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of a battery through it, which will completely melt it, and you now perceive the little globules of iron instead of the thin wire.

Charles. Will other wires besides iron be melted in the same manner?

Tutor. Yes, if the battery be large enough, and the wires, sufficiently thin, the experiment will succeed with them all even with a single jar, if it be pretty large, very slender wire may be fused. But the charges of batteries have been used to determine the different conducting powers of the several metals.

James. If the charge is not strong enough to melt the wire, will it make it red hot?

Tutor. It will: and when the experiment is properly done, the course of the fluid may be discerned by its effects: for if the wire is about three inches long, it will be seen that the end of it, which is connected with the inside of the battery, is

red-hot first, and the redness proceeds towards the other.

Charles. That is a clear proof that the superabundant, electricity accumulated in the inside is carried to the outside of the jars.

Tutor. Example 9. We have in the present volume discussed the subject of magnetism: and we may here observe that by discharging the battery through a small sewing needle, it will become magnetic, that is, if the needle be accurately suspended on a small piece of cork in a basin of water, one end will, of itself, point to the north, and the other to the south.

Example 10. I will lay this chain on a sheet of writing-paper, and send the charge of the battery through the chain; and you will see black marks will be left on the paper in those places where the rings of the chain touch each other.

Example 11. Place a small piece of very dry wood between the balls of the universal dischargers so that the fibres of

the wood may be in the direction of the wires, and pass the charge of the battery through them, the wood will be torn in pieces. The points of the wires being run into the wood, and the shock passed through them, will effect the same thing.

Example 12. Here is a glass tube, open at both ends, six inches long, and a quarter of an inch in diameter. These pieces of cork, with wires in them, exactly fit the ends of the tube. I put in one cork, and fill the tube with water, then put the other cork in, and push the wires so that they nearly touch, and pass the charge of the battery through them, you see the tube is broken, and the water dispersed in every direction.*

* To prevent accidents, a wire cage, such as is used in some experiments on the air-pump, should be put over the tube before the discharge is made young persons should not attempt this experiment by themselves.

Charles. If water is a good conductor, how is it that the charge did not run through it without breaking the tube?

Tutor. The electric fluid, like common fire, converts the water into a highly elastic vapour, which occupying very suddenly a much larger space than the water, bursts the tube before it can effect any means of escape.

CONVERSATION XXXVII.

Of the Electric Spark, and Miscellaneous Experiments.

TUTOR. I wish you to observe some facts connected with the electric spark. By means of the wire inserted in this ball, I fix it to the end of the conductor, and bring either another brass ball, or my knuckle to it, and if the machine act pretty powerfully, a long crooked, brilliant spark, will pass between the two balls, or between the knuckle and ball. If the conductor is negative, it receives the spark from the body; but if it is positive, the ball or the knuckle receives the spark from the conductor.

Charles. Does the size of the spark depend at all on the size of the conductor?

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