THE MONTHLY REVIEW, For DECEMBER, 1779. ART. I. PHILOSOPHICAL TRANSACTIONS of the Royal Society of For the Year 1778. Part 2. 4to. London. Vol. LXVIII. 10 s. 6 d. fewed. Davis. 1779. PAPERS relating to ELECTRICITY, Article 37. Experiments on Electricity, being an Attempt to her the Advantage of elevated pointed Conductors. By Mr. Edward Nairne, F. R. S. WE E are forry, that we cannot give a very particular account of thefe ingenious and well imagined experiments; in confequence of the want of the plates by which they are illuftrated. They are well adapted to prove, fo far as the question feems capable of being proved by our artificial electrical apparatus, that elevated and pointed conductors are preferable to those which do not rife above the building, and which terminate in balls. Among other particulars, they fhew that a pointed conductor was not struck, nor an explofion produced, when it was expofed to an artificial cloud (or prime conductor electrified), at any distance whatever beyond a twentieth of an inch; when the electric fire ran to it in a very small ftream:-that, beyond that distance, the point only appeared luminous, and continued fo, till it was removed to the diftance of fix feet-that this power of the point to prevent an explosion, depended on its having a perfect or uninterrupted metallic communication with the earth; for that an explofion to the point would enfue, if any difcontinuity, or interruptions, were made in the fubftance of the pointed conductor.. From fome other of thefe experiments, it appears that, though an artificial cloud, A, hanging over a pointed conductor, and which receives a charge, or fpark, from time to time, from another artificial cloud, B, placed near it, will in VOL. LXI. D d fact fact ftrike into the point, immediately on receiving fucceffive fparks from the cloud, B, if the artificial cloud, A, be fixed: yet, if the latter be moveable (as is the cafe in real clouds, which are not fixed but floating bodies), though the distance continue the fame, the artificial cloud, A, recedes from the point, and does not ftrike it. From fubfequent experiments, it appears, that when the pointed conductor had a fwift motion given to it, under the charged prime conductor, it was ftruck; but a small ball, moving as fwiftly, was ftruck at a greater distance; and a large ball at a distance still greater.-We fhall have occafion to reconfider these experiments in the fucceeding article. A fingular appearance prefented itfelf in fome of the Author's experiments. A ball of one inch diameter, communicating with the earth, being presented near the end of a large prime conductor, ftrongly charged; fparks appeared till the ball was gradually removed to the diftance of two inches. The fparks ceafed, and were fucceeded by a luminous appearance on the ball, attended with a hiffing noife, while the ball was gradually removed to the distance of ten inches. The noise then ceafed, and the light difappeared; and the fparks again began to strike the ball, and continued to do fo till the diftance was about fifteen or fixteen inches. Article 36. Reafons for diffenting from the Report of the Committee In this paper, which fhould have fucceeded the preceding, Dr. Mufgrave attempts to invalidate the conclufions drawn by Mr. Nairne, from the foregoing fet of experiments; and to reconcile fome of the appearances prefented in them, with those exhibited in the trials of Mr. Wilfon at the Pantheon. The moft material obfervations relate to the different circumstances which enable pointed conductors to prevent an explosion in one cafe, and to receive it in another. Before I attempt,' fays the Author, to specify the particular cafes in which the fharp and the blunt terminations are refpectively more liable to electrical explosion; it may be of ufe to fhew (what many gentlemen feem not to be thoroughly aware of) that fharp points having the most perfect communica tion with the earth, are not wholly exempt from receiving them.' My first authority,' he adds, fhall be Dr. Franklin himfelf;'-and he then gives the following paffage from his letters, in proof of his affertion. Let a perfon," fays he, p. 60. "ftanding on the floor, prefent the point of a needle at twelve or more inches from it, [the prime conductor] and while the needle is fo prefented, the conductor cannot be charged; the point drawing off the fire as faft 1 faft as it is thrown on by the electrical globe. Let it be charged, and then prefent the point at the fame distance, and it will fuddenly be difcharged.". The word fuddenly means, I fuppofe,' adds the Author, that it will receive an explosion; that being the most natural and obvious proof of the fuddennefs of the discharge.' Surely Dr. Mufgrave, on reconfidering this paffage, muft be fenfible of the unfairness of the conclufion, which he has drawn from this quotation from Dr. Franklin; and of this mifreprefentation of his meaning, obviously founded on a mere ambiguity of expreffion, if indeed it may be fo called. The Author, nevertheless, afterwards avails himself of this forced interpretation of the word "fuddenly;"-and fays, it has been already fhewn, from the acknowledgment of Dr. Franklin-that electricity, accumulated to a certain degree, will explode upon a point.' Dr. Franklin's meaning, in the preceding quotation, is obvious enough, even without confidering the context. He means, that the prime conductor, on presenting a point to it, will be fuddenly, or rapidly, but, at the fame time, filently discharged, and without explosion. In the very lines immediately preceding this quotation, the prime conductor mentioned in it, is reprefented as being of fuch a fize and power, as to ftrike the knuckle at near two inches diftance; and his principal object is to fhew, that a point prefented to it would not be struck at any distance, but would quickly rob it of its electricity. Dr. Mufgrave furely cannot be ferious when he preffes Dr. Franklin, the great discoverer of the power of points, into the service of his party; and extorts from him a declaration, that this very prime conductor would produce an explosion, at the diftance even of twelve inches, on presenting the point of a needle to it; when it would not give an explosion, or a spark, to the knuckle or a blunt conductor, at a greater diftance than two or three inches! The