How does electricity travel?
Why do you get a shock if your hands are wet when you touch a
live wire?
If you were to use a piece of string instead of a copper wire to go
from one pole of a battery to another or to spin between the poles
of the magnet of the dynamo, you could get no flow of electricity to
speak of. Electrons do not flow through string easily, but they flow
through a copper wire very easily. Anything that carries, or conducts,
electricity well is called a _good conductor_. Anything that
carries it poorly is called a _poor conductor_. Anything that
allows practically no electricity to pass through it is called an
_insulator_.
EXPERIMENT 65.[5] Turn on an electric lamp. Turn it off by
opening the knife switch. Cover the blade of the knife switch
with a fold of paper and close it. Will the lamp glow? Try a
fold of dry cloth; a fold of the same cloth wet. Connect the
blade to the slot with a piece of iron; with a piece of glass;
with porcelain; with rubber; with dry wood; with wood that is
soaking wet; with a coin. Which of these are good conductors
of electricity? Which could be used as insulators?
[Footnote 5: Read footnote, page 226, before doing this experiment.]
[Illustration: FIG. 119. Electricity flows through the coin.]
HOW YOU CAN GET AN ELECTRICAL SHOCK. A person's body is not a very
good conductor of electricity, but will conduct it somewhat. When
electricity goes through your body, you get a shock. The shock from
the ordinary current of electricity, 110 volts, is not enough to
injure you at all; in fact, if you were standing on dry wood, it would
be _safe_, although you would get a slight shock, to connect the blade
of a knife switch to the slot of the switch, through your hand or
body. Your body would not allow enough current to pass through it to
light the lamp. Stronger currents, like those of power lines and even
trolley wires, are extremely dangerous.
All the electric wires entering your house are made of copper. They
are all covered with cloth and rubber and are fastened with glass
or porcelain knobs. The reason is simple: Copper and practically all
other metals are very good conductors of electricity; that is, they
allow electricity to pass through them very easily. Cloth, rubber,
glass, and porcelain are very poor conductors, and they are therefore
used as insulators,--to keep the electricity from going where you do
not want it to go.
[Illustration: FIG. 120. Will electricity go through the glass?]
EXPERIMENT 66. To each binding post of an electric bell fasten
a piece of insulated copper wire with bare ends and at least 4
feet long. Connect the free end of one of these wires with one
pole of a battery, using a regular laboratory battery or one
you made yourself. Attach one end of another piece of wire
a foot or so long, with bare ends, to the other pole of the
battery. Touch the free end of this short wire to the free end
of the long wire, as shown in Figure 120. Does the bell ring?
If it does not, something is wrong with the connection or with
the battery; fix them so that the bell will ring. Now leave a
gap of about an inch between the free end of the long wire
and the free end of the short wire. Try making the electricity
flow from the short wire into the long one through a number of
different things, such as string, a key, a knife, a piece of
glass tubing, wet cloth, dry cloth, rubber, paper, a nail,
a dish of mercury (dip the ends of the wire into the dish so
that they both touch the mercury at the same time), a dish of
water, a stone, a pail, a pin, and anything else that you may
like to try.
[Illustration: FIG. 121. Electrical apparatus: _A_, plug fuse; _B_,
cartridge fuse; _C_, knife switch; _D_, snap switch; _E_, socket with
nail plug in it; _F_, fuse gap; _G_, flush switch; _H_, lamp socket;
_I_, _J_, _K_, resistance wire.]
Each thing that makes the bell ring is a good conductor. Each one that
will not make it ring is a poor conductor or an insulator. Make a list
of the things you have tried; in one column note the good conductors,
and in another column note the insulators and poor conductors.
The water and wet cloth did not ring the bell, but this is because the
pressure or voltage of the electricity in the batteries is not very
high. In dealing with high-power wires it is much safer to consider
water, or anything wet, as a pretty good conductor of electricity.
Absolutely pure, distilled water is an extremely poor conductor; but
most water has enough minerals dissolved in it to make it conduct
electricity fairly well. In your list you had better put water and wet
things in the column with the good conductors.
_APPLICATION 50._ Robbers had cut the telegraph line between two
railroad stations (Fig. 122). The broken ends of the wire fell to
the ground, a number of feet apart. A farmer caught sight of the men
speeding away in an automobile and he saw the cut wires on the
ground. He guessed that they had some evil purpose and decided to
repair the damage. He could not bring the two ends of the wire
together. He ran to his barn and found the following things there:
A ball of cord, a pickax, a crowbar, some harness, a wooden wagon
tongue, a whip, a piece of iron wire around a bale of hay (the wire
was not long enough to stretch the whole distance between the two
ends of the telegraph wire, even if you think he might have used it
to patch the gap), a barrel with four iron hoops, and a rope.
Which of these things could he have made use of in connecting the
broken ends of the telegraph wire?
[Illustration: FIG. 122. Which should he choose to connect the broken
wires?]
_APPLICATION 51._ A man was about to put in a new socket for an
electric lamp in his home. He did not want to turn off the current
for the whole house, as it was night and there was no gas to furnish
light while he worked.
"I've heard that if you keep your hands wet while you work, the film
of water on them will keep you from getting a shock," his wife said.
"Don't you wet your hands, Father," said his 12-year-old boy; "keep
them dry, and handle the wires with your pliers, so that you won't
have to touch it."
"I advise you to put on your canvas gloves while you work; then you
can't get a shock," added another member of the family.
"That's a good idea," said the wife, "but wet the gloves, then you
will have the double protection of the water and the cloth."
The man laughed and went to work on the socket. He did not get a
shock. Which advice, if any, do you think he followed?
INFERENCE EXERCISE
Explain the following:
311. A red postage stamp looks greenish gray in the green
light of a mercury-vapor lamp.
312. Cracks are left between sections of the roadbed in cement
auto highways.
313. Electricity goes up a mountain through a wire.
314. It is impossible to stand sidewise against a wall on one
foot, when that foot touches the wall.
315. A charged storage battery will run an electric
automobile.
316. An empty house is noisier to walk in and talk in than is
a furnished one.
317. Lightning rods are made of metal.
318. It is harder to hold a frying pan by the end of the
handle than by part of the handle close to the pan.
319. Diamonds flash many colors.
320. In swimming, if you have hold of a fastened rope and try
to pull it toward you, you go toward it.