me when the wood catches fire. After you have
done this two or three times, the inside of the wood below the
notches will be burned out so completely that you can pull it
off with your fingers, leaving the lead bare all the way up to
the wires.

Let the class stand well back and watch the teacher do the
next part of the experiment.

Connect two heavy insulated copper wires, about No. 12, to
the sides of the knife switch just as you connected the
fine wires. But this time bring the ends of the copper wires
themselves together for an instant, then draw them apart. Hold
the ends of the wires over the zinc of the table while you do
this, as melted copper will drop from them.

[Illustration: FIG. 133. The pencil points are touched together and
immediately drawn apart.]

[Illustration: FIG. 134. A brilliant arc light is the result.]

WHAT HAPPENS WHEN AN ARC IS FORMED. What happens when you form an
electric arc is this: As you draw the two ends of the pencils apart,
only a speck of the lead in each touches the other. The electricity
passing for an instant through the last speck at the end of the pencil
makes it so hot that it turns to vapor. The vapor will let electricity
go through it, and makes a bridge from one pencil point to the other.
But the vapor gets very hot, because it has a rather high resistance.
This heat vaporizes more carbon and makes more vapor for the
electricity to pass through, and so on. The electricity passing
through the carbon vapor makes it white hot, and that is what causes
the brilliant glow. Regular arc lights are made exactly like this
experimental one, except that the carbons used are much bigger and are
made to stand the heat better than the small carbons in your pencil.

Carbon is one of those substances that turn directly from a solid to
a gas without first melting. That is one reason why it is used for arc
lights. But copper melts when it becomes very hot, as you saw when you
made an arc light with the copper wires. So copper cannot be used for
practical arc lights.

FIRES CAUSED BY ARCS. There is one extremely important point about
this experiment with arcs: most fires that result from defective
wiring are caused by the forming of arcs. You see, if two wires touch
each other while the current is passing and then move apart a little,
an arc is formed. And you have seen how intensely hot such an arc is.
Two wires rubbing against each oth