Why does a doorbell ring when you push a button?
Why is it that when you touch one electric wire you feel no
shock, while if you touch two wires you sometimes get a shock?
When a wire is broken in an electric light, why does it not
light?
Suppose you have some water in an open circular trough like the one
shown in Figure 123. Then suppose you have a paddle and keep pushing
the water to your right from one point. The water you push pushes
the water next to it, that pushes the water next to it, and so on all
around the trough until the water just behind your paddle pushes in
toward the paddle; the water goes around and around the trough in
a complete circuit. There never is too much water in one place; you
never run out of water. But then suppose a partition is put across the
trough somewhere along the circuit. When the water reaches that, it
cannot pass; it has no place to flow to, and the current of water
stops.
THE ELECTRIC CIRCUIT. The flow of electricity in an electric circuit
may be compared to the flow of the water in the tank we have been
imagining. The long loop of wire extending out from the dynamo to
your house and back again corresponds to the tank. The electricity
corresponds to the water. Your dynamo pushes the electricity around
and around the circuit, as the paddle pushes the water. But let some
one break the circuit by putting a partition between two parts of
it, and the electricity immediately stops flowing. One of the most
effective partitions we can put into an electric circuit is a gap of
air. It is very difficult for any electricity to flow through the air;
so if we simply cut the wire in two, electricity can no longer flow
from one part to the other, and the current is broken.
[Illustration: FIG. 123. Electricity flows around a completed circuit
somewhat as water might be made to flow around this trough.]
BREAKING AND MAKING THE CIRCUIT. The most convenient way to put an air
partition into an electric circuit and so to break it, or to close the
circuit again so it will be complete, is to use a switch.
EXPERIMENT 67. In the laboratory, examine the three different
kinds of switches where the electricity flows into the lamp
and resistance wire and then out again. Trace the path the
electricity must take from the wire coming into the building
down to the first switch that it meets; then from one end
of the wire through the brass or copper to which the wire is
screwed, through the switch and on out into the end of the
next piece of wire. Turn the first switch off and see how
a partition of air is made between the place where the
electricity comes in and the place where it would get out if
it could. Turn the switch on and notice how this gives the
electricity a complete path through to the next piece of wire.
In this way follow the circuit on through all the switches to
the electric lamp.
If you examine the socket into which the lamp screws and examine the
lamp itself, you will see that electricity which goes to the outer
part of the socket passes into the rim of the lamp; from here it goes
into one end of the filament. It passes through the filament to the
other end, which is connected to the little brass disk at the end of
the lamp. From this you can see that it goes into the center point
of the socket, and then on into the second wire that connects to the
socket. Trace the current on back through this other wire until you
see where this wire leads toward the dynamo. You should understand
that the electric lamp, the switches, the fuses, all things along the
circuit, are simply parts of the long loop from the dynamo, as shown
in Figure 124.
CONNECTING IN PARALLEL. The trouble with Figure 124 is that it is a
little too simple. From looking at it you might think that the loop
entered only one building. And it might seem that turning off one
switch would shut off the electricity all along the line. It would,
too, if the circuit were arranged exactly as shown above. To avoid
this, and for other reasons, the main loop from the dynamo has
branches so that the electricity can go through any or all of them at
the same time and so that shutting off one branch will not affect the
others. Electricians call this _connecting in parallel_; there are
many parallel circuits from one power house.
[Illustration: FIG. 124. Diagram of the complete circuit through the
laboratory switches.]
Figure 125 illustrates the principle just explained. As there
diagrammed, the electricity passes out from the dynamo along the lower
wire and goes down the left-hand wire of circuit _A_ through one of
the electric lamps that is turned on, and then it goes back through
the right-hand wire of the _A_ circuit to the upper wire of the
main circuit and then on back to the dynamo. But only a part of the
electricity goes through the _A_ circuit; part goes on to the _B_
circuit, and there it passes partly through the electric iron. Then it
goes back through the other wire to the dynamo. No electricity can get
through the electric lamp on the _B_ circuit, because the switch
to the lamp is open. The switch on the _C_ circuit is open; so no
electricity can pass through it.
The purpose of the diagram is to show that electricity from the
dynamo may go through several branch circuits and then get back to the
dynamo, and that shutting off the electricity from one branch circuit
does not shut it off from the others. And the purpose of this section
is to make it clear that electricity can flow only through a complete
circuit; it must have an unbroken path from the dynamo back to the
dynamo again or from one pole of the battery back to the other pole.
If the electricity does not have a complete circuit, it will not flow.
_APPLICATION 52._ A small boy disconnected the doorbell
batteries from the wires that ran to them, and when he wanted
to put the wires back, he could not remember how they had been
connected. He tried fastening both wires to the carbon part of
the battery, connecting one wire to the carbon and one to the
zinc, and connecting both to the zinc. Then he decided that
one wire was all that had to be connected anyway, that the
second was simply to make it stronger. Which of the ways he
tried, if any, would have been right?
[Illustration: FIG. 125. Parallel circuits.]
[Illustration: FIG. 126. How should he connect them?]
_APPLICATION 53._ Dorothy was moving. "When they took out our
telephone," she said to her chum, Helen, "the electrician just
cut the wires right off."
"He must have turned off the electricity first," Helen
answered, "or else it would all have run out of the cut ends
of the wire and gone to waste."
"Why, it couldn't," Dorothy said. "Electricity won't flow off
into the air."
"Of course it can if there is nothing to hold it in," Helen
argued.
Which was right?
INFERENCE EXERCISE
Explain the following:
321. It is very easy to get chilled when one is perspiring.
322. Ice cream becomes liquid if you leave it in your dish too
long.
323. You should face forward when alighting from a street car.
324. There are always at least two electric wires going into a
building that is wired.
325. Woolen sweaters keep you warm.
326. Steel rails are not riveted to railroad ties but the
spikes are driven close to each rail so that the heads hook
over the edge and hold the rail down without absolutely
preventing its movement forward and backward. Why should rails
be laid in this way?
327. The earth keeps whirling around the sun without falling
into it, although the pull from the sun is very great.
328. Electricity is brought down from power houses in the
mountains by means of cables.
329. White clothes are cooler than black when the person
wearing them is out in the sun.
330. All the street cars along one line are stopped when a
trolley wire breaks.