Page:On the various forces of nature and their relations to each other.djvu/146

piece of brass and bring it near the machine, you see how the electricity leaves the latter, and passes to the brass cylinder. And, again, if I take a rod of metal and touch the machine with it, I lower the indicator; but when I touch it with a rod of glass, no power is drawn away,—shewing you that the electricity is conducted by the glass and the metal in a manner entirely different: and to make you see that more clearly, we will take one of our Leyden jars. Now, I must not embarrass your minds with this subject too much; but if I take a piece of metal, and bring it against the knob at the top and the metallic coating at the bottom, you will see the electricity passing through the air as a brilliant spark. It takes no sensible time to pass through this; and if I were to take a long metallic wire, no matter what the length—at least as far as we are concerned—and if I make one end of it touch the outside, and the other touch the knob at the top, see how the electricity passes!—it has flashed instantaneously through the whole length of this wire. Is not this different from the transmission of heat through this copper bar (fig. 42), which has