under_ it in the opposite direction; that is, the part

of the wire above _helps_ that below. Each turn of the wire increases

the strength of the magnetic field about the coil, and helps to deflect

the needle. In this way, by increasing the number of turns, detectors

may be made that will show the presence of very weak currents. The

magnetic fields about wires and coils will be studied in a later

chapter.

[Illustration: Fig. 72.]

=EXPERIMENT 105. To study the construction and use of the

simple galvanoscope.=

_Apparatus._ The galvanoscope, G V, complete (No. 58),

described in § 240-246; dry cell, D C (No. 51); current

reverser, C R (No. 57) (§ 235); wires, with spring connectors,

to join the different pieces of apparatus (§226). (See

Apparatus Book, Chapter XIII, for Home-Made Galvanoscopes.)

=240. The Galvanoscope= (Fig. 72) is more than a mere detector

of electricity. With it we shall be able to study, more fully,

cells, currents, etc., etc. We must first understand its

construction.

=241. The Coil-support=, C S, is fastened to the cross-piece,

C P, on which are the 3 binding-posts or coil-ends, L, M and

R (left, middle, right). The legs, G L, are screwed to C P in

such a way that C P is held a little above the table: this

allows C S to be tipped to the front or rear to adjust it

vertically. On account of the peculiar arrangement of the legs,

the galvanoscope can be made to stand firmly, even upon uneven

surfaces. The screws holding G L should not be put in far

enough to tear the threads in the wood, C P.

=242. The Galvanoscope Coils=, G C, are two in number, both

being fastened to the coil-support, C S. The first coil has

five turns of wire, its ends being fastened to L and M; the

other coil has _ten_ turns, with ends at M and R. The current

can, at will, be made to pass through 5, 10 or 15 turns of wire

by making the proper connections.

Suppose that we have two wires direct from a cell, or from the

current reverser, with spring connectors on them so that we

can slip them onto L, M or R, which stand for left, middle or

right. When the wires are joined to L and M the current can

pass through but 5 turns; when joined to M and R it will go

through 10 turns; and when to L and R it will pass through the

entire 15 turns. When the current enters the galvanoscope at

L and passes out at M or R, it will pass through the turns of

wire from left to right, at the top; that is, it will pass in a

"clockwise" direction.

=243. The Compass-needle=, furnished with O C (No. 18), will

do also for this galvanoscope. It should be placed upon the

pin-point after fixing on the pointers (§ 246). The length of

the needle should be parallel to the plane of the coil when no

current passes; that is, the coil and coil-support should be

in the N and S line.

The needle can be centered in regard to right and left, and in

regard to up and down, by properly adjusting the position of

the pin-point support, P P S; this is held firmly to C S by two

spring-connectors. By removing S C, the support, P P S, may be

raised or lowered.

=244. To place the coil= in the N and S line, simply swing the

galvanoscope bodily around, at the same time looking down upon

the needle, until the length of the needle becomes parallel

to the coil-support. When once carefully adjusted N and S, a

line may be drawn upon the table as a guide for its position in

future experiments. The coil should stand in a vertical plane,

and this straight up and down position can be easily adjusted.

To place the coil in the E and W line, turn it until the

pointers are at the 90° (90 degree) marks,--the 0° (zero

degree) marks remaining, of course, as described above.

=245. The Degree-Card, G D C= (Fig. 72) has a dot at its

center, to show where to make a pin-hole for the pin that

supports the compass-needle. With this you can tell how many

degrees the needle is deflected when the current passes. This

card, G D C, should be pressed down over the pin-point. The

zero points of G D C should be N and S, also, when the coil

is in that position; that is, they should be in the plane of

the coil. The pointers on the needle (§ 246) will then be at

O, when the needle is at rest, no current passing through the

coils. (See Apparatus Book § 272 for Home-Made Degree-Card.) G

D C may be held permanently in position after it is adjusted,

by sticking a short pin through it into P P S. Do not let this

pin interfere, however, with the swinging of the needle.

[Illustration: Fig. 73.]

=246. Pointers= (Fig. 73) should be fastened to the needle,

in order to make the readings of degrees accurate. Fasten to

the compass-needle a piece of No. 30 insulated copper wire, as

shown. It may be cut to the proper length after it is wound

around the needle. See that the wire does not touch the pin

when needle is in place; balance needle by cutting a little

from the heavier end of wire with shears; bend the ends of wire

so that they are at opposite sides of the degree-card, both

pointing at O, for example. The needle must swing freely, be

nicely balanced, and the wire must not touch pin or degree-card.

[Illustration: Fig. 74.]

=247. Directions.= (A) Arrange as in Fig. 74, the coil being N

and S (§ 244). Join the ends of the wires, 2 and 3, with the

5-turn coil of G V as shown. Wire, 2, is connected to L (Fig.

72). Press lever 2 of C R (Fig. 69) for an instant, watching

the compass-needle and noting how many degrees it swings the

first time. Get thoroughly in mind the direction in which the N

pole of the needle is deflected when the current passes around

G C in a "clockwise" direction. There must be no magnets, iron,

or pieces of steel within 3 feet of A G.

(B) Press lever, 3, for an instant, watching the needle. The

current will now pass in an "anti-clockwise" direction. Is the

needle deflected about the same number of degrees as in (A)?

(§ 242) and repeat (A) and (B).

(D) Change 2 and 3 to L and R (Fig. 72), thus allowing the

current to pass around 15 turns; then repeat (A) and (B).

_=248. Discussion; True Readings.=_ Is not possible to get the magnetic

needle, M, exactly in the center of G C; the pointers will not exactly

be in the axis of M; the coils will not be exactly N and S: hence, if

you pass a certain current through the coil and the pointer reads 20

degrees, you will find, if you reverse the current, that the pointer

may read 24 degrees on the other side of the zero mark. To get the

_true reading_, average the two, in this case the average being 22

degrees.

The galvanoscope gives us an instrument with which we can study, more

fully, cells, currents, etc.

=249. Note of Caution.= It has already been stated that the

compass-needle should be in the center of the coil (§ 243),

and that the coil should be in the N and S line (§ 244). In

addition to the above, see that there are no magnets near G V,

when using it; tap G V occasionally to be sure that the needle

swings freely, hold the eye directly over the pointers when

reading degrees; the pointers should be at zero when no current

passes through G V; be sure that the electrical connections are

good.

There are several sources of error in taking readings, and in

all the quantitative experiments given. The author takes it for

granted that such errors will be looked out for by the teacher.

[Illustration: Fig. 75.]

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

astatic needle.=

_Apparatus._ Two unmagnetized sewing-needles (No. 1); horseshoe

magnet, H M (No. 16); piece of stiff paper doubled and cut as

in Fig. 75; a pin-point on which to support the paper. The pin

may be stuck through a cork, or that of O C (No. 18) may be

used.

=250. Directions.= (A) Draw each needle across the N pole of H

M five times from point to head (Exp. 9). This should make them

of nearly equal strength, both points being N poles.

