An Account of Some New Experiments in Electricity, with the Description and Use of Two New Electrical Instruments. By Mr. Tiberius Cavallo, F. R. S. Communicated by the President

II. An Account of some new Experiments in Electricity, with the Description and Use of two new Electrical Instruments. By Mr. Tiberius Cavallo, F. R. S. communicated by the President. Read Nov. 25, 1779. Professor Lichtenberg, of Gottingen, some time ago made an experiment upon the electrophorus, an account of which was first received in London towards the latter end of the year 1777. The phenomena attending the experiment are very entertaining and various, but I do not know that any person ever offered a satisfactory explanation of them. The author himself, in his paper entitled "De nova methodo naturam ac motum Fluidi Electrici investigandi Commentatio prior," wherein he gives an account of the experiment, does not attempt any explanation of it; contenting himself with the account only of various particulars attending it. In brief, the experiment is as follows: The electrophorus, that is, a plate of some resinous substance, as sulphur, rosen, gum-lac, &c. is first excited, either by rubbing, or otherwise; then a piece of metal of any shape at pleasure, as, for instance, a three-legged compass, a piece of brass tube, or the like, is set upon the electrophorus; and to this piece of metal, so placed, a spark is given of the electricity contrary to that of the plate. This done, the piece of metal is removed by means of a stick of sealing-wax, or other electric; and some powder of rosen, kept in a linen bag, is shaken upon the electrophorus. This powder will be found to fall about those points upon the plate which the piece of metal touched, forming some radiated appearances much like the common representations of stars; at the same time upon the greatest part of the plate, that is, in all parts except where the stars are formed, there is hardly any powder at all. Now it is to be remarked, that if the plate be excited negatively, and the spark given to the metal set upon it, be positive, the appearance will be as above described; but if on the contrary the plate is positive, and the spark is negative, then the powder of rosen will be found to fall upon those parts of the plate, which in the other case are left uncovered; and to leave the stars clean: in short, it will do just the reverse of what it did in the other case: or, in other words, words, the powder of rosin will be attracted by those parts only of the electrophorus, which are electrified positively. When I first observed these phenomena, I thought that there was no apparent reason why the powder of rosin should be attracted by those parts of the electrophorus, which are in a positive state of electricity, and not by those, which are negative. The two electricities are certainly contrary to one another; but either of them attracts a non-electrified body. On this consideration I thought, that the experiment could be explained only upon the supposition, that the powder of rosin, on its falling from the linnen bag, was actually electrified negatively; in which case it would have been easy to account for the phenomena upon the well known principle of bodies attracting each other when they are contrarily electrified; and repelling one another when they are possessed of the same kind of electricity. In order to try the reality of my supposition by experiments, I insulated a brass plate upon a glass stand, and connected a very sensible electrometer with it; and then began shaking the powder of rosin upon it, in the same manner as I had done upon the electrophorus, and in a few seconds of time had the pleasure to see the electro- meter diverge with a very manifest degree of negative electricity, answering my expectations exactly. The explanation of the ingenious Professor Lichtenberg's experiment now became very easy and natural; for the powder of rosin, being actually electrified negatively, could not be attracted, except by those parts of the electrophorus, which are in a contrary state, that is, electrified positively. It is observed, that powder of rosin answers better for this experiment than the powders of other substances; and accordingly I find, that this powder, when shaken upon the insulated brass plate, shews a stronger degree of electricity than the other powders. Indeed the electricity of the powder of rosin, not only when shaken upon the brass plate in the manner above mentioned, but simply let fall upon it from a piece of paper, a spoon, &c. is remarkably great; half an ounce of it being sufficient to make the threads of the electrometer diverge as much as they possibly can. This discovery not only affords an easy explanation of Professor Lichtenberg's experiment upon the electrophorus, but shews a method of exciting powders, which has long been a desideratum in the science of electricity. The method is as follows; insulate a metal plate upon an electric stand, and connect within a cork-ball electrometer; then then the powder required to be tried being held in a spoon, or other thing, about six inches above the plate, is to be let fall gradually upon it. In this manner the electricity acquired by the powder, being communicated to the metal plate and to the electrometer, is rendered manifest by the divergence of the threads, and its quality may be ascertained in the usual manner. See fig. 4. It must be observed, that if the powder is of a conducting nature, like the amalgam of metals, sand, &c. it must be held in some electric substance, as a glass phial, a plate of sealing wax, or the like. The metal spoon that holds the powder may also be insulated; in which case, after the experiment, the spoon will be found possessed of an electricity contrary to that of the powder. In performing these experiments care must be had to render the powders, and whatever they are held in, as free from moisture as possible, it being sometimes necessary to make them very warm, otherwise the experiments are apt to fail. The following are the particulars I have observed with this new method, which, however, are neither numerous nor often repeated; but they may suffice to excite the curiosity of those persons who have leisure and the opportunity of repeating them more at large, and in a greater variety. Powder of rosin, whether it be let fall from paper, glass, or a metal spoon, electrifies the plate strongly negative; the spoon, if insulated, remaining strongly positive. Flower of sulphur produces the same effect, but in a less degree. Pounded glass, let fall from a piece of paper, made dry and warm, electrifies the plate negatively, but not in so strong a degree as rosin. If