Page:Popular Science Monthly Volume 9.djvu/183

Rh, will answer. Let a small weight suspended from a silk string rest on one end of the conductor, and hold your rubbed glass rod near the other end. You can predict beforehand what will occur when you remove the weight. It carries away with it electricity, which repels rubbed glass, and which attracts your balanced lath.

Stand on an insulating stool: make one, if necessary, by placing a board on four warm tumblers. Present the knuckles of your right hand to the end of the balanced lath, and stretch forth your left arm. There is no attraction. But let a friend or an assistant bring the rubbed glass tube over the left arm; the lath immediately follows the right hand.

While matters continue thus, touch the lath, which I suppose to be uninsulated; the "attractive virtue," as it was called by Gray, disappears. After this, as long as the excited tube is held over the arm there is no attraction. But when the tube is removed the attractive power of the hand is restored. Here, you will at once comprehend, the first attraction was by positive electricity driven to the right hand from the left, and the second attraction by negative electricity, liberated by the removal of the glass rod.

Stand on an insulating stool, and place your right hand on the electroscope: there is no action. Stretch forth the left arm and permit an assistant alternately to bring near, and to withdraw, an excited glass tube. The gold-leaves open and collapse in similar alternation. At every approach, positive electricity is driven over the gold-leaves; at every withdrawal, the equilibrium is restored.

I will now ask you to charge your Dutch gold electroscope positively by rubbed gutta-percha, and to charge it negatively by rubbed glass. A moment's reflection will enable you to do it. You bring your excited body near: the same electricity as that of the excited body is driven over the leaves, and they diverge by repulsion. Touch the electroscope, the leaves collapse. Withdraw your finger, and withdraw afterward the excited body: the leaves then diverge with the opposite electricity.

The simplest way of testing the quality of electricity is to charge the electroscope with electricity of a known kind. If, on the approach of the body to be tested, the leaves diverge still wider, the leaves and the body are similarly electrified. The reason is obvious.

The wealth of knowledge, and of interest, which these experiments involve, may be placed within any boy's reach by the wise expenditure of half a crown.

Once firmly possessed of the principle of induction and versed in its application, the difficulties of our subject will melt away before us. In fact, our subsequent work will consist mainly in unraveling phenomena by the aid of this principle.

Without a knowledge of this principle we could render no account