(B) Stick the needles through the paper as shown, the N poles

being at the same end of the paper. Balance the paper upon the

pin-point. Has this combination a strong or weak pointing-power?

pointing-power.

_=251. Discussion; Astatic Needles.=_ By arranging the needles so

that their poles oppose each other, the pointing-power becomes almost

nothing. This sort of a needle is needed in some experiments in

electricity. Their magnetic fields are still retained. The combination

is called an _astatic needle_; it is used to detect very feeble

currents. The more nearly equal the magnets are in strength, the

better. They are usually arranged with one above the other (Fig. 76).

[Illustration: Fig. 76.]

[Illustration: Fig. 77.]

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

astatic galvanoscope.=

_Apparatus._ An astatic galvanoscope, A G (No. 59) (§ 252-254);

dry cell, D C (No. 51); current reverser, C R (No. 57) (§ 235);

wires for connections (§ 226).

_Arrange_ as shown in Fig. 80, which is a top view. The wires

from C R are connected to the binding-posts of A G at the back,

the spring connectors being slipped into them (§ 229).

=252. Construction of the Astatic Galvanoscope.= When not to be

used for a long time, or for shipping, the legs, A (Fig. 77)

may be removed, and the whole packed inside of the box, B.

The _Coil_, C, has a resistance of about 5 ohms, and is

fastened to the coil-support, C S. The ends of the coil are

permanently fastened to the binding-posts, L and R (left and

right). The ends are so arranged that when the current enters

at L it will pass around the coil in a clockwise direction.

=253. The Astatic Needle= (Exp. 106) is supported by a small

thread, T, which is tied to the thread-wire, T W. This T W

springs into an eyelet, E, which, in turn, rests in a hole

made in the end of B. E should turn easily in the hole, but it

should not wabble.

Fig. 78 shows a sectional view of the coil and needle. The

wire, W, should be bent, as shown, so that the magnets can be

as near the center-line of C as possible. Fig. 79 shows a front

view of the needle. As a matter of convenience it will be best

to arrange the poles of the needles, as shown, to agree with

the descriptions of the experiments.

To keep the needle from being affected by air currents, the

glass plate (No. 38) may be placed in front of the box, B.

Stand it upon the legs, A, and tie a string around it, and B,

to hold it in place.

[Illustration: Fig. 78.]

[Illustration: Fig. 79.]

=254. Adjusting the Needle.= As T is tied to T W, the needle

may be swung completely around by turning T W. This should be

done until the length of the needle is parallel to the turns

of C. The up and down position of the needle should be fixed

as nearly as possible when fastening T to T W, the exact place

being finally fixed by raising or lowering T W through E. The

spring in T W should hold it firmly in E after adjustment. The

wire, W, joining the needle-magnets should not touch the coil.

It may be made to just swing free from C by tilting the box

forward or backward a little. The construction of the legs,

etc., makes it possible to tilt the box, and to make it stand

firmly upon an irregular surface.

[Illustration: Fig. 80.]

=255. Directions.= (A) See that there are no magnets within

3 feet of A G. Test the astatic needle, after you have it

properly suspended, to convince yourself that it does not try

to swing around in a N and S line. In case the needle-magnets

have been in contact with other magnets, or are not equally

magnetized, remagnetize them as directed in Exp. 106. They must

remain in any position given them by turning T W. Finally,

bring them parallel to the turns of the coil. (See § 254.)

Arrange as in Fig. 80.

(B) Press lever 2 of C R (§ 235) for an instant only. This

allows the current to enter A G at L. Repeat several times

until you thoroughly fix in your mind the direction in which

the right-hand end of the needle is deflected. Does the needle

jump suddenly when the current passes?

_=256. Astatic Galvanoscopes.=_ It is evident that in the ordinary

current detector (Exp. 104), the pointing power of the needle has to be

overcome by the magnetic field about the coil, before the needle can be

forced from its N and S line. Very weak currents will not visibly move

the needle in ordinary detectors. To make a sensitive instrument we

must have strong fields about both the needle and coil, and we must, at

the same time, decrease the pointing power of the needle. Both of these

things are accomplished by using an _astatic needle_ in connection

with a coil containing considerable wire. The uses of the astatic

galvanoscope will be studied more fully in later experiments.

CHAPTER XV.

GALVANIC CELLS AND BATTERIES.

=EXPERIMENT 108. To study the effect of dilute sulphuric acid

upon carbon and various metals.=

_Apparatus._ A piece each of copper and iron wire 4 in., (10

cm.) long; two narrow strips of sheet zinc (No. 60), one being

amalgamated (§ 257); a carbon rod (No. 64); a tumbler (No. 65),

partly filled with dilute sulphuric acid, (§ 258); mercury (No.

52).

=257. To amalgamate= one of the above zinc strips is to coat

it with mercury. Remove all jewelry from the hands before

proceeding. Wash the zinc with water, and with a cloth remove

all dirt from its surface. Amalgamated zinc is very brittle,

so lay it flat upon a piece of board or upon a plate, after

dipping it for an instant in the dilute acid. Place a small

drop or two of mercury upon the strip, and rub the mercury

about upon both sides of the zinc with a cloth made wet with

the dilute acid.

Mercury clings strongly to zinc or tin, so you may use a narrow

piece of either as a spoon to carry a small drop from the

supply to the zinc. Tap it upon the zinc to dislodge the drops.

Do not get on too much mercury, just enough to coat it, or, at

least, that part of it that will be under the acid. Be careful

not to break the thin zinc when amalgamating it, as it gets

very brittle. It should look bright. (See Apparatus Book § 32,

33.)

_Note._ Should any mercury get upon copper plates it may be

removed by heating them in a flame.

=258. Dilute sulphuric acid=, for these experiments, should be

made by mixing 1 part, by volume, of concentrated acid, with

20 parts of water. Do not let the acid get upon clothes or

carpets. Do not add water to acid. Mix by _slowly_ adding the

acid to the water in a glass or earthen dish, stirring at the

same time. Mix over a sink or out of doors. (For fuller details

see App. Book; § 21, 22, 23, 24, 25.) To save time, make at

least a quart of the mixture, bottle, and label it for use.

=259. Directions.= (A) Bend over one end of each of the wires

and metal strips, and hang them upon the edge of the tumbler

(Fig. 81), so that their lower ends shall be in the acid. Do

not let them touch each other. Stand the carbon rod in the acid.

If there is no visible action upon any of the above substances,

add a few drops of concentrated acid to the tumbler.

Note carefully what takes place in the tumbler, and state which

of the substances are dissolved, which simply made brighter,

and which not acted upon at all.

_=260. Discussion.=_ The bubbles of gas that arise from the zinc when

it dissolves are hydrogen, and they indicate that a vigorous chemical

action is going on in the tumbler, and that the zinc is being eaten

away.

[Illustration: Fig. 81.]

[Illustration: Fig. 82.]

=EXPERIMENT 109. To study the effect of dilute sulphuric acid

upon various combinations of metals.=

_Apparatus._ The same as in the last experiment. A small piece

of amalgamated zinc, however, will be better than the whole

strip.

