Those who have seen the arc-lamps will readily recognize them from the
picture in Fig. 22.
You will see that there are two sticks, or "pencils," of carbon. Now
you will remember that in the chapter on Magnetism we told you that _in
order to have electricity do work for us we must put some resistance or
opposition in its way_. When we get light from an electric lamp it is
because we make the electricity do some work in the lamp, and this work
is in pushing its way through a resistance or opposition which is in
the lamp.
[Illustration: Fig. 22]
When we generate electricity in the dynamo and put two wires for it to
travel in, the current goes away from the dynamo through one of the
wires and will go back to the dynamo through the other one if it can
possibly get a chance to get to this other one. Now, the electricity
which is constantly being made fills the wires and acts as a pressure
to force the current through the wires back to the dynamo, and, if we
put no resistance or opposition in the way, it would have a very easy
path to travel in and would do no work at all. The wires leading to an
electric lamp should have very little resistance, not sufficient to
require any work from the current in passing through.
So, if we bring the two carbons in an arc-lamp together they really
form part of the wire, and do not interrupt the current in its
travels, but, if we _separate the carbons_, we make a gap which the
current must jump across if it wants to go on. As the volts, or
pressure, is so great, the current must jump, and this _against the
resistance or opposition_ in an arc-lamp is that which gives the
current so much work to do. Indeed, so hard is it for the current to
jump across this gap that it breaks off from one carbon a shower of
tiny particles as fine as the finest dust, and makes them white hot in
passing to the other. This shower of fine carbon dust, together with
the ends of the carbons, being white hot, of course makes a light, and
this is the dazzling light which you see in the arc-lamp.
Of course, when the electricity has jumped over from one carbon to the
other, it goes through it to the wire, and so passes on to the next
lamp, where it has to jump again, and so on until it has gone through
the last lamp, then it has an easy path to get back to the dynamo.
Now, we want you to understand more thoroughly how that much resistance
or opposition will cause heat, so we will try to give you a simple
example.
Most of you know that if you were holding a rope tightly in your hands
and some one pulled it through them quickly and suddenly, it would get
very hot and your hands would feel as though they were being burned.
This is heat caused by your hands resisting or opposing the passage of
the rope through them, and if you could hold on tightly enough and the
rope was drawn through quickly enough, it would take fire. This fire
would, therefore, cause heat and light.
It is just this principle of resistance to the passage of the current
which causes the light in an arc-lamp, as we have shown you.