Abbildungen der Seite
PDF
EPUB

If it be asked, What thickness of a metalline rod may be supposed sufficient ? in answer, I would remark, that five large glass jars, such as I have described in my former papers, discharge a very great quantity of electricity, which nevertheless will be all conducted round the corner of a book, by the fine filleting of gold on the cover, it following the gold the farthest way about, rather than take the shorter course through the cover, that not being so good a conductor. Now, in this line of gold, the metal is so extremely thin as to be little more than the color of gold, and on an octavo book is not in the whole an inch square, and therefore not the thirty-sixth part of a grain, according to M. Réaumur; yet it is sufficient to conduct the charge of five large jars, and how many more I know not. Now, I suppose a wire of a quarter of an inch diameter to contain about five thousand times as much metal as there is in that gold line; and, if so, it will conduct the charge of twenty-five thousand such glass jars, which is a quantity, I imagine, far beyond what was ever contained in any one stroke of natural lightning. But a rod of half an inch diameter would conduct four times as much as one of a quarter.

And with regard to conducting, though a certain thickness of metal be required to conduct a great quantity of electricity, and, at the same time, keep its own substance firm and unseparated; and a less quantity, as a very small wire, for instance, will be destroyed by the explosion ; yet such small wire will have answered the end of conducting that stroke, though it become incapable of conducting another. And, considering the extreme rapidity with which the electric fluid moves without exploding, when it has a free passage, or complete metal communication, I should think a vast quantity would be conducted in a short time, either to or from a cloud, to restore its equilibrium with the earth, by means of a very small wire; and therefore thick rods should seem not so necessary. However, as the quantity of lightning discharged in one stroke cannot well be measured, and in different strokes is certainly very various, in some much greater than in others; and as iron (the best metal for the purpose, being least apt to fuse) is cheap, it may be well enough to provide a larger canal to guide that impetuous blast than we imagine necessary; før, though one middling wire may be sufficient, two or three can do no harm. And time, with careful observations well compared, will at length point out the proper size to greater certainty.

Pointed rods erected on edifices may likewise often prevent a stroke, in the following manner. , An eye so situated as to view horizontally the under side of a thunder-cloud, will see it very ragged, with a number of separate fragments, or petty clouds, one under another, the lowest sometimes not far from the earth. These, as so many stepping-stones, assist in conducting a stroke between the cloud and a building. To represent these by an experiment, take two or three locks of fine, loose cotton; connect one of them with the prime conductor by a fine thread of two inches (which may be spun out of the same lock by the fingers), another to that, and the third to the second, by like threads. Turn the globe, and you will see these locks extend themselves towards the table (as the lower small clouds do towards the earth), being attracted by it; but, on presenting a sharp point erect under the lowest, it will shrink up to the second, the second to the first, and all together to the prime conductor, where they will continue as long as the point continues under them. May not, in like manner, the small electrized clouds, whose equilibrium with the earth is soon restored VOL. V. 40

AA

by the point, rise up to the main body, and by that means occasion so large a vacancy, as that the grand cloud cannot strike in that place? These thoughts, my dear friend, are many of them crude and hasty; and, if I were merely ambitious of acquiring some reputation in philosophy, I ought to keep them by me, till corrected and improved by time and farther experience. But since even short hints and imperfect experiments in any new branch of science, being communicated, have oftentimes a good effect, in exciting the attention of the ingenious to the subject, and so become the occasion of more exact disquisition, and more complete discoveries, you are at liberty to communicate this paper to whom you please; it being of more importance that knowledge should increase, than that your friend should be thought an accurate philosopher.

B. FRANKLIN. — To PETER collinson.

Jyotice of another Packet of Letters.

Philadelphia, 23 November, 1753. DEAR FRIEND,

In my last, viá Virginia, I promised to send you per next ship, a small philosophical packet; but now, having got the materials (old letters and rough drafts) before me, I fear you will find it a great one. Nevertheless, as I am like to have a few days leisure before this ship sails, which I may not have again in a long time, I shall transcribe the whole, and send it; for you will be under no necessity of reading it all at once, but may take it a little at a time, now and then of a winter evening. When you happen to have nothing else to do (if that ever happens),it may afford you some amuse

ment.” B. FRANKLIN.

Proposal of an Experiment to measure the Time taken up by an Electric Spark in moving through any given Space. By James .Alexander, of .New York.f

READ AT THE Royal society, DEcEMBER 26TH, 1756.

IF I remember right, the Royal Society made one experiment to discover the velocity of the electric fire, by a wire of about four miles in length, supported by silk, and by turning it forwards and backwards in a field, so that the beginning and end of the wire were at only the distance of two people, the one holding the Leyden bottle and the beginning of the wire, and the other holding the end of the wire and touching the ring of the bottle; but by this experiment no discov

[ocr errors]

* These letters and papers are a philosophical correspondence between Mr. Franklin and some of his American friends. Mr. Collinson communicated them to the Royal Society, where they were read at different meetings during the year 1756. But, Mr. Franklin having particularly requested that they might not be printed, none of them were inserted in the Transactions. Mr. Franklin had at that time an intention of revising them, and pursuing some of the inquiries farther; but, finding that he is not like to have sufficient leisure, he has at length been induced, imperfect as they are, to permit their publication, as some of the hints they contain may possibly be useful to others in their philosophical researches. — Mote in Mr. Collinson's edition.

As some of the papers transmitted in the above letter to Mr. Collinson do not relate to electricity, they are transferred to the parts of the work in which they belong, according to their subjects. They are all inserted, as far as it can be ascertained, in the order of their dates.— Editor.

# This paper and the following one were among those communicated to Mr. Collinson, November 23d, 1753. Their dates are uncertain. —Editor.

ery was made, except that the velocity was extremely quick.

As water is a conductor as well as metals, it is to be considered, whether the velocity of the electric fire might not be discovered by means of water; whether a river, or lake, or sea, may not be made part of the circuit through which the electric fire passes, instead of the circuit all of wire, as in the above experiment.

Whether in a river, lake, or sea, the electric fire will not dissipate, and not return to the bottle ? or, will it proceed in straight lines through the water the shortest course possible back to the bottle ?

If the last, then suppose one brook that falls into Delaware doth head very near to a brook that falls into Schuylkill; and let a wire be stretched and supported as before, from the head of one brook to the head of the other; and let the one end communicate with the water; and let one person stand in the other brook, holding the Leyden bottle; and let another person hold that end of the wire not in the water, and touch the ring of the bottle. If the electric fire will go as in the last question, then will it go down the one brook to Delaware or Schuylkill, and down one of them to their meeting, and up the other and the other brook; the time of its doing this may possibly be observable, and the farther upwards the brooks are, chosen, the more observable it would be.

Should this be not observable, then suppose the two brooks falling into Susquehanna and Delaware, and proceeding 'as before, the electric fire may, by that means, make a circuit round the North Cape of Virginia, and go many hundreds of miles, and in doing that, it would seem, it must take some observable time.

If still no observable time is found in that experiment, then suppose the brooks falling the one into the

« ZurückWeiter »