=261. Directions.= (A) Twist one end of the clean copper wire

around the small piece of amalgamated zinc (Fig. 82). Hold

one end of the wire in the hand and dip the combination into

the acid. What takes place? Watch the surface of the copper,

remembering that each, alone, was not acted upon by the acid

(Exp. 108).

(B) Use the clean iron wire in place of the copper wire, and

repeat (A). Watch the surface of the iron.

amalgamated zinc to the carbon rod (Fig. 82), then dip the

combination into the acid. Watch the surface of the carbon.

_=262. Discussion.=_ While amalgamated zinc is not rapidly dissolved

by dilute sulphuric acid, a vigorous action of some kind takes place

when it is in contact with another metal or with carbon in the acid.

The bubbles of hydrogen that are liberated do not seem to come from the

zinc; they appear to grow, in the fluid, directly at the surface of

the copper, iron, or other metal used with the zinc. This shows that

something besides the mere dissolving of a metal takes place.

Can we arrange our apparatus so that we can get some useful results

from this action?

=EXPERIMENT 110. To study the construction of a simple Voltaic

or Galvanic cell.=

_Apparatus._ A narrow strip of zinc (No. 60), amalgamated as

directed in § 257. (An amalgamated zinc rod (No. 74) may be

used in place of the strip); a narrow strip of sheet copper

(No. 67); the tumbler of dilute acid of Exp. 108; a flexible

copper wire about 2 feet long, with spring connectors (No. 54)

attached to its ends. (See Electrical Connections, § 226.)

[Illustration: Fig. 83.]

=263. Directions.= (A) Holding the amalgamated zinc strip in

one hand and the copper strip in the other (Fig. 83), dip them

into the acid, but do not let them touch each other. Note any

chemical action.

(B) Touch the copper and zinc together, _below_ the surface of

the acid. Watch the copper.

together _above_ the acid. Anything still happen to the copper?

(D) Slip one spring connector with the attached wire upon the

zinc strip, then stand the strips in the tumbler, so that they

can not touch each other. Now touch the copper strip with the

free end of the wire, at the same time watching the copper.

(E) Raise the wire from Cu, touch it to Cu again, and repeat

several times until you are sure that something takes place

every time the wire touches Cu.

_=264. The Electric Current.=_ Something must happen in or through

the wire, and it can only happen when the two metals are joined in

some way. This peculiar, invisible action in the wire is called the

_electric current_, and such an arrangement is called a _Galvanic cell_.

_=265. Source of the Electrification.=_ When two different metals are

placed in acid they are electrified unequally by chemical action.

Each has a higher potential than the acid, but their potentials are

not the same. This electrification tends to pass from the place of

higher to the place of lower potential, and the conducting wire allows

this transfer to take place. As the difference of potential is kept

up by the continued chemical action, the current is continuous, and

not simply instantaneous, as in discharges of frictional electricity.

As heat is produced by the burning of coal, so electrification is

produced by the chemical burning of zinc. Chemical energy is the source

of electrification in the Galvanic cell, just as muscular energy was

the source of the electrification in the experiments with frictional

electricity.

_=266. The Electric Circuit; Open and Closed Circuits.=_ The simple

Galvanic cell just used, together with the wire which joined the metal

strips, is called an _electric circuit_. If the current should pass

through a telegraph instrument, for example, on its way from one strip

to the other, the telegraph instrument would also be in the circuit.

When the wire is cut or removed from one metal strip, the circuit is

said to be _open_--that is, we have an _open circuit_. When the current

passes, the circuit is _closed_. We also say _make_ and _break_ the

circuit, and that the circuit has been _broken_.

_=267. Plates or Elements.=_ The copper and zinc strips are called the

_plates or elements_ of the cell. The zinc, Zn, Fig. 84, is dissolved

by the acid, and is called the positive plate (+ plate). The copper,

Cu, is the negative plate (- plate). The copper is also called the

_cathode_, and the zinc the _anode_.

_=268. Direction of Current.=_ It has been agreed, for convenience,

that _in_ the cell the current passes from the zinc through the liquid

to the copper, where the hydrogen bubbles are deposited. It then passes

through the wire, or other conductor furnished, back to the zinc,

through the liquid to Cu again, and so on around and around thousands

of times per second. The current really starts at the surface of the

zinc, where the chemical action is. When carbon and zinc are used, the

action and direction of the current are the same, carbon being the -

plate.

[Illustration: Fig. 84.]

_=269. Poles or Electrodes.=_ If the wire were cut, the electricity

coming from the + plate would be stopped at the end of the wire marked

+, Fig. 84, after passing through the acid and up Cu. This end of the

wire is called the + _pole or electrode_ (positive). The end of the

wire joined to Zn is called the - _pole_ or _electrode_; that is, the +

electrode is the end of the wire attached to the - plate. The tops of

Cu and Zn may be considered electrodes. The top of Cu is the + _pole_

of the cell, while Cu is the - _plate_.

=270. Chemical Action in the Simple Galvanic Cell.= The

chemical formula of sulphuric acid is H_{2}SO_{4} (read H,

2, S, O, 4). This means that it is a compound of hydrogen

(H), sulphur (S), and oxygen (O). The H_{2}SO_{4} stands

for _molecule_ of acid, and the small figures show that the

molecule is made up of 2 _atoms_ of hydrogen (H_{2}), one

of sulphur (S), and 4 of oxygen (O_{4}). The atoms are held

together by _chemical attraction_ or _affinity_.

There is a stronger chemical affinity between zinc (Zn) and

SO_{4} than between H_{2} and SO_{4}; so, as soon as the Zn

gets a chance, as it does in the cell, it drives out the

H_{2}, and it takes its place in the molecule. This chemical

_reaction_ may be shown by the following chemical _equation_:

Zn + H_{2}SO_4 = ZnSO_4 + H_2.

Zinc and sulphuric acid produce zinc sulphate and hydrogen.

The zinc sulphate produced weakens the effect of the acid; in

fact, the acid has to be renewed occasionally, as it is changed

to the sulphate which remains in solution.

=271. Action in Cell Using Impure Zinc.= The above action

takes place in the cell when impure zinc is used, even when no

current passes, heat being produced by the reaction instead of

useful electricity. (See Local Currents.)

=Action in Cell Using Pure Zinc.= When pure zinc (or impure

zinc that has been properly amalgamated) is used in the cell,

it dissolves, or is eaten away, only when the current passes.

It should be noted that the bubbles of hydrogen do not even

then appear at the zinc; they are not seen throughout the body

of the liquid. There seems to be an unseen transfer of hydrogen

from the zinc, through the liquid, to the copper (or other -

plate used), and it appears there in the shape of bubbles. The

larger the quantity of pure zinc dissolved, the stronger the

current, and the larger the amount of hydrogen produced.

As the zinc dissolves it parts with its latent energy, and this

energy forces the electric current through the circuit. While

the hydrogen of the decomposed acid makes its way towards the -

plate, the SO_4 part of it travels towards the Zn plate, where

the ZNSO_4 is formed. (See § 270.)

