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or you to experience. Here is a brass wire with two little balls or knobs b s screwed to it (Plate vir. Fig. 7.) I will bring one of them, as s, to the outside, and the other, b, to the ball v on the wire.

James. What a brilliant spark, and what a loud noise!

Tutor. The electric fuid, that occasions the light and the noise, ran from the inside of the jar through the wire-to.s, and spread itself over the ouside.

Charles. Would it have gone through my arms if I had put one hand to the outside, and touched the wire communicating with the inside, with the other?

Tutor. It would, and you may conceive that the shock would have been in proportion to the quantity of the fluid collected. The instrument I used may be called a discharging-rod, But here is a more convenient one (Plate vir. Fig. 8.:) the handle o is solid glass, fastened into a brass socket, and the brass work is the same as Fig. 7, only by turning on a joint the arms may be opened to any extent

James. Why is the handle glass?

Tutor. Because glass being a non-conductor, the electric fluid passes through the brass work without affecting the hand ; whereas, with the other, a small sensation was perceived while I discharged the jar.

Charles. Would the jar never discharge itself?

Tutor. Yes: by exposure to the air for some time, the charge of the jar will be silently and gradually dissipated, for the superabundant electric fluid of the inside will escape, by means of the air, to the outside of the jar. But electricians make it a rule never to leave a jar in its charged state.

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Of the Leyden Jar-Lane's discharging Electrometer,

and the Electrical Battery.





CHARLES. In discharging the jar yes- the terday, I observed that when one of the loatin discharging-rods touched the outside of the thou jar, the flash and report took place the other end came in contact with the brassmes. wire that communicates with the insiderge, coating

Tutor. Yes, it acts in the same manneritor, as when you take a spark from the conduc lect, tor; you do not, for that purpose, bring wil your knuckle close to the tin.

James. Sometimes, when the machine acts very powerfully, you may get the spark a the distance of several inches.

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Tutor. By the same principle, the higher an electrical or Leyden jar is charged, the more easily, or at a greater distance, is it discharged.

Charles. From your experiments it does not seem that it will discharge at so great a distance as that in which a spark may be taken from the conductor.

Tutor. Very frequently a jar will discharge itself, after it has accumulated as much of the electrical fluid as it can contain ; that is, the fluid which is thrown on the inşide coating will make its way over the glass, though a non-conductor, on to the outside coating · James. In a Leyden jar, after the first discharge, you always, I perceive, take an, other and smaller onę.

Tutor. The tin foil on the jar not being a perfect conductor, the whole quantity of fuid will not pass at first from the inside to the out: what remains is called the resia duum, and this, in a large jar, would give you a considerable shock ; therefore, I advise you always, in discharging an electri

cal jar, to take away the residuum before
you venture to remove the apparatus. I
will now describe an electrometer, which
depends, for its action, on the principles
we have been describing.
Charles. Do

mean upon

the jars discharging before the outside and inside coating are actually brought into contact ?

Tutor. I do. (Plate vii. Fig. 10.) The arm p is made of glass, and proceeds from a socket on the wire of the electrical jar F. To the top of the glass arm is cemented another brass socket E, through which a wire, with balls B and c at each end, will slide backwards and forwards.

James. So that it may be brought to any distance from the ball A, which is on the wire, connected with the inside of the jar?

Tutor. Just so. When the jar F is set either in contact, or very near the conductor, as is represented in the figure, and the ball B is set at the distance of the eighth of an inch from the ball a, let a wire cik be fixed between the ball c-and the outside coating of the jar. Then as soon as the

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