Page:Popular Science Monthly Volume 8.djvu/630

 612 plain in any weather. It is rendered electrical by friction against the hair, and with it you can pull the lath quite round.

If you moisten the hair with oil, the comb will still be excited and exert attraction; but, if you moisten it with water, the excitement ceases; a comb passed through wetted hair has no power over the lath.

After its passage through dry or oiled hair, balance the comb itself upon the egg; it is attracted by the lath. You thus prove the attraction to be mutual: the comb attracts the lath, and the lath attracts the comb. Suspend your rubbed glass, rubbed gutta-percha, and rubbed sealing-wax in your wire loop. They are all just as much attracted by the lath as the lath was attracted by them. This is an extension of Boyle's experiment with the suspended amber.

How it is that the unelectrified lath attracts, and is attracted by the excited glass, sealing-wax, and gutta-percha, we shall learn by-and-by.

A very striking illustration of electric attraction may be obtained with the board and India-rubber mentioned in our list of materials. Place the board before the fire and make it hot; heat also a sheet of foolscap paper and place it on the board. There is no attraction between them. Pass the India-rubber briskly over the paper. It now clings firmly to the board. Tear it away, and hold it at arm's length, for it will move to your body if it can. Bring it near a door or wall, it will cling tenaciously to either. The electrified paper also powerfully attracts the balanced lath from a great distance.

The friction of the hand, of a cambric handkerchief, or of wash-leather, fails to electrify the paper in any high degree. It requires friction by a special substance to make the excitement strong. This we learn by experience. It is also experience that has taught us that resinous bodies are best excited by flannel, and vitreous bodies by silk.

Take nothing for granted in this inquiry, and neglect no effort to render your knowledge complete and sure. Try various rubbers, and satisfy yourself that differences like that first observed by Newton exist between them.

Lay bare, also, the true influence of heat in our last experiment. Spread a cold sheet of foolscap on a cold board—on a table, for example. If the air be not very dry, rubbing, even with the India-rubber, will not make them cling together. But is it because they were hot that they attracted each other in the first instance? No, for you may heat your board by plunging it into boiling water, and your paper by holding it in a cloud of steam. Thus heated they cannot be made to cling together. The heat really acts by expelling the moisture. Cold weather, if it be only dry, is highly favorable to electric excitation. During the late frost the whisking of the hand over silk or flannel, or over a cat's back, would have rendered it electrical.