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

the cell.=

_Apparatus._ Tumbler of dilute sulphuric acid. (§ 258); strip

of unamalgamated zinc; crystal of copper sulphate (blue

vitriol) (No. 86); a galvanized iron nail (No. 69), this being

iron covered with zinc.

=272. Directions.= (A) Hold the nail in the acid for a few

seconds, and note result.

(B) Rub the copper sulphate upon the zinc simply in one spot,

then place the zinc in the acid, noting the result at the spot.

_=273. Local action; Local Currents.=_ Ordinary commercial zinc

contains such impurities as carbon, iron, lead, etc., in small

quantities. It was seen, Exp. 109, that when different metals were in

contact with the zinc, the zinc was rapidly dissolved by the acid. The

impurities in the zinc act like the copper plate in the simple cell,

thus producing _local currents_ in the zinc, which rapidly destroy it

without doing any good. These currents pass from zinc to impurities,

and back to the zinc, without going out into the main wire. This local

action takes place even when the main circuit is open.

_=274. Reasons for Amalgamating Zinc Plates.=_ Pure zinc is not

affected by dilute sulphuric acid, but it is too expensive to use in

cells; so amalgamated zinc is used instead, because it is cheaper, and

acts the same as pure zinc. The impurities are removed from the surface

of the zinc, as the zinc alone is dissolved by the mercury. There is,

then, a liquid layer of pure zinc with mercury upon the surface of

the amalgamated plate. This is not acted upon by the acid when the

circuit is open. A stronger and more regular current is produced with

amalgamated zinc than with the impure unamalgamated zinc.

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

_Apparatus._ The galvanoscope G V (No. 58), (See § 240, etc.);

the simple cell arranged as described in § 275, the zinc being

amalgamated.

=275. The Simple Cell= should be arranged so that the plates

will be held firmly in position. The zinc, Zn (No. 60),

and copper, Cu (No. 67), should be fastened to the wooden

cross-piece, W C P (No. 70), as shown in Fig. 85. Care should

be taken not to use longer screws than those provided for (No.

72). If the screws touch each other they will short circuit the

cell. Partly fill the tumbler (No. 65) with dilute sulphuric

acid (§ 258), join wires with connectors to the plates. The

free ends of the wires are then ready to join to apparatus. The

ends of wires _may_ be fastened under the screw-heads instead

of using connectors on the plates. Do not put the plates into

the acid until you read the "directions."

=276. Directions.= (A) Arrange as in Fig. 86. Place the coil

of G V, N and S (§ 244). _Before_ putting the plates in

the acid join them to the 15-turn coil of G V (§ 242). The

compass-needle should point to zero. See that the needle swings

freely.

(B) Place the plates in the acid, and _quickly_ bring the

needle to rest with the aid of the hand, so that you can take

the reading at once before the hydrogen bubbles entirely

cover the copper plate. Watch the action of the needle for a

few minutes. Make a note of the reading, in degrees, at the

beginning of the experiment and at the end of five minutes.

(See Note.)

[Illustration: Fig. 85.]

[Illustration: Fig. 86.]

=Note.= If no change takes place in the position of the needle,

the change beginning inside of 10 seconds after the plates are

let down into the acid, withdraw the plates, then clean and

thoroughly dry the copper to remove all traces of hydrogen.

This may be done by heating the copper over a gas flame. Let

the copper remain in the air 15 minutes, then try again. In

taking the first reading you must work quickly. Catch the

needle during its _first_ swing. If you allow it to swing back

and forth until it comes to rest, your first reading will not

be what it should be.

change, for 2 or 3 minutes, take hold of the wooden

cross-piece, move the plates back and forth in the acid to

dislodge the hydrogen bubbles, and note carefully the action of

the needle. Does the current seem stronger when the plates are

moved? Can you get the needle back to the first reading?

(D) Remove the plates from the acid, dry and clean the copper,

let them stand in the air for 15 minutes, then take another

quick reading and compare it with the first.

_=278. Polarization of Cells.=_ The acid gets a little weaker, of

course, as it is decomposed by the zinc (§ 270), but the chief cause of

the weakening of the current is hydrogen, which forms a filmy coating

upon the copper plate. This coating even seems to soak into the copper,

and it takes some time for it to be thoroughly removed. The zinc plate

is kept comparatively free from hydrogen by amalgamation.

_=279. Effects of Polarization.=_ The hydrogen bubbles weaken the

current in at least two ways. In the _first_ place, hydrogen is not a

conductor of electricity; so it holds the current back, as any other

resistance would.

_Secondly_, acid acts upon hydrogen as it would upon another metal.

When the copper plate becomes well covered with hydrogen, the acid

cannot touch it; so we really have a _hydrogen plate_ in the cell.

Hydrogen acts very much like zinc in the acid; we say that it is more

electro-positive than copper. The result is, then, that a new current

starts up, and as this is towards the zinc, in the acid, it partially

destroys or neutralizes the main zinc-to-copper current. Practical use

is made of the principles of polarization (see Secondary Batteries).

_=280. Remedies for Polarization; Depolarizers.=_ Any scheme by which

the hydrogen may be destroyed and kept from the inactive, or negative

plate, will prevent polarization. _Mechanical_ means have been employed

to brush away the hydrogen by keeping up a constant circulation of

the liquid. _Chemical action_ is another means by which the hydrogen

may be side-tracked before it gets to the - plate in single-fluid

cells. Substances like nitric acid and bichromate of potash, called

_depolarizers_, contain large quantities of oxygen, and, during

the chemical changes that take place, this oxygen unites with the

hydrogen. These substances are used in zinc-carbon cells. (See § 286,

etc. for various forms of cells.)

There is another form of cell, the _two-fluid_ type, in which

_electro-chemical_ means are employed, and in which a metal is

deposited upon the - plate instead of hydrogen. The - plate is usually

copper, copper being deposited upon it.

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

_Apparatus._ The glass tumbler, G T, (No. 65); porous cup, P C,

(No. 73); the strip of zinc (No. 60), well amalgamated (§ 257),

or the amalgamated zinc rod (No. 74); piece of sheet copper

(No. 75), bent so that it will surround P C; copper wires, C

W, with connectors; a saturated solution of copper sulphate,

commonly called blue vitriol or blue stone (See § 283);

dilute sulphuric acid (See § 258). With the above, set up the

two-fluid cell (See § 281). The galvanoscope, G V, complete,

is also needed, and if quantitative work is to be done, a pair

of scales weighing to 0.1 gram is necessary. (See App. Book,

Chapter I, for Home-Made Two-Fluid Cells.)

[Illustration: Fig. 87.]

