placed between these will, when the conductor is electrified, appear like a rope-dancer.

This piece of leaf brass is called the electric fish one end is a sort of obtuse angle, the other is acute: if the large end be presented towards an electrified conductor, it will fix to it, and, from its wavering motion, it will appear to be animated.

This property of attraction and repulsion has led to many inventions of instruments called electrometers.

James. Is not an electrometer a machine to measure the strength of the electricity ?

Tutor. Yes; and this is one of the most simple, (Plate vII. Fig. 5.) and it depends entirely upon the repulsion which takes place between two bodies in a state of electrification. It consists of a light rod and a pith-ball, hanging parallel to the stem, but turning on the centre of a semicircle, so as to keep close to its graduated limb. This is to be placed in a hole a on the conductor L, and according as the conductor is more or less electrified, the ball will fly farther from the stem.

Charles. If the circular part be marked with degrees, you may ascertain, I suppose, pretty accurately, the strength of any given charge.

Tutor. Yes, you may; but you see how fast' the air carries away the electricity, it scarcely remains a single moment in the place to which

1

it was repelled. Two pith-balls may be suspended parallel to one another, on silken threads, and applied to any part of an electrical machine, and they will, by their repulsion, serve for an electrometer, for they will repel one another the more, as the machine acts more powerfully.

James. Has this any advantage over the other?

Tutor. It serves to show whether the electricity be negative or positive; for if it be positive, by applying an excited stick of sealingwax, the threads will fall together again; but if it be negative, excited sealing-wax, or resin, or sulphur, or even a rod of glass, the polish of which is taken off, will make them recede farther.

We have now perhaps said enough respecting electrical attraction and repulsion, at least for the present; I wish you, however, to commit the following results to your memory.

(1.) Bodies that are electrified positively repel each other.

(2.) Bodies that are electrified negatively repel each other.

Charles. Do you mean, that if two bodies have either more or less of the electric fluid than their natural share, they will repel each other if brought sufficiently near?

Tutor. That is exactly what I mean.

(3.) Bodies electrified by contrary powers ;

that is, two bodies, one having more, and the other less, than its natural share, attract each other very strongly.

(4.) Bodies that are electrified attract light substances which are not electrified.

These are facts which, I trust, have been made evident to your senses. To-morrow we will describe what is usually called the Leyden phial.

CONVERSATION XXXIII.

Of the Leyden Phial or Jar.

Tutor. I will take away the wires and the ball from the conductor, and then remove the conductor an inch or two farther from the cylinder. If the machine acts strongly, bring an insulated pith-ball, that is, you know, one hanging on silk, to the end of the conductor, nearest to the glass cylinder.

Charles. It is immediately attracted.

Tutor. Carry it to the other end of the conductor, and see what happens.

Charles. It is attracted again; but I thought it would have been repelled.

Tutor. Then as the ball was electrified before, and is still attracted, you are sure that the electricity of the two ends of the conductor are of different names; that is, one is plus, and the other minus.

James. Which is the positive, and which is the negative end?

Tutor. The end of the conductor which is nearest to the cylinder, becomes possessed of an electricity different from that of the cylinder itself.

James. Do you mean that if the cylinder is positively electrified, the end of the conductor next to it is electrified negatively?

Tutor. I do: and this you may see by holding an insulated pith-ball between them.

Charles. Yes, it is now very evident, for the ball fetches and carries as we have seen it before.

Tutor. What you have seen with regard to the conductor, is equally true with respect to non-conducting bodies. Here is a common glass tumbler: if I throw within-side it a greater portion of electricity than its natural share, and hold it in my hand, or place it on any conducting substance, as a table, a part of the electric fluid, that naturally belongs to the outside, will make its escape through my body, on the table.

Charles. Let me try this.
VOL. III.-Q

Tutor. But you must be careful that you do not break the glass.

Charles. I will hang the chain on the conductor, and let the other end lie on the bottom of the glass, and James will turn the machine, Tutor. You must take care that the chain does not touch the edge of the glass, because then the electric fluid will, by that means, run from one side of it to the other, and spoil the experiment.

James. If I have turned the machine enough, take the chain out, and try the two sides with the insulated pith-ball.

Charles. What is this? Something has pierced through my arms and shoulders.

Tutor. That is a trifling electric shock, which you might have avoided, if you had waited for my directions.

Charles. Indeed it was not trifling: I feel it

now.

Tutor. This leads us to the Leyden phial: so called, because the discovery was first made at Leyden, in Holland, and by means of a phial or small bottle.

James. Was it found out in the same manner as Charles has just discovered it?

Tutor. Nearly so. Mr. Cuneus, a Dutch philosopher, was holding a glass phial in his hand, about half filled with water, but the sides above the water, and the outside was quite dry, a wire