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Rh but its beginning was very small. For a couple of thousand years it made little or no progress, and then, during the course of little more than a century, developed into the giant which it now is. The ancient Greeks were aware that amber, when rubbed with silk, had the property of attracting light bodies; and Dr. Gilbert, about three hundred years ago, showed that many other things, such as sulphur, sealing-wax, and glass, have the same property as amber.

In the progress of the science it came to be known that certain substances are able to carry away the peculiar influence produced, while others are unable to do so; the former are called conductors, and the latter non-conductors, or insulators, of electricity. To make the distinction apparent, let us take a metal rod, having a glass stem attached to it, and rub the glass stem with a piece of silk, care being taken that both silk and glass are warm and dry. We shall find that the glass has now acquired the property of attracting little bits of paper, or elder pith; but only where it has been rubbed, for the peculiar influence acquired by the glass has not been able to spread itself over the surface.

If, however, we take hold of the glass stem, and rub the metal rod, we may, perhaps, produce the same property in the metal, but it will spread over the whole, not confining itself to the part rubbed. Thus we perceive that metal is a conductor, while glass is an insulator, or non-conductor, of electricity.