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 switc