Page:Popular Science Monthly Volume 9.djvu/179

Rh This is a fit place to say that you must keep a close eye upon the tumblers you employ for insulation. Some of them, made of common glass, are hardly to be accounted insulators at all. We shall prove this.

Our mastery over this subject of induction must be complete, for it underlies all our subsequent inquiries. Without reference to it nothing is to be explained; possessed of it you will enjoy, not only a wonderful power of explanation, but of prediction. We will attack it, therefore, with the determination to exhaust it.

And here a slight addition must be made to our apparatus. We must be in a condition to take samples of electricity, and to convey them, with the view of testing them, from place to place. For this purpose the little "carrier," shown in Fig. 10, will be found convenient. T is a bit of tin-foil, two or three inches square. A straw stem is stuck on to it by sealing-wax, the lower end of the stem being covered by sealing-wax. To make the insulation sure, the part between R and S'  is wholly of sealing-wax. You can have stems of ebonite, which are stronger, for a few pence; but you can have this one for a fraction of a penny. The end M'  is to be held in the hand; the electrified body is to be touched by T, and the electricity conveyed to an electroscope to be tested.

Touch your rubbed glass rod with T, and then touch your electroscope: the leaves diverge with positive electricity. Touch your rubbed gutta-percha or sealing-wax with T, and then touch your electroscope: the leaves diverge with negative electricity. If the electricity of any body augment the divergence produced by the glass, the electricity of that body is positive. If it augment the divergence produced by the gutta-percha, the electricity is negative. And now we are ready for further work.