=281. Setting Up the Two-Fluid Cell.= Fig. 87. Stand the

amalgamated zinc rod, Zn, in P C, then place P C in the

tumbler, G T; put in the copper plate as shown. Pour diluted

acid (§ 258) into P C until it stands about 2-1/2 in. deep;

then at once pour the copper solution (§ 283) into G T, on the

outside of P C, until it stands at the same height as the acid

_in_ P C. As soon as the liquids have soaked into P C the cell

will be ready for use; but it is better to connect it with the

galvanoscope at once and note the increase of current during

the first few minutes while the liquids soak through P C. A

crystal of copper sulphate should be put outside of P C to keep

the solution full strength. This is a form of the well-known

Daniell cell. Fig. 87 shows a form of home-made two-fluid cell

as described in Apparatus Book. If you have the one furnished,

use the rod instead of sheet zinc, and use connectors on the

plates.

=282. Care of Two-Fluid Cell.= This experimental cell should

be taken apart when not in use. It should not be left in open

circuit, even for half an hour. Even after the plates are

removed, copper may be deposited upon P C in case there are

any metallic impurities on it. Remove the plates and P C, and

thoroughly wash them. The copper solution should be put into a

bottle to prevent evaporation; the dilute acid may be thrown

away to be replaced by fresh acid for the next experiment.

=283. Copper Sulphate Solution= is made by adding the blue

crystals to water until no more will dissolve--that is, the

solution should be "saturated," extra crystals always being

left in the stock bottle. An ounce of the crystals to half a

tumbler of water will be about right, but a pint or so should

be made at a time and be kept bottled to save time.

=284. Directions.= (A) In case you have access to a pair of

scales that weigh to within 0.1 gram, carefully weigh both

copper and zinc before proceeding. They should be washed and

dried with a cloth. See that there are no drops of mercury upon

the zinc that may fall off during the experiment.

(B) Replace the plates in the cell, and connect them with the

15-turn coil of G V, placed N and S. Allow the circuit to

remain closed for half an hour, and record the position of the

needle every 5 minutes.

both the zinc and copper. Compare the new weights with those

found in (A).

_=285. Chemical Action in the Two-Fluid Cell.=_ In this form of cell

the hydrogen is not allowed to collect upon the copper plate. The

action inside of P C is like that explained in § 270, hydrogen being

set free. As soon as this hydrogen (H_{2}) comes in contact with the

copper sulphate (CuSO_{4}), and it begins to do this in the walls of

P C, a new chemical reaction takes place. Hydrogen has a stronger

attraction for SO_{4} than Cu has, so it unites with the SO_{4},

forming H_{2}SO_{4} (sulphuric acid), and this at the same time throws

out the Cu bodily. This Cu is then free, instead of hydrogen, to be

deposited upon the copper plate. The current is constant, as there is

no polarization.

_=286. Various Galvanic Cells; Open and Closed Circuit Cells.=_ There

is no one form of cell that is best for all kinds of work. If momentary

currents only are wanted, such as for bells, telephones, etc., in which

the cell has plenty of time to rest, _open circuit_ cells are used.

These cells polarize, however, when the circuit has to be closed for

any length of time. This form of cell is always ready for use, and may

not need attention for months at a time. The most common forms of the

open circuit cells are the _Leclanché_ (§ 287) and _dry_ cell (§ 288).

Open circuit cells polarize quickly, because the depolarizer (§ 280) is

slow in destroying the hydrogen.

When a strong current is needed for a considerable time, such as for

telegraph lines, motors, etc., a _closed circuit_ cell is necessary.

The depolarization must be rapid and constant. The _bichromate_ (§ 289)

and the _Daniell_ cell (§ 290) are very common forms of closed circuit

cells. (See, also, Storage Cells.)

=287. The Leclanché Cell= is an open circuit cell in which

carbon and zinc are the plates. The carbon is surrounded

with dioxide of manganese, a depolarizer; the two are either

packed together in a porous cup, or the latter is compressed

into blocks, which are fastened to the carbon. The porous cup

stands in a jar containing a solution of sal ammoniac (ammonium

chloride), which acts as the exciting fluid, and in which

stands a zinc rod. The zinc is not acted upon when the circuit

is open. The hydrogen given off by the decomposition of the

ammonium chloride is destroyed by the oxygen contained in the

manganese dioxide. The E. M. F. is nearly 1.5 volts.

=288. Dry Cells= are for open circuit work. Sheet zinc forms

at the same time the active plate and the outside cylinder

or case. A carbon plate acts as the inactive or - plate. The

exciting fluid is kept from spilling by its being absorbed by

one of the various substances used for that purpose.

=289. The Bichromate of Potash Cell= is a very common one

for laboratory use. It gives a strong current, and although

a single fluid cell, it does not readily polarize. Zinc and

carbon plates are used. In the sulphuric acid, which is the

exciting fluid, is dissolved bichromate of potash. This cell

is used for running small motors, induction coils, etc. It

has, with fresh solutions, an E. M. F. of about 2 volts. (See

Apparatus Book, Chapter I., for Home-Made Batteries.)

=290. The Daniell Cell=, of which the two-fluid cell used in Exp.

112 is a form, is noted for its constant current. The E. M. F.

is a little over 1 volt, and it should be kept working through

a resistance when not in regular use; it should not be left in

open circuit. The porous cup keeps the two fluids from mixing,

but it does not stop the current.

=291. The Gravity Cell= is a form of the above. As one of the

fluids is heavier than the other, no porous cup is needed.

Gravity, together with the action of the current, tends to

keep the fluids separated. A copper plate is placed in the

bottom of the jar, and upon this is put the copper sulphate

solution. The zinc plate is supported by the top of the jar and

rests in a solution of zinc sulphate, which is lighter than

the blue solution below. An insulated wire extends from the

copper through the liquids. This cell is used for telegraph and

similar work. (See Apparatus Book for Home-Made Gravity Cell,

its Regulation, etc.)

CHAPTER XVI.

THE ELECTRIC CIRCUIT.

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

"shunts."=

_Apparatus._ The galvanoscope, G V (No. 58); astatic

galvanoscope, A G (No. 59); two-fluid cell, 2-F C (see § 281);

6 wires with connectors; small thin pieces of tin or other

metal, M P, for rapidly making connections (§ 226). _Arrange_

as in Fig. 88. The wires, 1 and 4, from 2-F C, lead to the

metal plates M P-A and M P-B, for convenience. The wires, 2 and

3, from G V, are also connected with these plates. The wires,

5 and 6 (dotted lines), lead from A G, to be used as directed

in part (B) of the experiment. See that G V is properly placed.

See that A G is adjusted.

[Illustration: Fig. 88.]

=292. Directions.= (A) Without A G in place, take the reading

of G V. The current now passes from Cu through 1, M P-B, 2, G

V, 3, M P-A, 4 to Zn.

(B) Connect wires 5 and 6 to the plates, as shown by the dotted

lines. Again take reading of G V, and compare it with the first

reading. Does some of the current pass through A G?

_=293. Divided Circuits; Shunts.=_ The current divides at M P-B into

two parts; one part may be called a _shunt_ of the other. The circuit

is said to be _divided_; it has two branches. If the two ends of a wire

be fastened to another as in Fig. 101, the circuit is also divided.

When two or more conductors lead side by side from one point to

another, they are called _parallel_ circuits; that is, the conductors

are joined in parallel.

