hes, 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