Fig. 31. The Electric Arc.
"Why doesn't it burn out?"
"It does burn out in time. What preserves it, however, for a long time, is that most of the oxygen has been exhausted from the bulb——"
"Oh, yes; I know, there must be oxygen to support combustion, so that the carbon is merely heated up?"
"I am glad you remembered that. The arc light, on the other hand, depends on an entirely different thing. You have seen, no doubt, the long black pencils used in the large lamps. That is carbon also, made out of ground coke, molded and compressed into shape."
"What does arc mean!"
"Did you notice that when we put together the two circuiting ends of the wires in our battery this morning, we could not notice the existence of a current, but whenever we pulled them apart we had a spark?