You will notice that the two wires running out from the dynamo (which
are called the main wires) do not form one continuous loop as in the
arc-light system, but that a smaller wire is attached to one of the
main wires and then connected with the screw-ring in the lamp-socket;
then another wire is connected with the button in the socket and
afterward to the other main wire. Every lamp forms an independent path
through which the current can travel back to the dynamo.
Now, if we turn one of these incandescent lamps out, we simply shut off
one of these paths and the electricity travels through the other lamps,
and, if we wish, we can turn out all the lamps but one and there will
still be a way for the electricity to go back to the dynamo.
In the arc-lamps we must have a very high number of volts pressure,
because the electricity has only one path, and it all has to pass
through the first and other lamps till it comes to the last one. In
the incandescent light the electricity has as many paths as there are
lamps, so we only need to keep _one_ certain _pressure_ in volts in the
main wires all the time. This pressure is _even_ all the way through
the main wires, and, therefore, it is ready to light a lamp the instant
it is turned on, because, as you have seen, electricity will always get
back to the dynamo if there is a possible chance, and the lamp opens a
path.
The volts pressure used to operate any number of incandescent lamps is
altogether very much less than for a number of arc-lights. For example,
in the Edison system the pressure (sometimes called "electromotive
force") is only about 110 volts, which is very mild and not at all
dangerous. This electromotive force would be _the same_ if there were
_one lamp or ten thousand_ lighted.
While this Edison current would not hurt any one, you should remember
that it is much the better plan not to touch _any_ electric-light wires
until you have learned a great deal more on this subject.
We may add that each of the standard incandescent lamps requires only
about one-quarter of an ampère of current to make them give a light of
16 candle-power, which is about the light given by a very good gas-jet,
and while the electromotive force, or pressure, would only be about
110 volts, whether there were one lamp or ten thousand lighted, there
must be sufficient ampères in the wires to give each lamp its proper
quantity.