Excerpt from The Principles of Radiography

William Gilbert but the effect itself was known to the Greek philosophers. In Spite of the immense amount of electrical research, we know very little more as to its real nature than they did.

Although both a rubbed glass rod and a rubbed ebonite rod attract light objects, there is a difference between the electricity excited in the two cases. If an ebonite rod is rubbed with flannel and suspended on a stirrup so as to be free to turn, and a second ebonite rod, electrified in the same way, is brought near it, it will be found that the two rods repel each other. If, however, the charged glass rod is brought near the suspended ebonite rod, the latter will be attracted. Similarly an electrified glass rod is repelled by a second charged glass rod, while it is attracted by a charged ebonite rod. There are thus two kinds Of electricity, each of which repels other electrical charges of the same kind as itself, while it attracts charges of the other kind. This is usually expressed by saying that like charges repel; unlike charges attract.

The kind of electricity excited on a glass rod by rubbing it with Silk was called vitreous, but is now known as positive electricity while the kind excited on an ebonite rod by rubbing it with flannel was called resinous, but is now always called negative electricity. The terms positive and negative arose from the fact that if equal quantities of vitreous and resinous electricity are placed on the same body, the latter shows no Signs of electrification. The result of adding together equal quantities of vitreous and resinous electricity is thus zero, and this result can be expressed mathematically by assign ing a positive sign to the one and a negative Sign to the other. It is therefore a convenience in the mathematical theory of electricity to regard the two kinds Of electricity as being of opposite Sign.

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