Page:Electrical Engineering Volume 1.djvu/18

1450 quite similar to those of well-known mechanical and natural phenomena, and will be best understood by comparison.

2204. Electricity may appear either to reside upon the surfaces of bodies as a charge, under high pressure, or flow through their substance as a current, under comparatively low pressure.

That branch of the science which treats of charges upon the surfaces of bodies is termed electrostatics, and the charges are said to be static charges.

Electrodynamics is that branch which treats of the action of electric currents.

2205. When a glass rod or a piece of amber is rubbed with a piece of silk or fur, the parts rubbed will be found to have the property of attracting light bodies, such as pieces of silk, wool, feathers, gold-leaf, pith, etc., which, after momentary contact, are again repelled. These attractions and repulsions are caused by a static charge of electricity residing upon the surfaces of those bodies. A body in this condition is said to be electrified.

A better experiment for demonstrating this action is to suspend a small pith-ball by a silk thread from a support or bracket, as shown in Fig. 901. Such an apparatus is spoken of as an electric pendulum. If a static charge of electricity be developed on a glass rod by rubbing it with silk, and the rod be brought near the pendulum, the ball will be attracted to the rod, but after momentary contact will be repelled. By this contact the ball becomes electrified, and so long as the two bodies retain their