[Illustration: Fig. 214.—Discharging rod.]
The _Electric Battery_ (_see_ fig. 206) is formed by a collection of Leyden jars. The inside and outside coatings are connected in a box divided into partitions lined with tinfoil. The rods of the jars are also connected, as in the illustration, by brass rods, and when this battery is charged people should be careful how they handle it, for a shock may be produced which would cause serious injury, if not death. The battery can be charged from the machine by a chain fastened to the central ball, while a second chain connects the exterior of the box and all the outside of the jars, by means of the handle, to the ground.
When electricity is at rest it is termed “static electricity,” and when in motion “dynamic” electricity. The latter treats of electric currents which can be sent through wires or chains. We can keep this current moving by means of a machine, and the battery called a Voltaic battery, from Volta. We will describe it presently. Electric currents can be measured, for they may be of different strengths according to the battery, and they are measured by the GALVANOMETER. Electricity can therefore be transferred and carried by the conducting substances, and much heat will be engendered as the “electric fluid” passes along a wire. Lightning frequently fuses bell-wires as it passes, and when we touch upon Galvanism or Dynamical Electricity we shall hear more about it.
By the Electric Machine we can obtain some very powerful currents of electricity; we can produce many pleasing effects, and perform a number of experiments, such as making balls or figures of pith dance, and several other easy and entertaining tricks, which will be found in books more specially devoted to the entertainment of young people.
We have now given some explanation of the manner in which electrical phenomena can be produced,—viz., by the Electric Machine and by the Leyden Jar,—but we must not expect to find any electricity inside any charged body. It has been proved that all the electricity is upon the surface of bodies, even if in varying quantity, and that equal quantities of electricity are always produced when bodies are excited by friction, but the _kinds_ are different. The rubbing body is of one kind, the body rubbed another, and consequently the forces neutralize each other. The two forces or kinds of electricity we have seen repel or attract each other, and we can imagine the farther they are apart the less will be the force, and the _rate of diminution of force_, according to distance, is ascertained by an ingenious apparatus called a “Torsion” Electromoter, which was constructed by Coulomb, and was frequently used by Faraday.