outside coating is connected with an upright metallic support, on which is a similar bell, e. Between the two bells a light copper ball is suspended by a silk thread. The jar is then charged in the usual manner and placed on the support, m. The internal armature contains a quantity of free electricity; the pendulum is attracted and immediately repelled, striking against the second bell, to which it imparts its free electricity. Being now neutralised it is again attracted by the first bell, and so on for some time, especially if the air be dry, and the jar pretty large.

Fig. 337.

the thickness of the insulator.

416. Electric batteries.-The charge which a Leyden jar can take depends on the extent of the coated surface, and for small thicknesses is inversely proportional to

Hence the larger and thinner

the jar the more powerful the charge. But very large jars are ex

pensive, and liable to break; and, when too thin, the accumulated electricities are apt to discharge themselves through the glass, es

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435

pecially if it is not quite homogeneous. Leyden jars have usually from 1 to 3 square feet of coated surface. For more powerful charges electric batteries are used.

An electric battery consists of a series of Leyden jars, whose internal and external coatings are respectively connected with each other (fig. 338). They are usually placed in a wooden box lined on the bottom with tinfoil. This lining is connected with two metallic handles in the sides of the box. The internal coatings are connected with each other by metallic rods, and the battery is charged by placing the internal coatings in connection with the prime conductor, while the external coatings are connected with the ground by means of a chain fixed to the handles. A quadrant electrometer fixed to the jar serves to indicate the charge of the battery. Although there is a large quantity of electricity accumulated in the apparatus the divergence is not great, for it is simply due to the free electricity on the internal coating. The number of jars is usually four, six, or nine. The larger and more numerous they are, the longer is the time required to charge the battery, but the effects are so much the more powerful.

When a battery is to be discharged, the coatings are connected by means of the discharging rod, the outside coating being touched first. Great care is required, for with large batteries serious accidents may be produced, resulting even in death.

417. Condensing electroscope. We shall conclude the study of condensers by an application which Volta made of this principle to the ordinary gold leaf electroscope, by which a far greater degree of delicacy is attained (fig. 339). The rod to which the gold leaves are affixed, terminates in a disc instead of in a knob, and there is another disc of the same size provided with an insulating glass handle. The discs are covered with a layer of insulating shellac varnish (fig. 339).

To render very small quantities of electricity perceptible by this apparatus, one of the plates, which thus becomes the collecting plate, is touched with the body under examination. The other plate, the condensing plate, is connected with the ground, by touching it with the finger. The electricity of the body, being diffused over the collecting plate, acts inductively through the varnish on the neutral fluid of the other plate, attracting the opposite electricity, but repelling that of like kind. The two electricities thus become accumulated on the two plates just as in Epinus's condenser, but there is no divergence of the leaves, for the

opposite electricities counteract each other. The finger is now removed, and then the source of electricity, and still there is no

divergence; but if the upper plate be raised (fig. 340), the neutralisation ceases, and the electricity being free to move diffuses itself over the rod and the leaves, which then diverge widely. The delicacy of the apparatus is increased by adapting to the foot of the apparatus two metallic rods, terminating in knobs, for these knobs being excited by induction from the gold leaves react upon them.

CHAPTER V.

VARIOUS EFFECTS OF ACCUMULATED ELECTRICITY.

418. Effects of the electric discharge.-The recombination of the two electricities which constitutes the electrical discharge may be either continuous or sudden; continuous, or of the nature of a current, as when the two conductors of a cylinder machine are

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Physiological Effects of Electricity.

437

joined by a chain or a wire; and sudden, as when the opposite electricities accumulate on the surface of two adjacent conductors, till their mutual attraction is strong enough to overcome the intervening resistances, whatever they may be. But the difference between a sudden and a continuous discharge is one of degree and not of kind, for there is no such thing as an absolute nonconductor, and the very best conductors, the metals, offer an appreciable resistance to the passage of electricity. Still, the difference at the two extremes of the scale is sufficiently great to give rise to a wide range of phenomena.

The phenomena of the discharge are usually divided into the physiological, luminous, mechanical, magnetical, and chemical effects.

419. Physiological effects.-The physiological effects are those produced on living beings, or on those recently deprived of life. In the first case they consist of a violent excitement which the electric

fluid exerts on the sensibility and contractibility of the organic tissues through which it passes; and in the latter, of violent muscular convulsions which resemble a return to life.

The shock from the electrical machine has been already noticed (403).. The shock taken from a charged Leyden jar, by grasping the external coating with one hand and touching the inner with the other, is much more violent, and has a peculiar character. With a small jar the shock is felt in the elbow; with a jar of about a quart capacity it is felt across the chest, and with jars of still larger dimensions in the stomach.

A shock may be given to a large number of persons simultaneously by means of the Leyden jar. For this purpose they must form a chain by joining hands. If then the first touches the outside coating of a charged jar, while the last at the same time touches the knob, all receive a simultaneous shock, the intensity of which depends on the charge, and on the number of persons receiving it. Those in the centre of the chain are found to receive a less violent shock than those near the extremities. The Abbè Nolled discharged a Leyden jar through an entire regiment of 1,500 men, all of whom received a violent shock in the arms and shoulders.

With large Leyden jars and batteries the shock is sometimes very dangerous. Priestley killed rats with batteries of 7 feet coated surface, and cats with a battery of about 4 square yards coating.

420. Luminous effects. Luminous jar.-The luminous effects of electricity are in all cases due to the combination of the two fluids, positive and negative. Some of these effects have already been made known in describing the electrical egg and the magic pane. We here give a description of another one.

The luminous jar (fig. 342) is a Leyden jar, whose outer coating consists of a layer of varnish strewed over with metallic powder. A strip of tin fitted on the bottom is connected with the ground by means of a chain; a second band at the upper part of the coating has a projecting part, and the rod of the bottle is curved so that the knob is about 3 of an inch distant from the projection. This bottle is suspended from the machine, and as rapidly as this is worked, large and brilliant sparks pass between the knob and the outer coating, illuminating the outside of the apparatus.

421. Calorific effects. Besides being luminous, the electric spark is a source of intense heat. When it passes through inflammable