fluid which is thrown on the inside coating will make its way over the glass, though a nonconductor, 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 residuum, and this, in a large jar, would give you a considerable shock; therefore, I advise you always, in discharging an electrical 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 eighth of an inch from the ball A, let a wire c K be fixed between the ball c and the outside coating of the jar. Then as soon as the 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 work. The electric fluid passes from the conductor to the inside of all the jars, till it is charged sufficiently high for the purpose. Great caution, however, must be used when you come to make experiments with a battery, for fear of an accident, either to yourself, or to spectators.

Charles. Would a shock from this be attended with any bad consequences?

Tutor. Yes: very serious accidents may happen from the electricity accumulated in a large battery, and even with a battery such as is re

presented in the plate, which is one of the smallest made; a shock may be given, which, if passed through the head, or other vital parts of the body, may be attended with very mischievous effects.

James. How do you know when the battery is properly charged?

Tutor. The quadrant electrometer (Plate VII. Fig. 5.) is the best guide, and this may be fixed either on the conductor, or upon one of the rods of the battery. But if it is fixed on the battery, the stem of it should be of a good length, not less than twelve or fifteen inches.

Charles. How high will the index stand, when the battery is charged?

Tutor. It will seldom rise so high as 90°, because a machine, under the most favourable circumstances, cannot charge a battery so high, in proportion, as a single jar. You may reckon that a battery is well charged when the index rises as high as 60°, or between that and 70°.

James. Is there no danger of breaking the jars when the battery is very highly charged? Tutor. Yes, there is; and if one jar be cracked, it is impossible to charge the others till the broken one be removed. To prevent accidents, it is recommended, not to discharge a battery through a good conductor, except the circuit is at least five feet long.

Charles. Do you mean the wire should be so long?

F

Tutor. Yes, if you pass the charge through that; but you may carry it through any conductor.

Before a battery be used, the uncoated part of the jars must be made perfectly clean and dry; the smallest particles of dust will carry away the electric fluid. And after an explosion, always connect the wire from the hook, with the ball, to prevent any residuum from remaining.

CONVERSATION XXXV.

Experiments made with the Electrical Battery.

Tutor. I will now show you some experiments with this large battery. To perform these in perfect safety, I must beg you to stand a good distance from it: this will prevent accidents.

Example 1. I take this quire of writingpaper, and place it against the hook or wire that comes out of the box; and when the battery is charged, I put one ball of the dischargingrod to a knob of one of the wires F, and bring