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FROM E. KINNERSLEY TO BENJAMIN FRANKLIN.
Experiments on boiling Water, and Glass heated by boiling Water. — Doctrine of Repulsion in electrized Bodies doubted. — Electricity of the Atmosphere at different Heights. — Electrical Horse-race. — Electrical Thermometer. — In what Cases the Electrical Fire produces Heat. — Wire lengthened by Electricity. — Good Effect of a Rod on the House of Mr. West, of Philadelphia. ,.
Philadelphia, 12 March, 1761.
Having lately made the following experiments, I very cheerfully communicate them, in hopes of giving you some degree of pleasure, and exciting you to further explore your favorite, but not quite exhausted subject, electricity.
I placed myself on an electric stand, and, being well electrized, threw my hat to an unelectrized person, at a considerable distance, on another stand, and found that the hat carried some of the electricity with it; for, upon going immediately to the person who received it, and holding a flaxen thread near him, I perceived he was electrized sufficiently to attract the thread.
I then suspended by silk a broad plate of metal, and electrized some boiling water under it, at about four feet distance, expecting that the vapor, which ascended plentifully to the plate, would, upon the principle of the foregoing experiment, carry up some of the electricity with it; but was at length fully convinced, by several repeated trials, that it left all its share thereof behind. This I know not how to account for; but does it not seem to corroborate your hypothesis, that the vapors of which the clouds are formed, leave their share of electricity behind, in the common stock, and ascend in the negative state?
I put boiling water into a coated Florence flask, and found that the heat so enlarged the pores of the glass, that it could not be charged. The electricity passed through as readily, to all appearance, as through metal; the charge of a three-pint bottle went freely through, without injuring the flask in the least. When it became almost cold, I could charge it as usual. Would not this experiment convince the Abbe Nollet of his egregious mistake? For, while the electricity went fairly through the glass, as he contends it always does, the glass could not be charged at all.
I took a slender piece of cedar, about eighteen inches long, fixed a brass cap in the middle, thrust a pin horizontally and at right angles, through each end (the points in contrary directions), and hung it, nicely balanced, like the needle of a compass, on a pin, about six inches long, fixed in the centre of an electric stand. Then, electrizing the stand, I had the pleasure of seeing what I expected; the wooden needle turned round, carrying the pins with their heads foremost. I then electrized the stand negatively, expecting the needle to turn the contrary way, but was extremely disappointed, for it went still the same way as before. When the stand was electrized positively, I suppose that the natural quantity of electricity in the air, being increased on one side by what issued from the points, the needle was attracted by the lesser quantity on the other side. When electrized negatively, I suppose that the natural quantity of electricity in the air was diminished near the points; in consequence whereof, the equilibrium being destroyed, the needle was attracted by the greater quantity on the opposite side.
The doctrine of repulsion, in electrized bodies, I begin to be somewhat doubtful of. I think all the phenomena on which it is founded may be well enough accounted for Without it. Will not cork balls, electrized negatively, separate as far as when electrized positively? And may not their separation in both cases be accounted for upon the same principle, namely, the mutual attraction of the natural quantity in the air, and that which is denser or rarer in the cork balls? it being one of the established laws of this fluid, that quantities of different densities shall mutually attract each other, in order to restore the equilibrium.
I can see no reason to conclude that the air has not its share of the common stock of electricity, as well as glass, and, perhaps, all other electrics per se. For, though the air will admit bodies to be electrized in it, either positively or negatively, and will not readily carry off the redundancy in the one case, or supply the deficiency in the other; yet, let a person in the negative state, out of doors in the dark, when the air is dry, hold, with his arm extended, a long sharp needle, pointing upwards, and he will soon be convinced that electricity may be drawn out of the air; not very plentifully, for, being a bad conductor, it seems loath to part with it, but yet some will evidently be collected. The air near the person's body, having less than its natural quantity, will have none to spare; but, his arm being extended, as above, some will be collected from the remoter air, and will appear luminous, as it converges to the point of the needle.
Let a person electrized negatively present the point of a needle, horizontally, to a cork ball, suspended by silk, and the ball will be attracted towards the point, till it has parted with so much of its natural quantity of electricity, as to be in the negative state in the same degree with the person who holds the needle; then it
Vol. v. 47
will recede from the point, being, as I suppose, attracted the contrary way by the electricity of greater density in the air behind it. But, as this opinion seems to deviate from electrical orthodoxy, I should be glad to see these phenomena better accounted for by your superior and more penetrating genius.
Whether the electricity in the air, in clear, dry weather, be of the same density at the height of two or three hundred yards, as near the surface of the earth, may be satisfactorily determined by your old experiment of the kite. The twine should have throughout a very small wire in it, and the ends of the wire, where the several lengths are united, ought to be tied dowm with a waxed thread, to prevent their acting in the manner of points. I have tried the experiment twice, when the air was as dry as we ever have it, and so clear that not a cloud could be seen, and found the twine each time in a small degree electrized positively. The kite had three metalline points fixed to it; one on the top, and one on each side. That the twine was electrized, appeared by the separating of two small cork balls, suspended on the twine by fine flaxen threads, just above where the silk was tied to it, and sheltered from the wind. That the twine was electrized positively, was proved by applying to it the wire of a charged bottle, which caused the balls to separate further, without first coming nearer together. This experiment showed, that the electricity in the air, at those times, was denser above than below. But that cannot be always the case; for, you know, we have frequently found the thunder-clouds in the negative state, attracting electricity from the earth; which state, it is probable, they are always in when first formed, and till they have received a sufficient supply. How they come afterwards, towards the latter end of the gust, to be in the positive state, which is sometimes the case, is a subject for further inquiry.
After the above experiments with the wooden needle, I formed a cross, of two pieces of wood of equal length, intersecting each other at right angles in the middle, hung it horizontally upon a central pin, and set a light horse, with his rider, upon each extremity; whereupon, the whole being nicely balanced, and each courser urged on by an electrized point of a pair of spurs, I was entertained with an electrical horse-race.
I have contrived an electrical air-thermometer, and made several experiments with it, that have afforded me much satisfaction and pleasure. It is extremely sensible of any alteration in the state of the included air, and fully determines that controverted point, whether there be any heat in the electric fire. By the enclosed draft, and the following description, you will readily apprehend the construction of it . (See Plate II.)
A, B is a glass tube, about eleven inches long, and one inch diameter in the bore. It has a brass ferrule cemented on each end, with a top and bottom part, C and D, to be screwed on, air-tight, and taken off at pleasure. In the centre of the bottom part, D, is a male screw, which goes into a brass nut, in the mahogany pedestal E. The wires, F and G, are for the electric fire to pass through, darting from one to the other. The wire G extends through the pedestal to H, and may be raised and lowered by means of a male screw on it. The wire F may be taken out, and the hook / be screwed into its place. K is a glass tube, with a small bore, open at both ends, cemented in the brass tube L, which screws into the top part C. The lower end of the tube K is immersed in water, colored with cochineal, at the bottom of the tube A, B. (I used, at first, colored spirits of wine; but, in one experiment