In order to get long dense sparks, a small Leyden jar is hung on each of the rods R, R', with their outer coatings in metallic contact. With these condensers a splendid series of long, intensely brilliant sparks of 6 in. in length are got, each accompanied with a snap painfully loud, quite eclipsing anything shown by friction machines. The conddnsed spark does not kindle paper, but gives a very powerful shock.
We have not space to enter into the theory of Holtz's machine. Indeed it may be questioned if, in all points, its action has been fully accounted for. We would only say that reciprocal action of the armatures on each other is common to all induction machines, and is quite similar to Siemens and Wheatstone's principle for magneto-elec tricity (see MAGNETO•ELECTRIC MACHINES).
Leyden is a glass jar, with a coating of tin-foil pasted carefully inside and out, extending to within a few inches of the mouth. This last is generally closed by a wooden stopper, through which passes the stalk of a brass knob or ball, surmounting the whole. The connection between the inside coating and the ball is completed by a chain extending from the stalk to the bottom of the jar. If this jar be put on all insu lating stool, so that sparks can pass from the prime conductor of a machine to the knob, when the jar isthus insulated, one or two sparks pass, and then the charge seems com plete, for no more sparks will follow, though the action of the machine is continued; or if they do, they are immediately dissipated from the knob in a brush discharge. If then, however, the knuckle of the experimenter be brought near the outer coating, sparks begin again to pass freely; and for every spark of + E. that passes between the machine and the knob, a corresponding spark of the same name passes between the knuckle and outer coating. This continues for some time, and then the jar appears to be again saturated. It is now said to be fully charged. The outside of the jar can, in this state, be handled freely, and if it be still on the insulating stool, so may also the knob, although, when the hand first approaches, it receives a slight spark. But if, when the experimenter has one hand on the outer coating, he bring the other hand to the knob, before it can reach it, a straight, highly brilliant spark passes between the knob and his hand, and he experiences a shock of great violence. If he try the same thing again, a feeble spark and shock again ensue, and the jar is now thoroughly dis charged. As it is highly inconvenient, if not dangerous, to discharge the jar through the body, discharging tongs are used for that purpose, which consist of two brass arms ending in balls, and moved on a hinge by glass handles.
A. very instructive experiment may be made when the coatings are fitted to the jar so as to be removed at pleasure. After the jar is charged, it is put on an insulating stand. The inside coating is lifted out by the knob, and a slight spark is got by the hand in doing so. The jar is now taken up by one hand, and the outside coating is
removed by the other, and, as before, another feeble spark is got. The whole is now built up in inverse order and discharged, when the spark is nearly as brilliant as when it is discharged at once without such dissection. From this it may be argued that the charge of the jar lies in the glass and not in the coatings, and that it is likely that in all cases it is in the dielectric the charge resides; that the conductors, which are usually looked upon as the seat of the charge, are merely the limiting surfaces or expo nents of it. A portion of the total charge may reside in them, but no more than is found on two similar contiguous dielectric surfaces. Taking this for granted, it is easy to explain the action of the leyden jar. The electrifying power of the charging machine is exerted on two dielectrics—the glass of the jar and the air—the external limit in both cases being the surrounding objects which constitute the ground. The action on the air through the inside coating or the knob is quite similar to what we find in the case of any body to be charged. The action through the glass is peculiar, because we are shut out from it. The limits of this action are the inside surfaces of the inside and outside coatings. The air charge we participate in as we move in it. We are, however, quite external to the action on the glass; hut if we could move about in it between the coatings, we should End things there exactly similar, so far, at least, as kind of action is concerned, to what we find in charged air. Seeing that the glass of the jar is a thin and good dielectric, and the air much thicker and more difficult to polarity, the charging power of the machine is exerted for the most part in the glass, the polarifica tion in the air being comparatively slight. Assuming E. to be a polarification of molecules, the E. of the jar resides thus in glass, and to a much less extent in the air. The potential at the inner coating is the same as that at the knob, for any con nected system of good conductors must be at the same potential. We judge of poten tial by the air charge, and thus we take the potential at the knob as the potential at the inner coating. The spark got from the knob of the insulated jar is small compared wtilt that of the charging machine, and as sparking distance is, generally speaking, propor tional to potential, the potential of the jar is much below that of the charging machine. Thus, a Leyden jar is a contrivance for accumulating large quantities of electricity at a low. potential. The thinner the glass, the greater will be the accumulation of electricity, but the feebler will be the potential of the jar. When the electric field is limited, as in the glass of the Leyden jar, it is sometimes said to be bound, as distinguished from the free charge of an ordinary air field.