« ZurückWeiter »
times the sparks would follow extremely quick, and once I had a continual stream from bell to bell, the size of a crow-quill; even during the same gust there were considerable variations.
In the winter following I conceived an experiment, to try whether the clouds were electrified positively or negatively; but my pointed rod, with its apparatus, becoming out of order, I did not refit it till towards the spring, when I expected the warm weather would bring on more frequent thunder-clouds.
The experiment was this; to take two phials; charge one of them with lightning from the iron rod, and give the other an equal charge by the electric glass globe, through the prime conductor; when charged, to place them on a table within three or four inches of each other, a small cork ball being suspended by a fine silk thread from the ceiling, so as it might play between the wires. If both bottles then were electrified positively, the ball, being attracted and repelled by one, must be also repelled by the other. If the one positively, and the other negatively, then the ball would be attracted and repelled alternately by each, and continue to play between them as long as any considerable charge remained, i
Being very intent on making this experiment, it was no small mortification to me, that I happened to be abroad during two of the greatest thunder-storms we had early in the spring; and, though I had given orders in the family, that, if the bells rang when I was from home, they should catch some of the lightning for me in electrical phials, and they did so, yet it was mostly dissipated before my return; and, in some of the other gusts, the quantity of lightning I was able to obtain was so small, and the charge so weak, that I could not satisfy myself; yet I sometimes saw what heightened my suspicions, and inflamed my curiosity.
At last, on the 12th of April, 1753, there being a smart gust of some continuance, I charged one phial pretty well with lightning, and the other equally, as near as I could judge, with electricity from my glass globe; and, having placed them properly, I beheld, with great surprise and pleasure, the cork ball play briskly between them, and was convinced, that one bottle was electrized negatively.
I repeated this experiment several times during the gust, and in eight succeeding gusts, always with the same success; and being of opinion (for reasons I formerly gave in my letter to Mr. Kinnersley, since printed b London), that the glass globe electrizes positively, I concluded, that the clouds are always electrized negatively, or have always in them less than their natural quantity of the electric fluid.
Yet, notwithstanding so many experiments, it seems I concluded too soon; for at last, June the 6th, in a gust which continued from five o'clock, P. M., to seven, I met with one cloud that was electrized positively, though several that passed over my rod before, during the same gust, were in the negative state. This was thus discovered. .
I had another concurring experiment, which I often repeated, to prove the negative state of the clouds, viz., while the bells were ringing, I took the phiaj, charged from the glass globe, and applied its wire to the erected rod, considering, that if the clouds were electrized positively, the rod, which received its electricity from them, must be so too; and then the additional positive electricity of the phial would make the bells ring faster; but, if the clouds-were in a negative state, they must exhaust the electric fluid from my rod, and bring that into the same negative state with themselves, and then the wire of a positively charged phial, supplying the rod with what it wanted (which it was obliged otherwise to draw from the earth by means of the pendulous brass ball playing between the two bells), the ringing would cease till the bottle was discharged.
In this manner I quite discharged into the rod several phials, that were charged from the glass globe, the electric fluid streaming from the wire to the rod, till the wire would receive no spark from the finger; and, during this supply to the rod from the phial, the bells stopped ringing; but, by continuing the application of the phial wire to the rod, I exhausted the natural quantity from the inside surface of the same phials, or, as I call it, charged them negatively.
At length, while I was charging a phial by my glass globe, to repeat this experiment, my bells of themselves stopped ringing, and, after some pause, began to ring again. But now, when I approached the wire of the charged phial to the rod, instead of the usual stream, that I expected from the wire to the rod, there was no spark; not even when I brought the wire and the rod to touch; yet the bells continued ringing vigorously, which proved to me, that the rod was then positively electrified, as well as the wire of the phial, and equally so; and, consequently, that the particular cloud then over the rod was in the same positive state. This was near the end of the gust.
But this was a single experiment, which, however, destroys my first too general conclusion, and reduces me to this; That the clouds of a thunder-gust are most commonly in a negative state of electricity, but sometimes in a positive state.
The latter I believe is rare; for, though I, soon after the last experiment, set out on a journey to Boston, and was from home most part of the summer, which prevented my making farther trials and observations; yet Mr. Kinnersley, returning from the Islands just as I left home, pursued the experiments during my absence, and informs me, that he always found the clouds in the negative state.
So that, for the most part, in thunder-strokes, it is the earth that stinkes into the clouds, and not the clouds that strike into the earth.
Those who are versed in electric experiments, will easily conceive, that the effects and appearances must be nearly the same in either case; the same explosion, and the same flash between one cloud and another, and between the clouds and mountains, &c., the same rending of trees, walls, &,c., which the electric fluid meets with in its passage, and the same fatal shock to animal bodies; and that pointed rods fixed on buildings, or masts of ships, and communicating with the earth or sea, must be of the same service in restoring the equilibrium silently between the earth and clouds, or in conducting a flash or stroke, if one should be, so as to save harmless the house or vessel; for points have equal power to throw off, as to draw on, the electric fire, and- rods will conduct up as well as down.
But, though the light gained from these experiments makes no alteration in the practice, it makes a considerable one in the theory. And now we as much need an hypothesis to explain by what means the clouds become negatively, as before to show how they became positively, electrified.
I cannot forbear venturing some few conjectures on this occasion; they are what occur to me at present, and, though future discoveries should prove them not wholly right, yet they may in the mean time be of some use, by stirring up the curious to make more experiments, and occasion more exact disquisitions.
I conceive, then, that this globe of earth and water,
Vol. v. 39 z*
with its plants, animals, and buildings, have diffused throughout their substance, a quantity of the electric fluid, just as much as they can contain, which I call the natural quantity.
That this natural quantity is not the same in all kinds of common matter under the same dimensions, nor in the same kind of common matter in all circumstances; but a solid foot, for instance, of one kind of common matter may contain more of the electric fluid than a solid foot of some other kind of common matter; and a pound weight of the same kind of common matter may, when in a rarer state, contain more of the electric fluid than when in a denser ptate.
For, the electric fluid being "attracted by any portion of common matter, the parts of that fluid (which have among themselves a mutual repulsion) are brought so near to each other,: by the attraction of the common matter that absorbs them, as that their repulsion is equal to the condensing power of attraction in common matter; and then such portion of common matter will absorb no more.
Bodies of different kinds, having thus attracted and absorbed what I call their natural quantity, that is, just as much of the electric fluid as is suited to their circumstances of density, rarity, and power of attracting, do not then show any signs of electricity among each other.
And, if more electric fluid be added to one of these bodies, it does not enter, but spreads on the surface, forming an atmosphere; and then such body shows signs of electricity.
I have, in a former paper, compared common matter to a sponge, and the electric fluid to water; I beg leave once more to make use of the same comparison, to illustrate farther my meaning in this particular,