As strong currents would injure delicate galvanometers, a small part

only of the current may be allowed to pass through the galvanometer by

using a shunt. Fig. 89 shows such an arrangement, in which most of the

current passes through the shunt, S. There are many practical uses of

shunts.

[Illustration: Fig. 89.]

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

_Apparatus._ About the same as in Exp. 114, Fig. 88. The

astatic galvanoscope is not needed; in place of it provide

a short piece of metal, such as a battery-plate, or even a

jack-knife. _Arrange_ as in Fig. 88, but without A G.

=294. Directions.= (A) With the current passing as described in

Exp. 114 (A), take the reading of G V.

(B) Lay the ends of the metal, or other thick conductor, upon M

P-A and M P-B. Compare the new reading of G V with that in part

(A).

reading in degrees, then touch M P-A to M P-B; watch G V.

_=295. Short Circuits=_ are very apt to occur unless care is taken. Do

not allow uninsulated wires to touch each other. As shown by the above

experiment, practically the whole of the current may be side-tracked

by a _shunt of low resistance_. A galvanic cell is short-circuited by

connecting the plates directly by a wire or other conductor.

CHAPTER XVII.

ELECTROMOTIVE FORCE.

_=296. Electromotive Force.=_ It has been stated that a galvanic cell

has the _power_ to charge one of its plates positively and the other

negatively; this power is called _electromotive force_, and, for short,

E. M. F. is written. The E. M. F. of a cell depends upon the kinds of

plates used and their condition, the chemicals used in the exciting

fluids, etc. The greater the E. M. F. of a cell the greater its power

to force the current through wires, etc. The E. M. F. of a cell does

not depend upon the size of its plates, as will be seen by later

experiments.

_=297. Unit of E. M. F.; The Volt.=_ A certain amount of E. M. F. has

been taken as the standard, and, in honor of Volta, it has been called

the volt. The E. M. F. of the two-fluid cell used in Exp. 113 is not

far from 1 volt. If a certain cell has the power to keep up twice the

difference of potential between its terminals that the Daniell cell

has, we say that it has an E. M. F. of about 2 volts.

_=Voltmeters=_ are instruments to measure E. M. F.

=EXPERIMENT 116. To see if the E. M. F. of a cell depends upon

the materials used in its construction.=

_Apparatus._ Tumbler two-thirds full of dilute sulphuric acid

(258); strips of zinc, Zn (No. 60); copper strips, Cu (No. 67);

iron strip, I (No. 76); lead strip, L (No. 77); carbon rod (No.

64); the galvanoscope, G V (No. 58); 2 wires with connectors

(§ 226), so that the plates can be changed quickly; the wooden

cross-piece, W C P (No. 70).

_Arrange_ as in Fig. 90. The metal strips are all of the same

size; they may be held with the hand firmly against W C P, in

order to have them the same distance apart in each trial. They

should be lowered to the bottom of the tumbler in each case, in

order to have them acted upon by the same amount of acid. Place

G V properly.

[Illustration: Fig. 90.]

=298. Directions.= (A) With Zn and Cu connected to G V as shown

(Fig. 90), take the reading in degrees, and note in which

direction (east or west) the N end of the needle is deflected.

Tabulate results, as shown in Fig. 91, filling in each column

of your table made out on paper.

(B) In like manner try the following combinations in the order

given, in each case connecting the first-mentioned plate with

the left-hand binding-post, L, of G V. For (B) use zinc-iron.

lead-copper; (G) copper-carbon.

[Illustration:

+------+---------------+------------+-------------+------------+

+------+-------+-------+------------+-------------+------------+

| (B) | | | | | |

| | | | | | |

| (C) | | | | | |

Fig. 91.]

=299. Note.= Some of the combinations produce but slight

currents. In case G V is not delicate enough to show clearly

which way the current passes, use the astatic galvanoscope in

its place for such combinations.

_=300. Discussion.=_ Exp. 116 clearly showed that different

combinations of metals in the acid have different powers of pushing

electricity through the galvanoscope. Although some of the pairs of

metals furnished so weak a current that it was necessary to use the

astatic galvanoscope to study the current, all produced _some_ current,

and from the results can be formed an electromotive series (§ 301). The

strength of acid, condition of plates, etc., affect the E. M. F. of a

cell.

=301. Electromotive Series.= All metals are not acted upon to

the same degree by dilute acid. From the results of Exp. 116

it is seen, part (B), that iron is electronegative to zinc;

that is, the current in the cell flows from zinc to iron.

Part (D) showed that iron is electropositive to copper, as the

current flowed from iron to copper in the cell. It is possible

to arrange the metals in a series, one below the other, in such

a way that any one will be electronegative to those above it

and electropositive to those below it; that is, the list should

have the most electropositive metal at the top, and the one

least acted upon by the acid at the bottom. Make such a list

from your results. The farther the metals used are apart in

the _list_, the greater will be the E. M. F. of the cell. Good

carbon is acted upon the least of all, so zinc and carbon are

better than zinc and copper.

=EXPERIMENT 117. To see whether the electromotive force of a

cell depends upon its size.=

_Apparatus._ Galvanoscope; two glass tumblers; dilute acid; two

wooden cross-pieces; two copper and two zinc strips, the same

size as those used for Exp. 112. (See § 275). These materials

will form two simple cells like Fig. 85. Have about 3 in. of

acid in one tumbler, and but 1 in. in the other. The plates of

one cell will then be about 2-1/2 in. in acid, and those of the

other cell only 1/2 in. in acid. This gives us the same effect

as a large and a small cell.

=302. Directions.= (A) Join the large and small cells with G V

so that their currents will oppose each other. To do this, join

the two zinc plates by means of a wire and connectors. With two

other wires connect the two copper plates with the galvanoscope

binding-posts, and watch for any indication of current. Does

one cell oppose the other?

_=303. Discussion.=_ The E. M. F. of a cell, then, does not depend upon

the size of its plates. The small piece of zinc--that is, the one in

but little acid--had the same potential as the large piece; they must

have had, as they were joined. The large cell will give a stronger

current, under certain conditions, than the small one; but this

depends upon other things than E. M. F. (See experiments under Current

Strength.) A zinc-copper cell, like the one just used (Exp. 117), has

the same _voltage_ as one of the same kind would have, even though it

were made as large as a barrel.

CHAPTER XVIII.

ELECTRICAL RESISTANCE.

_=305. Resistance.=_ It is harder for a horse to draw a wagon through

deep sand than over a smooth pavement. We may say that the sand holds

the horse back--that is, it offers a resistance. The electric current

does not pass through all sorts of substances with the same ease, and

when it succeeds in pushing its way through a circuit of considerable

resistance, we cannot expect it to arrive at the end of its journey

without being weaker than when it started. Do we expect this of a man

or horse? We shall soon see that there is a definite relation between

resistance and the strength of the current at the end of its journey.

=EXPERIMENT 118. To study the general effect of "resistance"

upon a current.=

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

C (No. 79) (§ 310); two-fluid cell, 2-F C (§ 281); 4 wires

with connectors (§ 226). _Arrange_ as in Fig. 92. The current

passes as shown by the arrow, and the circuit may be opened and

closed at the metal plate, M P, or by using a key in its place.

