Page:Popular Science Monthly Volume 9.djvu/51

Rh law. Repeat, for example, Otto von Guericke's experiment. Hang a leather by a silk thread, and bring your rubbed glass tube near it: the feather is attracted, touches the rod, charges itself with the electricity of the rod, and is then repelled. Cause it to retreat from the rod in various directions.

Hang your feather by a common thread: if no insulating substance intervenes between the feather and the earth, you can get no repulsion. Why? you ought to be able to answer. Obviously it is because the charge of positive electricity communicated by the rod is not retained by the feather, but passes away to the earth. Hence, you have not positive acting against positive at all. Why you should have the attraction of the neutral body by the electrified one will, as already stated, appear by-and-by.

Attract your straw needle by your rubbed glass rod. Let the straw strike the rod, so that the one shall rub against the other. The straw accepts the electricity of the rod, and repulsion immediately follows attraction, as shown in Fig. 7.



Mr. Cottrell has devised the simple electroscope represented in Fig. 8 to show repulsion. A is a stem of sealing-wax, with a small circle of tin, T, at the top. W is a bent wire proceeding from T, with a small disk attached to it by wax. I I'  is a little straw index, supported by the needle, N, as shown in the figure. The stem, A, is not quite vertical, the object being to cause the bit of paper, I, to rest close to W when the apparatus is not electrified. When electricity is imparted to T, it flows through the wires, W and w, over both disk and index: immediate repulsion of the straw is the consequence.

No better experiment can be made to illustrate the self-repulsive character of electricity than the following one: Heat your square board again, and warm, as before, your sheet of foolscap. Spread the paper upon the board, and excite it by the friction of India-rubber. Cut from the sheet two long strips with your penknife. Hold the strips together at one end. Separate them from the board, and