Page:Popular Science Monthly Volume 9.djvu/50

 32 surface. Rub your sealing-wax with vulcanized India-rubber, the electricity of glass will be found upon the resinous surface.

We now use the term positive electricity to denote that developed on glass by the friction of silk; and negative electricity to denote that developed on sealing-wax by the friction of flannel. These terms are adopted purely for the sake of convenience. There is no reason in Nature why the resinous electricity should not be called positive, and the vitreous electricity negative. Once agreed, however, to apply the terms as here fixed, we must adhere to this agreement throughout.

. 10. Fundamental Law of Electric Action.—In all the experiments which we have hitherto made, one of the substances has been electrified by friction, and the other not. But, once engaged in inquiries of this description, questions incessantly occur to the mind, the answering of which extends our knowledge, and suggests other questions. Suppose, instead of exciting only one of the bodies presented to each other, we were to excite both of them, what would occur? This is the question which was asked and answered by Du Fay, and which we must answer for ourselves.

Here your wire loop (Fig. 1), comes again into play. Place an unrubbed gutta-percha tube, or a stick of sealing-wax, in the loop, and be sure that it is unrubbed—that no electricity adheres to it from former experiments. If it fail to attract light bodies, it is unexcited; if it attract them, pass your hand over it several times, or, better still, pass it over or through the flame of a spirit-lamp or candle. This will remove every trace of electricity. Attract the unrubbed guttapercha tube by a rubbed one.

Remove the unrubbed tube from the loop, and excite it with its flannel rubber. One end of the tube is held in your hand, and is therefore unexcited. Return the tube to the loop, keeping your eye upon the excited end. Bring a second rubbed tube near the excited end of the suspended one: strong repulsion is the consequence. Drive the suspended tube round and round by this force of repulsion.

Bring a rubbed glass tube near the excited end of the gutta-percha tube: strong attraction is the result.

Repeat this experiment step by step with two glass tubes. Prove that the rubbed glass tube attracts the unrubbed one. Remove the unrubbed tube from the loop, excite it by its rubber, return it to the loop, and establish the repulsion of glass by glass. Bring rubbed gutta-percha or sealing-wax near the rubbed glass: strong attraction is the consequence.

These experiments lead us directly to the fundamental law of electric action, which is this: Bodies charged with the same electricity repel each other, while bodies charged with opposite electricities attract each other. Positive repels positive, and attracts negative. Negative repels negative, and attracts positive.

Devise experiments which shall still further illustrate this