CONVERSATION XXXIV.

Of the Leyden Jar-Lane's discharging Electrometer, and the Electrical Battery.

CHARLES. In discharging the jar yesterday, I observed that when one of the discharging-rods touched the outside of the jar, the flash and report took place before the other end came in contact with the brass wire that communicates with the inside coating.

Tutor. Yes, it acts in the same manner as when you take a spark from the conductor; you do not, for that purpose, bring 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.

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 inside 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 another and smaller one.

Tutor. The tin foil on the jar not being a perfect conductor, the whole quantity of fluid will not pass at first from the inside to the out: what remains is called the resi 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 you 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 D 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 of an inch from the ball A, let a wire c be fixed between the ball c and the outse coating of the jar. Then as soon as e

machine is worked, the jar cannot be charged beyond a certain point, for when the charge is strong enough to pass from A to the ball B, the discharge will take place, and the electric fluid collected in the inside will pass through the wire c K to the outside coating.

Charles. If you remove the balls to a greater distance from one another, will a stronger charge be required before the fluid can pass from the inside of the jar to the ball B, of the electrometer?

Tutor. Certainly and therefore the discharge will be much stronger. This machine is called Lane's Discharging Electrometer, from the name of the person who invented it. It is very useful in applying the electric shock to medical purposes, as we shall see hereafter.

This box contains nine jars or Leyden phials; (Plate VIII. Fig 9.) the wires which proceed from the inside of each three of these jars, are screwed or fastened to a common horizontal wire E, which is knobbed at each extremity, and by means of the

wires FF, the inside coatings of 3 or 6, or the whole 9, may be connected.

James. Is it a common box in which the jars are placed?

Tutor. The inside of the box is lined with tin foil; sometimes very thin tin-plates are used, for the purpose of connecting more effectually the outside coatings of all the jars,

Charles. What is the hook c on one of the sides of the box for?

Tutor. To this hook is fastened a strong wire, which communicates with the inside lining of the box, and, of course, with the outside coating of the jars. And, as you, see, to the hook a wire is also fastened, which connects it with one branch of the discharging rod.

James. Is there any particular art to be used in charging a battery?

Tutor. No: the best way is, to bring a chain, or piece of wire, from the conductor to one of the balls on the rods that rest upon the jars; and then set the machine to