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received in the arms and breast a shock so violent that it was two days before he recovered from the effects; and writing to his friend Reaumur he said, he would not repeat the experiment for the whole kingdom of France.

The fact thus discovered caused probably a greater sensation throughout Europe than any other one has ever done. It was repeated innumerable times, and the apparatus, after successive modifications and improvements, acquired its present form. It is not difficult to see that the above experiment is a case of condensation. The liquid in the flask acts as a collector, the hand acts as a condensing plate, and the insulating plate is formed by the material of the flask itself.

The ordinary form of the Leyden jar consists of a glass bottle of

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any convenient size, the interior of which is either coated with tinfoil or filled with thin leaves of copper, or with gold leaf. Up to a certain distance from the neck the outside is coated with tinfoil. The neck is provided with a cork, through which passes a brass rod, which terminates at one end in a knob, and communicates with the metal in the interior. The metallic coatings are called respectively the internal and external armatures or coatings. Like the condenser, the jar is charged by connecting one of the armatures with the ground, and the other with the source of electricity. When it is held in the hand by the external coating, and the knob presented to the conductor of the machine, positive electricity is accumulated on the inner, and negative electricity on the outer coating. The reverse is the case if the jar is held by the knob, and the

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external coating presented to the machine.

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The theory of the jar is identical with that of the condenser, and all that has been said of this applies to the jar, substituting the two armatures for the two plates, A and B, of the condenser.

To charge the jar it is held in the hand as represented in fig. 335, and the knob is applied to an electrical machine, which is at work. The positive electricity of the machine acting inductively through the sides of the glass on the tinfoil and on the hand, condenses a large quantity of electricity.

Like the condenser, the Leyden jar may be discharged either

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slowly or instantaneously. For the latter it is held in the hand by the outside coating (fig. 336), and the two coatings are then connected by means of the simple discharger. Care must be taken to touch first the external coating with the discharger, otherwise a smart shock will be felt. To discharge it slowly the jar is placed on an insulated plate, and first the internal and then the external coating touched, either with the hand or with a metallic conductor. A slight spark is seen at each discharge.

Fig. 337 represents a very pretty experiment for illustrating the slow discharge. The rod terminates in a small bell, d, and the

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outside coating is connected with an upright metallic support, on which is a similar bell, e. Between the two bells a light copper ball

Fig. 337.

the thickness of the insulator.

is suspended by a silk thread. The

jar is then charged in the usual manner and placed on the support,

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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.

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

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

-417]

Condensing Electroscope.

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pecially if it is not quite homogeneous. Leyden jars have usually from 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

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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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