Before closing the subject of electric light you would perhaps like to
know something about the way in which we place the wires leading to
the lamps.
[Illustration: Fig. 25]
If you remember what we told you about measurements in the beginning of
this book, it will be easy to understand what follows:
You know that if you have a very great pressure you can force a
quantity through a small conductor. This is the principle upon which
the arc-lamps are run. Every arc-lamp takes about 40 to 50 volts and
from 5 to 10 ampères to produce the light, and they are connected with
the wires as shown in Fig. 25.
This is called running lamps in "series," and, as you will see from
the sketch, the wire starts out from the dynamo and connects with one
carbon of the first arc-lamp, and to the other carbon is connected
another wire which goes on to the next lamp, and so on until the last
lamp is reached, and then the wire goes back to the dynamo. This forms,
practically, one continuous loop from one brush to the other of the
dynamo.
The current starts out, makes its way through the first lamp, goes on
to the next, makes its way through that, and so on till it has jumped
the last one; then it goes back to the dynamo.
Now, as each of these jumps requires a pressure of 40 or 50 volts, you
will easily see that the total pressure, in volts, of the electricity
must be as many times 40 or 50 volts as there are lamps to be lighted;
so, if there were 60 lamps in circuit, there would be 2,400 to 3,000
volts pressure, which, while it gives very fine lights, might cause
instant death to any one touching the wires.
Suppose anything happened to the first lamp, which stopped the current
from jumping through it. There would be no path for the current to
travel farther, and, consequently, all the lights would go out. To get
over this difficulty there is sometimes used what is called a "shunt,"
which only acts when the lamp will not light. This shunt carries the
current round the lamp to the other wire, so that it may travel on and
light up the other lamps.