Properly place G V.

=306. Directions.= (A) Take the reading of G V in degrees, the

current passing through the entire length of R C. (See § 310.)

(B) Change the end of wire 4 from binding-post R to M, on R C,

so that the current will pass through one-half only of R C.

Note the reading of G V.

metal plate. Compare the readings of (A), (B) and (C). What do

they show?

[Illustration: Fig. 92.]

_=307. External Resistance; Internal Resistance.=_ When we consider

a circuit like that shown in Fig. 92 we see that it is composed of

two parts, and that we have two kinds of resistances. The wires,

instruments, etc., make up what is called the _external resistance_

of the circuit; that is, the part that is external to the cell. The

liquids in the cell offer a resistance to the current; this is called

_internal resistance_. (See § 314.) The strength of the current depends

upon the relation between these two resistances, as will be seen by

future experiments, as well as upon the E. M. F. of the cell. As

liquids are not as good conductors as metals, the internal resistance

of cells may be quite high.

_=308. Unit of Resistance; The Ohm.=_ Whenever anything is to be

measured, a standard, or unit, is necessary. The unit of resistance is

called the ohm, in honor of Ohm, who made careful investigations upon

this subject. A column of mercury having a length of a little over 3

feet has been taken as a unit. (The column taken is 106.3 cm. with a

weight of 14.4521 grams; it has a cross-section of about 1 sq. mm.,

at a temperature of 0°C.) Mercury is a liquid, and has no "grain" to

affect the resistance. For the use of students, 9 ft. 9 in. of No. 30

copper wire, or 39 ft. 1 in. of No. 24 copper wire will make a fairly

good ohm. We might, of course, take any other length as _our_ standard;

the above, however, will give results that are approximately correct.

(See wire tables at the end of this book.)

_=309. Resistance Coils; Resistance Boxes.=_ Coils of wire, having

carefully-measured resistances, are called _resistance coils_. The wire

for any coil is doubled at the center before it is wound into coils or

upon spools (Fig. 93) to avoid the magnetic effect. The ends of the

coils are attached to binding-posts, or to brass blocks, in regular

instruments, so that one or more coils can be used at a time; that is,

so that they may be handled in a manner similar to that in which the

different coils on the galvanoscope are used.

If we have 4 coils of 1, 2, 2, and 5 ohms resistance, we shall be

able to use any number of ohms from 1 to 10 by making the proper

connections. (See Apparatus Book, chapter XVII, for Home-made

Resistance Coils.)

For protection and convenience, coils are usually placed in a box, the

whole being called a _resistance box_. The ends of the coils are joined

to brass blocks, placed near each other on the top of the box, and

between which may be pressed plugs when it is desired to short circuit

the coils. By removing a plug, the coil, whose ends are joined to the

blocks touching it, is brought into the circuit.

[Illustration: Fig. 93.]

[Illustration: Fig. 94.]

=310. Simple Resistance Coil.= Fig. 94 shows a simple form of

coil, R C (No. 79). The total resistance is 2 ohms, L (left)

and R (right) being binding-posts to which the ends of the

coil, C, are joined. M (middle) connects with the middle of the

wire, at which point the wire is doubled. The coil is fastened

to a stiff pasteboard base, B.

=Connections.= When 2 ohms resistance are wanted, let the

current enter at L and leave at R (or the reverse). When 1 ohm

is wanted, let the current leave or enter at M, the other wire

being joined to L or to R. Connections should be made with

spring connectors. See § 229.

=EXPERIMENT 119. To test the power of various substances to

conduct galvanic electricity.=

_Apparatus._ Galvanoscope, G V (No. 58); dry cell, D C, or

two-fluid cell, 2-F C; pieces of different metals; wood,

dry and damp; tumbler of pure water; rubber; ebonite; silk;

glass, etc., etc. _Arrange_ as in Fig. 92, leaving out R C,

and instead of having M P between wires 1 and 2, use their

free ends to press firmly upon the ends of the substance to be

tested; that is, the body under test should take the place of M

P in the Fig. G V will show a deflection, of course, when the

particular thing under test is a conductor.

=311. Directions.= (A) Make tests with the above substances,

and with any others at hand, and note which are conductors and

which are not.

_=312. Conductors and Nonconductors.=_ It is evident, from the

experiments, that bodies which conduct static electricity do not

necessarily conduct galvanic electricity. The greater the E. M. F.

of a current, the greater its power to overcome resistance. Some

bodies, like dry wood, that readily conduct the high potential static

electricity, make fairly good insulators for the low potential galvanic

currents. For convenience, substances may be divided into _good

conductors_, _partial conductors_, and _insulators_, or nonconductors.

_=Good Conductors.=_ Metals, charcoal, graphite,

acids, etc.

_=Partial Conductors.=_ Dry wood, paper, cotton, etc.

_=Insulators.=_ Oils, porcelain, silk, resin, shellac,

ebonite, paraffine, glass, dry air.

[Illustration: Fig. 95.]

=EXPERIMENT 120. To find the effect of sulphuric acid upon the

conductivity of water.=

_Apparatus._ Galvanoscope, G V; cell; 2-F C; connecting wires;

saucer or tumbler, S; a little sulphuric acid.

_Arrange_ as in Fig. 95.

=313. Directions.= (A) Put a little pure water in S, and see

if enough current can pass through it to deflect the needle of

G V. The ends of the wires, 1 and 2, should be gradually moved

toward each other, the needle being watched.

(B) Put 4 or 5 drops of concentrated acid into the water; stir

it, then repeat the test. What effect has the acid?

_=314. Internal Resistance.=_ As found in Exp. 120, pure water is

not a good conductor of galvanic electricity. The acid in the simple

cell, and in other single-fluid cells, acts upon the zinc and at the

same time makes it possible for the current to pass, as it reduces the

internal resistance.

As seen later, this resistance in cells is greatly diminished by

bringing the plates near each other, and by increasing the surface of

the plates that are in contact with the acid. The larger the plates

the less the internal resistance, other things remaining the same. The

internal resistance of a _battery_ can be changed by connecting the

cells differently. (See Chap. on Arrangement of Cells.)

[Illustration: Fig. 96.]

=EXPERIMENT 121. To find what effect the length of a wire has

upon its electrical resistance.=

_Apparatus._ A No. 30 German-silver wire, G-S W, a little over

two meters long, un-insulated (No. 81); the two-fluid cell, 2-F

C (Exp. 113); galvanoscope, G V (No. 58); plate binding-posts,

X, Y and Z (No. 83-84-85); copper washers (No. 87).

_Arrange_ as in Fig. 96, so that the current will flow, at

first, as shown by the arrow. The metal plates, M P 1 and M

P 2, are used so that the connections may be changed without

disturbing G V. The binding-posts may be fastened directly

to the top of the table; but it will be more convenient to

permanently fix them to a board, B, as shown, so that the

same arrangement can be used for future experiments. The

binding-posts, X and Y, should be about 1/8 in. apart, just far

enough so that their edges do not touch each other.

