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add a foot to the semidiameter of the circle. By quite dry air, I mean the dryest we have; for perhaps we never have any perfectly free from moisture. An electrical atmosphere raised round a thick wire, inserted in a phial of air, drives out none of the air, nor on withdrawing that atmosphere will any air rush in, as I have found by a curious experiment * accurately made, whence we concluded that the air's elasticity was not affected thereby. • <
An Experiment towards discovering more of the Qualities of the Electric Fluid.
From the prime conductor, hang a bullet by a wire hook; under the bullet, at half an inch distance, place a bright piece of silver to receive the sparks; then let the wheel be turned, and in a few minutes (if the repeated sparks continually strike in the same spot) the silver will receive a blue stain, nearly the color of a watch-spring.
* The experiment here mentioned was thus made. An empty phial was stopped with a cork. Through the cork passed a thick wire, as usual in the Leyden experiment, which wire almost reached the bottom. Through another part of the cork passed one leg of a small glass siphon, the other leg on the outside came down almost to the bottom of the phial. This phial was first held a short time in the hand, which, warming and of course rarefying the air within, drove a small part of it out through the siphon. Then a little red. ink in a tea-spoon was applied to the opening of the outer leg of the siphon; so that as the air within cooled, a little of the ink might rise in that leg. When the air within the bottle came to be of the same temperature of that without, the drop of red ink would rest in a certain part of the leg. But the warmth of a finger applied to the phial would cause that drop to descend, as the least outward coolness applied would make it ascend. When it had found its situation, and was at rest, the wire was electrified by a communication from the prime conductor. This was supposed to give an electric atmosphere to the wire within the bottle, which might likewise rarefy the included air, and of course depress the drop of ink in the siphon. But no such efiect followed.
A bright piece of iron will also be spotted, but not with that color; it rather seems corroded.
"On gold, brass, or tin, I have not perceived it makes any impression. But the spots on the silver or iron will be the same, whether the bullet be lead, brass, gold, or silver.
On a silver bullet there will also appear a small spot, as well as on the plate below it .
FROM JAMES BOWDOIN TO BENJAMIN FRANKLIN.
On the Causes of the crooked Direction of Lightning. — Objections to the Hypothesis that the Sea is the Source of Lightning. — On the Swiftness of the Electrical Fire.
Boston, 21 December, 1751.
The experiments Mr. Kinnersley has exhibited here, have been greatly pleasing to all sorts of people, that have seen them; and I hope, by the time he returns to Philadelphia, his tour this way will turn to good account. His experiments are very curious, and I think prove most effectually your doctrine of electricity; that it is a real element annexed to, and diffused among, all bodies we are acquainted with; that it differs in nothing from lightning, the effects of both being similar, and their properties, so far as they are known, the same.
The remarkable effect of lightning on iron lately discovered in giving it the magnetic virtue, and the same effect produced on small needles by the electrical fire, are a further and convincing proof that they are both the same element; but, which is very unaccountable, Mr. Kinnersley tells me it is necessary, to produce this effect, that the direction of the needle and the electrical fire should be north and south, from either to the other; and that, just so far as they deviate therefrom, the magnetic power in the needle is less, till, their direction being at right angles with north and south, the effect entirely ceases.
We made at Faneuil Hall, where Mr. Kinnersley's apparatus is, several experiments to give some small needles the magnetic virtue, previously examining, by putting them in water, on which they will be supported, whether or not they had any of that virtue; and I think we found all of them to have some small degree of it, their points turning to the north. We had nothing to do then, but to invert the poles, which accordingly was done by sending through them the charge of two large glass jars; the eye of the needle turning to the north, as the point before had done. That end of the needle, which the fire is thrown upon, Mr. Kinnersley tells me, always points to the north.
The electrical fire, passing through the air, has the same crooked direction as lightning.* This appearance I endeavour to account for thus. Air is an electric per se; therefore there must be a mutual repulsion between air and the electrical fire. A column or cylinder of air, having the diameter of its base equal to the diameter of the electrical spark, intervenes between that part of the body which the spark is drawn from and that of the body it aims at. The spark acts upon this column, and is acted upon by it, more strongly than any other neighbouring portion of air. The column, being thus acted upon, becomes more dense, and, being more dense, repels the spark more strongly; its repellency being in proportion to its density. Having acquired, by being condensed, a degree of repellency greater than its natural, it turns the spark out of its straight course; the neighbouring air, which must be Jess dense, and therefore has a smaller degree of repellency, giving it a more ready passage. The spark having taken a new direction must now act on, or most strongly repel, the column of air which lies in that direction, and consequently must condense that column in the same manner as the former, when the spark must again change its course, which course will be repeatedly changed, till the spark reaches the body that attracted it.
* This is most easily observed in large strong sparks, taken at some inches distance.
To this account one objection occurs; that, as air is very fluid and elastic, and so endeavours to diffuse itself equally, the supposed accumulated air within the column aforesaid would be immediately diffused among the contiguous air, and circulate to fill the space it was driven from, and consequently that the said column, on the greater density of which the phenomenon is supposed to depend, would not repel the spark more strongly than the neighbouring air.
This might be an objection, if the electrical fire was as sluggish and inactive as air. Air takes a sensible time to diffuse itself equally, as is manifest from winds, which often blow for a considerable time together from the same point, and with a velocity, even in the greatest storms, not exceeding, as it is said, sixty miles an hour; but the electrical fire seems propagated instantaneously, taking up no perceptible time in going very great distances. It must be, then, an inconceivably short time in its progress from an electrified to an unelectrified body, which, in the present case, can be but a few inches apart. But this small, portion of time is not sufficient for the elasticity of the air to exert itself, and therefore the column aforesaid must be in a denser state than its neighbouring air.
Vol. v. 34 w
About the velocity of the electrical fire more is said below, which perhaps may more fully obviate this objection. But let lis have recourse to experiments. Experiments will obviate all objections, or confound the hypothesis. The electrical spark, if the foregoing be true, will pass through a vacuum in a right line.
To try this, let a wire be fixed perpendicular on the plate of an air-pump, having a leaden ball on its upper end; let another wire, passing through the top of a receiver, have on each end a leaden ball; let the leaden balls within the receiver, when put on the- air-pump, be within two or three inches of each other; the receiver being exhausted, the spark given from a charged phial to the upper wire will pass through rarefied air, nearly approaching to a vacuum, to the lower wire, and I suppose in a right line, or nearly so; the small portion of air remaining in the receiver, which cannot be entirely exhausted, may possibly cause it to deviate a little, but perhaps not sensibly, from a right line. The spark also might be made to pass through air greatly condensed, which perhaps would" give it a still more crooked direction. I have not had an opportunity to make any experiments of this sort, not knowing of an air-pump nearer than Cambridge; but you can easily make them at your State-house, where there is one. If these experiments answer, I think the crooked direction of lightning will be also accounted for.
With respect to your letters on electricity, it will be no new thing to you to be told, that they are very curious and entertaining, and by far the best and most rational that have been written on that subject. Your hypothesis, in particular, for explaining the phenomena of lightning is very ingenious. That some clouds are highly charged with electrical fire, and that their communicating it to those that have less, to mountains, and