Science for the School and Family, Part I. Natural Philosophy

by Worthington Hooker

Fig. 272.

If there were no communication of the outside with the earth the jar would not be charged. No electricity would pass to it, because the positive electricity which is on the outside can not be driven off, and no negative electricity can be received. To make this plain, suppose that the jar, a, Fig. 272, having a bent rod, is suspended to the prime conductor, b. Here you have the inside tin-foil connected with the source of positive electricity. But the outside is insulated. No electricity can pass from it or to it. It has both positive and negative electricity, but they are in equilibrium. If there were a preponderance of negative electricity there, it would attract positive electricity to it as near as possible, and so the latter would enter the jar from the conductor. But there is no such preponderance, and so, though a little may enter—a spark or two—there will not be enough to charge the jar sensibly, because there is no attraction in that direction. But bring now another jar, c, near to the outside coating of a, and there is a movement at once in the electricities. The positive electricity has a chance now to pass off from the outside of a to the inside of c, leaving therefore a preponderance of negative electricity on the outside of a, which exerts an attractive influence on the positive electricity of the conductor drawing it to the inside of the jar.

Fig. 273.

385. Discharge of the Leyden Jar.—The jar may be discharged by making a communication between the inside and outside by means of any conductor. It may be done with the discharging-rod (Fig. 273). This has two slender metallic rods, with brass knobs at their ends, and jointed at a, so that the knobs can be separated to different distances. The handle is glass, so that as the electricity passes through the rods none of it may be communicated to the hand. In discharging the jar one knob is placed upon the outside foil, and the other is brought near to the knob of the jar. The two fluids now rush together from their attraction, and in doing so a bright flash is produced, going from the knob of the jar to that of the discharging-rod, and with this a report. You can yourself be the conductor to discharge the jar. If, having one hand upon the outside of the jar, you bring the other near its knob, the fluids meet in you as they do in the discharging-rod, and a shock will be experienced in proportion to the amount of charge in the jar. Any number of persons can together receive the same shock. To do this they must join hands, and the person at one end of the row must touch the knob of the jar while the person at the other end has his hand upon the outside.

You may touch either the knob of the jar or the outside coating separately, and the power that is in it remains quiet; but the moment that you touch both it bursts forth, because a bridge is made upon which the two fluids can meet.

In a dry air the charge in the jar can be retained for some time, the communication between the two electric fluids being very slow through the medium of air. It is otherwise when there is much moisture in the air, for water is a good conductor. For this reason, if you let the moisture from your breath come upon the jar between the outside coating and the rod, the jar will be discharged soon, though imperceptibly, the moisture making a medium of communication between the inner and outer electricities.

Fig. 274.

386. The Electrical Sportsman.—In this contrivance, Fig. 274, the discharge of the Leyden jar is very prettily exhibited. The jar, c, has a rod with two branches. On the end of one of these, B, are suspended pith balls cut in the shape of birds. On the other is a knob by which the jar can receive its charge from the prime conductor. After it is charged it is placed on the stand with its knob, b, near the gun, a, of a metallic figure. The suspended birds, you observe, stand out from each other, because they are charged with the same fluid, positive electricity, and therefore are repellent. Now when the chain, e, which is connected with the outside of the jar, is made to touch the foot of the metallic image, the connection between the inside and outside of the jar is established. Of course there is an instantaneous flash between a and b, and the birds, losing their electricity, fall, and hang as they did before the jar was charged.