CONVERSATION XXIX.

Of the Electrical Machine.

Tutor. I will now explain to you the con struction of the electrical machine, and show you how to use it.

Charles. For what purpose is it used?

Tutor. Soon after the subject of the electric fluid engaged the attention of men of science, they began to contrive the readiest methods of collecting large quantities of it. By rubbing this stick of sealing-wax, I can collect a small portion if I excite or rub the glass tube, I get still more. The object, therefore, was, to find out a machine by which the largest quantities can be collected, with as little trouble and expense as may be.

James. You get more electricity from the tube than from the sealing-wax, because it is five or six times as large: by increasing the size of the tube, you would increase the quantity of the electric fluid, I should think.

Tutor. That is a natural conclusion. But if you look to the table of electrics, which I made out yesterday, you will see that had the wax been as large as the glass tube, it would not have collected so much of the electric fluid, be

cause, in its own nature, it is not so good an electric.

Charles. By the table, glass stands as the most perfect electric, but there are several substances between it and wax, all of which are, I believe, more perfect electrics than wax.

Tutor. They are: Electricians, therefore, had no hesitation as to the nature of the substance: they fixed on glass, which being easily melted and run, or blown into all sorts of forms, is, on that account, very valuable.

The most common form that is now used is that of a glass cylinder, from five or six inches in diameter to ten or twelve. Here is one completely fitted up (Plate VII. Fig. 2.) the cylinder A B is about eight inches in diameter, and twelve or fourteen in length: this I turn round in the frame-work, with the handle D c.

James. What is the piece of black silk к for? Tutor. The cylinder would be of no use without a rubber, you know: on which account you see the glass pillar R s, which, being cemented into a piece of hard wood, is made to screw into the bottom of the machine; on the pillar is a cushion, to which is attached a picce of black silk.

Charles. And I perceive the cushion is made to press very hard against the glass.

Tutor. This pressure, when the cylinder is turned round fast, acts precisely like the rub

bing of the tube by the hand, though in a still more perfect manner. I will turn it round. James. Here is not much sign of electricity yet.

Tutor. No: the machine is complete, but it has no means of collecting the fluid from the surrounding bodies: for you see the cushion or rubber is fixed on a glass pillar, and glass will not conduct the electric fluid.

Charles. Nevertheless it does, by turning round, show some signs of attraction.

Tutor. Every body in nature with which we are acquainted possesses a portion of this fluid, and therefore the signs which are now evident arise from the small quantity which exists in the rubber itself, and the atmosphere that immediately surrounds the machine.

Charles. Would the case be different, if the rubber were fixed on a conducting substance, instead of glass?

Tutor. It would; but there is a much easier method: I will attach one end of this brass chain to the cushion at R, which being several feet long, lies on the table, or on the floor, and this you know is connected, by means of other objects, with the earth, which is the grand reservoir of the electric fluid. Now see the effect of turning round the cylinder: but I must make every part of it dry and rather warm, by rubbing it with a dry warm cloth.

James. It is indeed very powerful. What a crackling noise it makes!

Tutor. Shut the window-shutters.

Charles. The appearance is very beautiful: the flashes from the silk dart all round the cylinder.

Tutor. I will now bring to the cylinder the tin conductor L, which is also placed on a glass pillar, F N, fixed in the stand at F.

James. What are the points in the tin conductor for?

Tutor. They are intended to collect the fluid from the cylinder. I will turn the cylinder, and do you hold your knuckle within four or five inches of the conductor.

Charles. The painful sensations which these sparks occasion, prove that the electric fluid is a very powerful agent, when collected in large quantities.

Tutor. To show you the nature of conducting bodies, I will now throw another brass chain over the conductor, so that one end of it may lie on the floor. See now if you can get any sparks, while I turn the machine.

James. No, I can get none, put my knuckle as near to it as I will.-Does it all run away by the chain?

Tutor. It does; a piece of brass or iron wire would do as well; and so would any conducting substance, which touched the conductor with one end, and the floor with the other: your body

would do as well as the chain. Place your hand on the conductor, while I turn round the cylinder: and let your brother bring his knuckle near the conductor.

Charles. I can get no spark.

Tutor. It runs through James to the earth, and you see his body is a conductor as well as the chain. With a very little contrivance, I can take sparks from you or James, as well as you did from the conductor.

James. I should like to see how that is done. Tutor. Here is a small stool, having a mahogany top and glass legs. If you stand on that, and put your hand on the conductor, the electricity will pass from the conductor to your body. Charles. Will the glass legs prevent it from running from him to the earth?

Tutor. They will: and therefore what he receives from the conductor, he will be ready to part with to any of the surrounding bodies, or to you, if you bring your hand near enough to any part of him.

James. The sparks are more painful in coming through my clothes, than when I received them on my bare hand.

Tutor. They are: you understand, I hope, the process.

Charles. By means of the chain trailing on the ground, the electric fluid is collected from the earth on the glass cylinder, which gives it through the points to the conductor: from this