Charles. I did, but it was near enough to get a strong spark from you.

Tutor. When a person has less electricity than his natural share, he is said to be electrified minus, or negatively: but if he has more than his natural share, he is said to be electrified plus. or positively.

James. Then before Charles gave me the spark, I was electrified minus; and when he had given it to me, he was minus till he received it from you.

Tutor. That is right. Suppose you stand on a stool and hold the rubber, and Charles stand on another stool, and touch the prime conductor L, while I turn the machine, which of you will be plus, and which minus, electrified?

:

James. I shall be minus, because I give to the rubber and Charles will be plus, because he receives from the conductor what I gave to the rubber, and which is carried by the cylinder to the conductor.

Tutor. You then have less than your share, and your brother has more than he ought to have. Now if I get another glass-legged stool,

I can take from Charles what he has too much, and give it to you who have too little. Charles. Is it necessary that you should be insulated for this purpose?

Tutor. By being insulated I may perhaps carry back to James the very electricity which passed from him to you. But if I stand on the ground, the quantity which I take from you will pass into the earth, because I cannot, unless I am insulated, retain more than my natural share.

James. And what is given by you to me is likewise instantaneously supplied by the earth.

Tutor. It is. Let us make another experiment to show that the electric fluid is taken from the earth. Here are some little balls (Plate vii. Fig. 3.) made of the pith of elder: they are put on thread, and being very light, are well adapted to our purpose.

While the chain is on the cushion, and I work the machine, do you bring the balls near the conductor by holding the thread

at D.

James. They are attracted by it, and now the two balls repel each other, as in the fi gure x.

Tutor. I ought to have told you, that the upper part D of the line is silk, by which means you know the balls are insulated, as silk is a non-conductor. I take the chain off from the cushion, and put it on the conductor, so as to hang on the ground, while I turn the machine. Will the balls be affectif you hold them to the conductor?

ed now,
James. No, they are not.

Tutor. Take them to the cushion.

Charles. They are attracted and repelled now by being brought near the cushion, as they were before, by being carried to the conductor.

Tutor. Yes, and you may now take sparks from the cushion as you did just now from the conductor: in both cases, it must be evident that the electric fluid is brought from the earth.

Some machines are furnished with two conductors, one of which is connected with the cushion, the other such as we have de

scribed. Turn the cylinder, and both conductors will be electrified; but any body which is brought within the influence of these, will be attracted by one of the conductors, and repelled by the other and if a chain or wire be made to connect the two together, neither will exhibit any electric appearances: they seem, therefore, to be in opposite states; accordingly electricians say, that the conductor connected with the cushion is negatively electrified, and the other is positively electrified.

CONVERSATION XXXI.

Of Electrical Attraction and Repulsion.

JAMES. What is this large roll of sealing-wax for?

Tutor. As I mean to explain, this morning, the principles of electrical attraction and repulsion, I have, besides the electrical machine, brought out for use a roll of sealing wax, which is about fifteen inches long, and an inch and a quarter in diameter; and the long glass tube.

Charles. Are they not both electrics, and capable of being excited?

Tutor. They are; but the electricity pro