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.
Fig. 215.—Leyden Jar.
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.
Perhaps some people may not be aware of the term “torsion.” It means twisting, and “the torsion of a thread suspended vertically is the force tending to twist the lower extremity when the upper end is turned through an angle.” This instrument is really an Electromoter, and is not considered suited to beginners, and it is scarcely accurate in its workings. We need not therefore describe it in detail. There are some excellent Electromoters, the Elliott being, we believe, the best for use. A full and detailed description of the Quadrant Electromoter will be found in Mr. Gordon’s treatise on Electricity.
Recapitulation of foregoing Chapter. So far, we have seen there is electricity in everything, although some bodies are termed conductors and others non-conductors; though, as in applying the terms heat and cold, we must remember that no body is entirely devoid of electricity, and no body is therefore an absolute non-conductor any more than any object is absolutely devoid of heat. Faraday, indeed, was of opinion that “conduction and insulation are only extreme degrees of one common condition”; they are identical both in principle and action, except that in conduction an effect common to both is raised to the highest degree, and in the case of insulation it occurs in an almost insensible quantity.
We have also read of positive and negative electricities, and we must not fancy there is any particular reason for this distinction. It was Du Fay, whom we have mentioned, who gave the names “vitreous” and “resinous” to the two kinds, as one was developed by rubbing glass, and the other by rubbing resin. But, as shown by our experiments, either kind of electricity can be excited in glass or sealing-wax, and both kinds are produced at once. You cannot get “positive” without negative electricity. “Positive” is the term applied to the kind produced by rubbing glass with silk or wool; “negative” is the term applied to the kind developed by rubbing sealing-wax, but the kind developed by friction depends on the rubbing substance and certain conditions.