The binding-post, Z, should be fastened to B with its inside

end 1 meter(100 centimeters, cm.) from the ends of X and Y.

Marks should be made upon B, 10 centimeters apart, as indicated

by the cross lines. This distance may be taken from the scale

on the rule (No. 88).

Fasten one end of the No. 30 wire, G-S W, to X. To do this

twist its end around the screw, S, between X and the copper

washer, then turn the screw in with a screw-driver until it

firmly holds X to the board. Pass the wire around the screw in

Z, and bring its free end to the other binding-post, Y, to be

fastened (Fig. 96). Two meters of wire then form a path for the

current from X to Y. Have the board wide enough so that another

set of binding-posts can be put by the side of Y. It will be

best to permanently leave the No. 30 wire upon the board, and

to fasten the No. 28 wire (next experiment) to another set of

binding-posts, placed in the same manner as those in Fig. 96.

Make holes in the wood with an awl before forcing in the screws.

=315. Note.= This experiment is usually done with a reverser in

the circuit, first taking readings with the current passing in

one direction, and then in the opposite direction. Considerable

time will be saved by taking all the readings for one direction

of the current at a time, simply using different lengths of

German-silver wire, and allowing the current to flow constantly

during each part. This obviates all danger of poor contacts

in the reverser, etc.; it saves the trouble of handling the

reverser, and much of the time needed for the needle to come to

rest.

[Illustration:

+--------------------+----+----+----+---+---+---+---+---+----+---+

|LENGTH OF CIRC., CM.|200 |180 |160 |140|120|100| 80| 60|200 | O |

+--------------------+----+----+----+---+---+---+---+---+----+---+

|DEFLECTION; WEST |26° |28° | 30°| | | | | | 26°|67°|

+--------------------+----+----+----+---+---+---+---+---+----+---+

|DEFLECTION; EAST |25° |27° | 30°| | | | | | 25°|67°|

+--------------------+----+----+----+---+---+---+---+---+----+---+

| AVERAGE |25.5|27.5| 30 | | | | | |25.5|67 |

+--------------------+----+----+----+---+---+---+---+---+----+---+

Fig. 97.]

=316. Directions.= (A) With the circuit arranged as in Fig.

96, and with G V properly placed, take the reading of G V, the

current passing through 200 cm. of No. 30 G-S W. Record your

results in a diagram made like Fig. 97. The row of figures

across the top shows the length of the circuit. The table is

started with results from one experiment. Your results will

probably be different from these.

(B) Get the deflection with the current passing through 180

cm. of wire. To do this press a piece of copper (O, Fig. 96)

upon the wire at the mark 10 cm. from Z, another thin piece of

metal, U, having been slipped under the wire. This will allow

the current to pass across from one wire to the other. Record

the deflection in the col. marked 180.

100, 80, and 60; then repeat (A) to be sure that the cell has

been working uniformly. This deflection should agree with that

in (A).

(D) Change the direction of the current through G V; to do

this, change wire, 1, from M P 2 to M P 1, and wire 5 to M P 2.

This must be done without disturbing G V.

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

the different lengths.

(F) Get the average deflections.

(G) Take, for future use, the deflection produced without G-S

W being in the circuit. Swing the end of wire, 3, that is

joined to Y, around to M P 2. The current will then pass simply

through G V. Record deflection in col. marked O.

=Note.= It is best to do the next experiment at once with the

same cell, so that the results of the two experiments can be

compared. In case this is impossible, get your cell to produce

the same deflection when you use it again, as shown in col. O,

Fig. 97. You can regulate the deflection of the needle of G V

by varying the strength and quantity of the acid in P C.

_=317. Discussion.=_ The resistance of a wire evidently depends

(Exp. 121) upon its length. The _exact_ relation between

resistance and length cannot be seen from these results,

however, which are used in the next experiment. It will be

shown later that in a wire, other things remaining the same,

the resistance varies directly as its length.

=EXPERIMENT 122. To find what effect the size (area of

cross-section) of a wire has upon its electrical resistance.=

_Apparatus._ Same as in last experiment, with one change,

however. Replace the No. 30 G-silver wire with a No. 28

G-silver wire (No. 82), or, what is better still, fasten it to

another set of binding-posts on the board and leave the No. 30

for future use. The two should be stretched side by side for

constant use.

=318. Directions.= (A) See that your cell is in the same

condition as for Exp. 121; that is, it should produce the same

deflection of the needle of G V as before, when the two, only,

are in the circuit. (See Exp. 121, G.) The deflection may be

changed by changing the strength and quantity of the dilute

acid and copper solution.

(B) Find the average deflection of the needle with the 2 meters

of No. 28 G-s wire in the circuit, arranged as in Fig. 96.

in Exp. 121, in order to find what length of the No. 30

wire has the same resistance as 2 meters of No. 28 wire. To

find how many times greater one length is than another, we

divide the larger length by the smaller; hence, to find the

relation between the two lengths of wire that gave the same

deflection,--lengths of equal resistance,--we divide the 200

centimeters (the length of the No. 28) by the length of No. 30

found as directed.

(D) From the wire tables it will be found that the area of

cross-section of No. 28 wire is about 1.59 times that of No.

30 wire. How does this quotient, or ratio, compare with that

found in part (C)? What is the relation between the area of

cross-section of a wire and its resistance? (See § 319, also

Exp. 136.)

_=319. Discussion.=_ If we find that a certain wire, X, which is 576

feet long, has the same resistance as a shorter one, Y, 360 feet long,

we see (576 divided by 360) that the ratio of their lengths is 1.6.

This means that the longer one, X, is 1.6 times as good a _conductor_

as Y; or, in other words, that the _resistance_ of Y is 1.6 times that

of X.

It is easier for water to flow through a large pipe than it is through

a small one. The same general principle is true of electricity. A large

wire offers less resistance to the current than a small one of the same

material. If one wire is twice the size of another of equal length, it

will be twice as good a conductor as the other; that is, it will have

one-half the resistance of the smaller, provided they are of the same

material. (See Laws.)

=EXPERIMENT 123. To compare the resistance of a divided circuit

with the resistance of one of its branches.=

_Apparatus._ Same as in last experiment. Arrange as in Fig. 98.

=320. Directions.= (A) Note the deflection of the needle when

the current passes through 1 meter of G-s wire, as shown. This

will be considered as one branch of the divided circuit.

(B) Still allowing the current to pass as in part (A), press

a piece of copper firmly across the binding-posts X and Y, to

electrically connect them, and note the reading of the needle.

In this case the current divides at Z through the two branches.

What is learned from the results of (A) and (B)?

arranged as in Fig. 99.

[Illustration: Fig. 98.]

[Illustration: Fig. 99.]

The Study of Elementary Electricity and Magnetism by Experiment Containing Two Hundred Experiments Performed with Simple, Home-made Apparatus · 第 17 / 19 章

=EXPERIMENT 80. To study the action of the electrophorus.=