This is one of the most important sections in the book.
Do you know that you can make an arc light with two ordinary pencils? The next experiment, which should be done by the class with the help of the teacher, shows how to do it.
Experiment 71. Sharpen two pencils. About halfway between the point and the other end of each pencil cut a notch all the way around and down to the "lead," or burn a notch down by means of the glowing resistance wire. What you call the "lead" of the pencil is really graphite, a form of carbon. The leads of your two pencils are almost exactly like the carbons used in arc lights, except, of course, that they are much smaller. Turn off the electricity both at the snap switch and at the knife switch. Fasten the bare end of a 2-foot piece of fine insulated wire (about No. 24) around the center of the lead in each pencil so that you get a good contact, as shown in Figure 132. Fasten the other bare end of each wire to either side of the open knife switch so that when this switch is open the electricity will have to pass down one wire to the lead of one pencil, from that to the lead of the other pencil, and from that back through the second wire to the other side of the knife switch and on around the circuit, as shown in Figure 133. Keep the two pencils apart and off the desk, while some one turns on the snap switch and the "flush" switch that lets the electricity through the resistance wire. Now bring the pencil points together for an instant, immediately drawing them apart about half an inch. You should get a brilliant white arc light.
Fig. 133. The pencil points are touched together and immediately drawn apart.
Caution: Do not look at this brilliant arc for more than a fraction of a second unless you look through a piece of smoked or colored glass.
Blow out the flame 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.
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.