« ZurückWeiter »
TO PETER COLLINSON.
Additional Proofs of the Positive and Negative State of Electricity in the Clouds.-New Method of ascertaining it.
Philadelphia, 18 April, 1754.
Since September last, having been abroad on two long journeys, and otherwise much engaged, I have made but few observations on the positive and negative state of electricity in the clouds. But Mr. Kinnersley kept his rod and bells in good order, and has made many.
Once this winter the bells rang a long time during a fall of snow, though no thunder was heard, or lightning seen. Sometimes the flashes and cracks of the electric matter between bell and bell were so large and loud as to be heard all over the house; but, by all his observations, the clouds were constantly`in a negative state, till about six weeks ago, when he found them once to change in a few minutes from the negative to the positive. About a fortnight after that, he made another observation of the same kind; and last Monday afternoon, the wind blowing hard at southeast, and veering round to northeast, with many thick, driving clouds, there were five or six successive changes from negative to positive, and from positive to negative, the bells stopping a minute or two between every change. Besides the methods mentioned in my paper of September last, of discovering the electrical state of the clouds, the following may be used. When your bells are ringing, pass a rubbed tube by the edge of the bell, connected with your pointed rod; if the cloud is then in a negative state, the ringing will stop; if in a positive state, it
will continue, and perhaps be quicker. Or, suspend a very small cork ball by a fine silk thread, so that it may hang close to the edge of the rod-bell; then, whenever the bell is electrified, whether positively or negatively, the little ball will be repelled, and continue at some distance from the bell. Have ready a round-headed glass stopper of a decanter, rub it on your side till it is electrified, then present it to the cork ball. If the electricity in the ball is positive, it will be repelled from the glass stopper, as well as from the bell; if negative, it will fly to the stopper.
Made in Pursuance of those made by Mr. Canton, dated December 6th, 1753; with Explanations, by Benjamin Franklin.
READ AT THE ROYAL SOCIETY, DECEMBER 18TH, 1755.
Philadelphia, 14 March, 1755.
I. ELECTRIC atmospheres that flow round non-electric bodies, being brought near each other, do not readily mix and unite into one atmosphere, but remain separate, and repel each other.
This is plainly seen in suspended cork balls, and other bodies electrified.
II. An electric atmosphere not only repels another electric atmosphere, but will also repel the electric matter contained in the substance of a body approaching it; and, without joining or mixing with it, force it to other parts of the body that contained it.
This is shown by some of the following experiments. III. Bodies electrified negatively, or deprived of their natural quantity of electricity, repel each other (or at least appear to do so, by a mutual receding), as well as those electrified positively, or which have electric atmospheres.
This is shown by applying the negatively charged wire of a phial to two cork balls, suspended by silk threads, and many other experiments.
Fix a tassel of fifteen or twenty threads, three inches long, at one end of a tin prime conductor (mine is about five feet long and four inches diameter), supported by silk lines.
Let the threads be a little damp, but not wet.
Pass an excited glass tube near the other end of the prime conductor, so as to give it some sparks, and the threads will diverge.
Because each thread, as well as the prime conductor, has acquired an electric atmosphere, which repels and is repelled by the atmospheres of the other threads; if those several atmospheres would readily mix, the threads might unite, and hang in the middle of one atmosphere, common to them all.
Rub the tube afresh, and approach the prime conductor therewith, crosswise, near that end, but not nigh enough to give sparks; and the threads will diverge a little
Because the atmosphere of the prime conductor is pressed by the atmosphere of the excited tube, and driven towards the end where the threads are, by which each thread acquires more atmosphere.
Withdraw the tube, and they will close as much. They close as much, and no more; because the atmosphere of the glass tube, not having mixed with the atmosphere of the prime conductor, is withdrawn entire, having made no addition to, or diminution from it. Bring the excited tube under the tuft of threads, and they will close a little.
They close, because the atmosphere of the glass tube repels their atmospheres, and drives part of them back on the prime conductor.
Withdraw it, and they will diverge as much.
For the portion of atmosphere which they had lost, returns to them again.
Excite the glass tube, and approach the prime conductor with it, holding it across, near the end opposite to that on which the threads hang, at the distance of five or six inches. Keep it there a few seconds, and the threads of the tassels will diverge. Withdraw it, and they will close.
They diverge, because they have received electric atmospheres from the electric matter before contained. in the substance of the prime conductor; but which is now repelled and driven away, by the atmosphere of the glass tube, from the parts of the prime conductor opposite and nearest to that atmosphere, and forced out upon the surface of the prime conductor at its other end, and upon the threads hanging thereto. Were it any part of the atmosphere of the glass tube that flowed over and along the prime conductor to the threads, and gave them atmospheres (as is the case when a spark is given to the prime conductor from the glass tube), such part of the tube's atmosphere would have remained,
and the threads continue to diverge; but they close on withdrawing the tube, because the tube takes with it all its own atmosphere, and the electric matter, which had been driven out of the substance of the prime conductor, and formed atmospheres round the threads, is thereby permitted to return to its place.
Take a spark from the prime conductor near the threads, when they are diverged as before, and they will close.
For by so doing you take away their atmospheres, composed of the electric matter driven out of the substance of the prime conductor, as aforesaid, by the repellency of the atmosphere of the glass tube. By taking this spark you rob the prime conductor of part of its natural quantity of the electric matter; which part so taken is not supplied by the glass tube, for, when that is afterwards withdrawn, it takes with it its whole atmosphere, and leaves the prime conductor electrized negatively, as appears by the next operation.
Then withdraw the tube, and they will open again.
For now, the electric matter in the prime conductor returning to its equilibrium, or equal diffusion, in all parts of its substance, and the prime conductor having lost some of its natural quantity, the threads connected with it lose part of theirs, and so are electrized negatively, and therefore repel each other, by Principle III.
Approach the prime conductor with the tube, near the same place as at first, and they will close again.
Because the part of their natural quantity of electric fluid, which they had lost, is now restored to them again, by the repulsion of the glass tube forcing that fluid to them, from other parts of the prime conductor; so they are now again in their natural state.
Withdraw it, and they will open again.