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Tutor. Whatever body is supported, or prevented from touching the earth, or communicating with it, by means of glass or other non-conducting substances, is said to be insulated. Thus a body suspended on a silk line is insulated, and so is any substance that stands on glass, or resin, or wax, provided that these are in a dry state, for moisture will conduct away the electric fluid from any charged body.

CONVERSATION XXX.

Of the Electrical Machine.

CHARLES. What is that shining stuff which I saw you put to the rubber yesterday? Tutor. It is called amalgam: the rubber, by itself, would produce but a slight excitation: its power, however, is greatly increased by laying upon it a little of this amalgam, which is made of quicksilver, zinc, and tinfoil, with a little tallow or mutton suet.

James. Is their any art required in using this amalgam?

Tutor. When the rubber and silk flap are very clean and dry, and in their place, then spread a little of the amalgam upon a piece

of leather, and apply it to the upper part of the glass cylinder, while it is revolving from you; by this means, particles of the amalgam will be carried by the glass itself to the lower part of the rubber, and will increase the excitation.

Charles. I think I once saw a globe, instead of a cylinder, for an electrical machine.

Tutor. You might: globes were used before cylinders, but the latter are the most convenient of the two. The most powerful electrical machines are fitted with flat plates of glass. In our experiments we shall be content with the cylinder, which will answer every purpose of explaining the principles of the science.

James. As I was able to conduct the electricity from the tin conductor to the ground, could I likewise act the part of the chain, by conducting the fluid from the earth to the cushion?

Futor. Undoubtedly: I will take off the chain, and now do you keep your hand on the cushion while I turn the handle.

James. I see the machine works as well as when the chain was on the ground.

Tutor. Keep your present position, but stand on the stool with glass legs; by which means there is now all communication cut off between the cushion and the earth; in other words, the cushion is completely insulated, and can only take from you what electricity it can get from your body. Go, Charles, and shake hands with your brother.

Charles. It does not appear that the machine had taken all the electricity from him, for he gave me a smart spark.

Tutor. You are mistaken; he gave you nothing, but he took a spark from you. Charles. I stood on the ground; I was not electrified: how then could I give him a spark?

Tutor. The machine had taken from James the electricity that was in his body and by standing on the stool, that is, by being insulated, he had no means of receiving any more from the earth, or any surrounding objects; the moment, therefore, you

brought your hand near him, the electricity passed from you to him.

Charles. I certainly felt the spark, but whether it went out of, or entered into, my hand, I cannot tell : have I then less than my share now

?

Tutor. No: what you gave to your brother was supplied immediately from the earth. Here is another glass-legged stool; do you stand on this, but at the distance of a foot or two from your brother, who still keeps his place. I take the electricity from him by turning the machine, and as he stands on the stool, he has now less than his share. But you have your natural share, because though you also are insulated, yet you are out of the influence of the machine; extend, therefore, your hand, and give him of the electric fluid that is in you. Charles. I have given him a spark. Tutor. And being yourself insulated, you have now less than your natural quantity, to supply which you shall have some from me give me your hand. You draw it back without my touching it!

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