queftion respecting the most advantageous method of terminating conductors may, in our opinion, be thus moft clearly, or, at leaft, briefly ftated. It is agreed, we believe, on all hands, nay, it is certain, that pointed conductors draw off the matter of lightning from a cloud, as it gradually approaches the zenith of the conductor; and, by thus diminishing the quantity, may, in many cafes, prevent an explosion, which might have taken place, had a blunt or rounded conductor been prefented to it; which no one pretends to poffefs any power of that kind. But another cafe exifts, or may exift; in which the fuperiority of the pointed over the blunt conductor may not appear quite fo manifeft, or, at leaft, fo confiderable. Dd 2 This This cafe will occur whenever a cloud hanging over a building receives fuddenly fuch a charge, by an explofion from another cloud at a diftance, as to enable it inftantly to strike into the earth. In this cafe, confidering the inftantaneity (as to sense) of the charge, the advantage which is juftly afcribed to the pointed conductor, from its power of gradually drawing off the electric matter, vanishes; as this gradual depletion implies time, which, from the circumstances, is not granted in the prefent cafe. This cafe has been imitated in thofe experiments, which have been made with what is called the interrupted conductor; or a fecond infulated conductor, or fubftitute, placed at fuch a distance from a prime conductor, as to receive ftrong sparks from it, or to become fuddenly charged by it. The queftion then is reduced to this ;-whether a vertical cloud may not fuddenly receive fuch a charge, by an explosion from a neighbouring cloud, as may enable it inftantly to ftrike into a pointed conductor, which, it is fuppofed, may invite the ftroke; and yet may be refifted by one that terminates in a ball, so that it fhall not explode upon it. An appeal has been made to experiment; and both parties allege, that experiment declares in their favour. The Author attempts to reconcile thefe different refults by the following diftinction. He affirms, that the experiment made with the interrupted or fecond conductor may be fo managed, as that the ball fhall receive the explofion at a greater distance than the point; or the point be ftruck at a greater diftance than the ball, at the pleafure of the operator; and that this diversity of effect depends on a fingle circumftance; viz. the height of the charge. If the fecond conductor be placed at fo great a distance from the first or prime conductor, as to receive from it the fulleft and ftrongeft fpark that it is capable of giving; the point placed under the fecond conductor will be ftruck preferably to the ball, and at a much greater diftance. In a particular cafe, mentioned by the Author, the point is faid to have been ftruck at a distance fix times greater than that at which the ball would receive a ftroke. He accounts for this effect, from the greater fufceptibility of pointed bodies to admit the electric fluid, or their giving lefs refiftance to it; combined with their incapacity, in the prefent cafe, of drawing off the electric matter filently. On the contrary, if the fecond conductor be placed confiderably within the diftance above defcribed, or fo near the firft conductor, as to receive only a weak fpark from it; the ball will be ftruck in preference to the point, or at a much greater diftance; becaufe, in this cafe, the point has an opportunity, on account of the fmalinefs of the charge, of ftealing away the electricity electricity filently; which the ball, from its greater refiftance, is incapable of effecting. Without expreffing any doubts with refpect to thefe experiments of the Author, and this diftinction made by him; we shall obferve, that whether the preceding cafe frequently occurs in thunder ftorms, or not, it is very remarkable, that though almost all his obfervations relate to this particular cafe, yet we do not find it exemplified, or even noticed, when he afterwards makes a practical application of his doctrine, and enumerates the various ways in which buildings may become expofed to a ftroke of lightning. And yet, to us, this appears to be the only cafe in which a doubt can reasonably be entertained of the fuperior utility of pointed conductors. In this enumeration, he obferves firft, that lightning may either accumulate directly (and gradually) over a building; or it may be brought toward it by a fmall cloud, fetching it in feveral fucceffive trips from a large cloud at fome distance;-or laftly, a large electrified cloud may be carried rapidly towards a building by the wind. To none of these three cafes are the experiments made with the fecond conductor applicable; though the Author's principal objections are made to fuch of Mr. Nairne's experiments as relate to this particular cafe. Further, in the two first of thefe three cafes, he acknowledges the probable efficacy of pointed conductors, in preventing an accumulation of the electric matter, by exhaufting the electrified cloud, fo as to prevent an explofion. On the third cafe, he reafons thus: But if, according to the third fuppofition, a cloud of great extent, and highly electrified, fhould be driven with great velocity in fuch a direction, fo as to pafs directly over the fharppointed conductor; there can be no doubt, but that such a point, from its fuperior readiness to admit electricity, would take the explosion at a much greater diftance than a rounded end, and, in proportion to the difference of that striking diftance, would do mifchief instead of good.' Even in this laft cafe, and in all cafes whatever, where time is allowed for the pointed conductor to operate;-and there is no cafe in which time is not allowed, except in that where the vertical cloud fuddenly receives an explosion from another cloud; --we muft think that the danger of an explofion must be greatly leffened by the action of the point: for the fame reasons that take place in the two firft of the three cafes; even though we were to admit the truth of the Author's pofition-that a pointed termination will, cæteris paribus, receive an inftantaneous explofion at a much greater diftance than a rounded end. In other words, we think it highly probable, that the disadvantage arising from the fuppofed rapidity of the cloud's motion may Dd 3 be |