Why can a bird sit on a live wire without getting a shock,
while a man would get a shock if he reached up and took hold
of the same wire?
We have just been laying emphasis on the fact that for electricity to
flow out of a dynamo or battery, it must have a complete circuit back
to the battery or dynamo. Yet only one wire is needed in order to
telegraph between two stations. Likewise, a single wire could be made
to carry into a building the current for electric lights. This is
because the ground can carry electricity.
If you make all connections from a battery or dynamo just as for any
complete circuit, but use the earth for one wire, the electricity will
flow perfectly well (Fig. 127). To connect an electric wire with the
earth, the wire must go down deep into the ground and be well packed
with earth; but since water pipes go down deep and the earth is
already packed around them, the most convenient way to ground a
circuit is to connect the wire that should go into the ground with the
water pipe. The next experiment, the grounding of a circuit, should be
done by the class with the help of the teacher.
[Illustration: FIG. 127. The ground can be used in place of a wire to
complete the circuit.]
EXPERIMENT 68. _Caution: Keep the switches turned off throughout this
experiment._[6]
[Footnote 6: All through this chapter it is assumed that the
electrical apparatus described in the appendix is being used. In this
apparatus all the switches are on one wire, the other wire being alive
even when the switches are turned off.]
(a) Put a piece of fuse wire across the fuse gap. Screw the
plug with nails in it into the lamp socket. Connect the bare
end of a piece of insulated wire to the water faucet and touch
the other end to one nail of the plug. If nothing happens,
touch it to the other nail instead. The electricity has gone
down into the ground through the water pipe, instead of into
the other wire. The ground carries the electricity back to the
dynamo just as a wire would.
(b) Put a new piece of fuse wire across the gap. _Keep
switches turned off._ Touch the brass disk at the bottom of
an electric lamp to the nail which worked, and touch the wire
from the faucet to the other brass part of the lamp (Fig.
129). What happens?
_Caution: Under no circumstances allow the switch to be turned
on while you are doing any part of this experiment. Under no
circumstances touch the wire from the faucet to the binding
posts of the fuse gap. Do only as directed._ Explain what
would happen if you disobeyed these rules.
[Illustration: FIG. 128. Grounding the circuit. The faucet and water
pipe lead the electricity to the ground.]
WHY A BIRD IS NOT ELECTROCUTED WHEN IT SITS ON A LIVE WIRE. If a man
accidentally touches a live wire that carries a strong current of
electricity he is electrocuted; yet birds perch on such a wire in
perfect safety. If a man should leap into the air and grasp a live
wire, hanging from it without touching the ground, he would be no more
hurt by it than a bird is. A person who is electrocuted by touching
such a wire must at the same time be standing on the ground or on
something connected with it. The ground completes the electric circuit
which passes through the body. An electric circuit can always be
completed through the ground, and when this is done, it is called
_grounding a circuit_.
[Illustration: FIG. 129. How the lamp and wire are held to ground the
circuit.]
_APPLICATION 54._ Explain why only one wire is needed to
telegraph between two stations; why you should not turn an
electric light on or off while standing in a tub of water.
_APPLICATION 55._ In a house in the country, the electric wires
passed through a double wall. They were separated from each
other and well covered with insulation, but they were not
within an iron pipe, as is now required in many cities. The
current was alternating. One night when the lights were out a
rat in the wall gnawed through the insulation of the wire
and also gnawed clear through one of the wires. Did he get a
shock? The next morning, the woman of the house wanted to use
the electric iron in the kitchen and it would not work. The
kitchen had in it a gas stove, a sink with running water, a
table, a couple of chairs, and the usual kitchen utensils.
There was also a piece of wire about long enough to reach
across the kitchen. The electrician could not come out for
several hours, and the woman wanted very much to do her
ironing. Figure 130 is a diagram of the wires and the kitchen.
Show what the woman might have done in order to use her iron
until the electrician arrived.
[Illustration: FIG. 130. How can the electric iron be used after one
wire has been cut?]
_APPLICATION 56._ A man wanted to change the location of the
wiring in his cement-floored garage. While he was working,
would it have been best for him to stand on the bare cement
floor, on a wire mat, on an old automobile tire, on a wet rug,
or on some skid chains that were there?
INFERENCE EXERCISE
Explain the following:
331. An ungreased wheel squeaks.
332. Lightning rods extend into the earth.
333. A banjo player moves his fingers toward the drum end of
the banjo when he plays high notes.
334. When the filament breaks, an electric lamp will no longer
glow.
335. An inverted image is formed by the lens of a camera.
336. A blown-out fuse may be replaced temporarily with a
hairpin or with a copper cent.
337. Sparks fly when a horse's shoe hits a stone.
338. A room requires less artificial light if the wall paper
is light than if it is dark.
339. Phonographs usually have horns, either inside or outside.
340. An electric car needs only one wire to make it go.