Abbildungen der Seite
PDF
EPUB

I made, it took fire.) On the top of the tube K is cemented, for ornament, a brass ferrule, with a head screwed on it, which has a small air-hole through its side, at a. The wire b is a small round spring, that embraces the tube K, so as to stay wherever it is placed. The weight M is to keep straight whatever may be suspended, in the tube A, B, on the hook I. Air must be blown through the tube K, into the tube A,B, till enough is intruded to raise, by its elastic force, a column of the colored water in the tube K, up to c, or thereabouts; and then, the gage-wire b being slipped down to the top of the column, the thermometer is ready for use.

I set the thermometer on an electric stand, with the chain N fixed to the prime conductor, and kept it well electrized a considerable time; but this produced no sensible effect; which shows, that the electric fire, when in a state of rest, has no more heat than the air, and other matter wherein it resides.

When the wires F and G are in contact, a large charge of electricity sent through them, even that of my case of five and thirty bottles, containing above thirty square feet of coated glass, will produce no rarefaction of the air included in the tube A, B; which shows that the wires are not heated by the fire's passing through them.

When the wires are about two inches apart, the charge of a three-pint bottle, darting from one to the other, rarefies the air very evidently; which shows, I think, that the electric fire must produce heat in itself, as well as in the air, by its rapid motion.

The charge of one of my glass jars, (which will contain about five gallons and a half, wine measure,) darting from wire to wire, will, by the disturbance it gives the air, repelling it in all directions, raise the column in

[ocr errors]
[graphic]
[graphic][ocr errors]

the tube K, up to d, or thereabouts; and the charge of the abovementioned case of bottles will raise it to the top of the tube. Upon the air's coalescing, the column, by its gravity, instantly subsides, till it is in equilibrio with the rarefied air; it then gradually descends as the air cools, and settles where it stood before. By carefully observing at what height above the gage-wire b the descending column first stops, the degree of rarefaction is discovered, which, in great explosions, is very considerable.

I hung in the thermometer, successively, a strip of wet writing-paper, a wet flaxen and woollen thread, a blade of green grass, a filament of green wood, a fine silver thread, a very small brass wire, and a strip of gilt paper; and found that the charge of the abovementioned glass jar, passing through each of these, especially the last, produced heat enough to rarefy the air very perceptibly.

I then suspended, out of the thermometer, a piece of small harpsichord wire, about twenty-four inches long, with a pound weight at the lower end, and sent the charge of the case of five and thirty bottles through it, whereby I discovered a new method of wire-drawing. The wire was red-hot the whole length, well annealed, and above an inch longer than before. A second charge melted it; it parted near the middle, and measured, when the ends were put together, four inches longer than at first. This experiment, I remember, you proposed to me before you left Philadelphia; but I never tried it till now. That I might have no doubt of the wire's being hot as well as red, I repeated the experiment on another piece of the same wire, encompassed with a goose-quill, filled with loose grains of gunpowder; which took fire as readily as if it had been touched with a red-hot poker. Also tinder, tied to another

VOL. V. FF

piece of the wire, kindled by it. I tried a wire about three times as big, but could produce no such effects with that.

Hence it appears that the electric fire, though it has no sensible heat when in a state of rest, will, by its violent motion, and the resistance it meets with, produce heat in other bodies when passing through them, provided they be small enough. A large quantity will pass through a large wire without producing any sensible heat; when the same quantity, passing through a very small one, being there confined to a narrower passage, the particles crowding closer together and meeting with greater resistance, will make it red-hot, and even melt it.

Hence lightning does not melt metal by a cold fusion, as we formerly supposed; but, when it passes through the blade of a sword, if the quantity be not very great, it may heat the point so as to melt it, while the broadest and thickest part may not be sensibly warmer than before.

And, when trees or houses are set on fire by the dreadful quantity which a cloud, or the earth, sometimes discharges, must not the heat, by which the wood is first kindled, be generated by the lightning's violent motion, through the resisting combustible matter?

If lightning, by its rapid motion, produces heat in itself, as well as in other bodies, (and that it does, I think is evident from some of the foregoing experiments made with the thermometer), then its sometimes singeing the hair of animals killed by it, may easily be accounted for. And the reason of its not always doing so, may perhaps be this; the quantity, though sufficient to kill a large animal, may sometimes not be great enough, or not have met with resistance enough, to become, by its motion, burning hot.

« ZurückWeiter »