The electric field. It was known to the Greeks and Romans that when pieces of amber are rubbed they acquire the power of attracting to themselves light bodies. There are other substances which possess the same property; thus, if a stick of sealing-wax is rubbed on a piece of dry cloth it will attract bran or small scraps of paper sufficiently near to it. The same result is obtained if a glass rod is rubbed with a dry piece of silk. It is also found that the cloth and the silk acquire the same property as the sealing-wax and the glass rod. Further, if the sealing-wax is suspended so that it is free to move, it is found that the cloth attracts the sealing-wax, but two pieces of sealing-wax similarly treated repel one another. In the same way the glass rod and the silk attract one another, but two such glass rods repel one another.

We describe bodies in such a state as electrified or charged with electricity. The word was derived by William Gilbert from ἤλ∊κτρον or amber, the first substance upon which such experiments were performed.

If we experiment further we find that an electrified stick of sealing-wax is attracted by an electrified glass rod, but repelled by the piece of silk with which the rod has been rubbed.

These and kindred phenomena are explained by the statement that electricity is of two kinds, and that charges of the same kind repel while charges of opposite kinds attract one another.