it is let fall from a brass cup, it electrifies the plate positively, but in a very small degree. Steel filings, let fall either from a glass phial or paper, electrified the plate negatively; but brass filings, treated in the same manner, electrified the plate positively. The amalgam of tin-foil and quicksilver, gunpowder, or very fine emery, electrify the plate negatively when they are let fall upon it from a glass phial. Quicksilver poured from a glass phial electrifies the plate positively. Soot from the chimney, or ashes from common pit coals mixed with small cinders, electrify the plate negatively, when they are let fall from a piece of paper. Description of the improved atmospherical electrometer. Fig. 2. is a geometrical representation of my new atmospherical electrometer in its real size. This instrument, the first hint of which I received from my ingenious friend Thomas Ronayne, Esq. after various trials, I brought to the present state of perfection as long ago as the year 1777; and immediately after several of them were made after my pattern by Mr. Adams, philosophical instrument maker in Fleet-street. The great difficulty attending the construction of this instrument has long dissuaded my publishing any description of it; nor had I ever troubled the Royal Society with it if the observations of several of my friends, who have used it in England and abroad, joined to my own repeated experiments, had not indisputably confirmed its superiority over any other instrument of that kind. Its particular advantages are, 1. The smallness of the size. 2. Its being always ready for experiments, without fear of entangling the threads, or having an equivocal result by the fluggishness of its motion. 3. Its being not disturbed by the wind. 4. Its great sensibility: and 5, its keeping the communicated electricity longer than any other electrometer hitherto used. The principal part of this instrument is a glass tube \( \text{CDMN} \), cemented at the bottom into the wooden piece \( \text{AB} \), by which part the instrument is to be held when used for the atmosphere, and it also serves to screw the instrument into its wooden case \( \text{ABO} \), fig. 1, when it is not to be used. The upper part of the tube \( \text{CDMN} \) is shaped tapering to a smaller extremity, which is entirely covered with sealing wax melted by heat, and not dissolved in spirits. Into this tapering part a small tube is cemented, which, with its under extremity, touches the flat piece of ivory \( \text{H} \), fastened to the tube by means of cork. The upper extremity of the wire projects about a quarter of an inch above the tube, and screws into the brass cap \( \text{EF} \), which cap is open at the bottom, and serves to defend the waxed part of the instrument from the rain, &c. In fig. 3, this brass cap is represented as transparent, in order to shew its internal shape, and the manner in which it is screwed to the wire projecting above the tube \( \text{L} \). The small tube \( \text{L} \) and the upper extremity of the large tube \( \text{CDMN} \) appear like one continued piece, on account of the sealing wax which covers them both. The conical corks \( \text{P} \) of this electrometer, which by their repulsion shew the electricity, &c. are as small as they can conveniently be made, and they are suspended by exceedingly fine silver wires: these wires are shaped in a ring ring at the top, by which they hang very loosely to the flat piece of ivory H, which has two holes for that purpose. By this method of suspension the friction is lessened almost to nothing, and thence the instrument is sensible of a very small degree of electricity. IM and KN are two narrow slips of tin-foil stuck to the inside of the glass CDMN, and communicating with the wooden bottom AB; they serve to convey off that electricity, which, when the corks touch the glass, is communicated to it, and being accumulated might disturb the free motion of the corks. In regard to its use, this instrument may be used to observe the artificial as well as the atmospherical electricity. When it is to be used for artificial electricity, this electrometer is set upon a table or other convenient support; then it is electrified by touching the brass cap EF with an electrified body, which electricity will sometimes be preserved for more than an hour; in this state, if any electrified substance is brought near the cap EF, the corks of the electrometer by their converging or diverging more, will shew the species of that body's electricity. Before we proceed farther, it is necessary to remark, that the communication of any electricity to this electrometer, by means of an excited electric, for instance, a piece of sealing wax (which we suppose as always elec- trified negatively) is not very readily done in the usual manner, because of the cap EF being well rounded, and free from points or sharp edges. By the approach of the wax the electrometer will be caused to diverge; but as soon as the wax is removed, the wires immediately collapse. The best method to electrify it is, to bring the excited wax so near the cap that one or both the corks may touch the side of the bottle CDMN; after which they will soon collapse and appear unelectrified. If now the wax is removed, they will again diverge and remain electrified positively. In this operation the wax does not impart any of its electricity to the electrometer, but only acts by means of its atmosphere, viz. when the excited wax is first brought near the brass cap EF (agreeable to the well known law of electricity, and according to Dr. Franklin's hypothesis) it determines the electric fluid, naturally belonging to the corks, towards the cap; hence the corks repel each other. Now if in this state they touch the sides of the glass CDMN, they acquire from it a quantity of electric fluid equal to that, which, by the action of the excited wax, was driven towards the cap, consequently they collapse and appear unelectrified. Notwithstanding this appearance, the cap is actually overcharged, so that when the wax is removed, the overplus of the electric fluid, which the corks had acquired from from the glass, and tin-foil stuck upon it, and which was crowded upon the cap, because the negative atmosphere of the wax now diffuses itself equally through the cap, the wires, the corks, &c.; and, therefore, the corks repel each other with positive electricity. If, instead of the sealing wax excited negatively, an electric possessed of positive electricity be used, the electrometer acquires the negative electricity, and the explanation, mutatis mutandis, is the same as above. By considering this remark it will appear, that when this electrometer is electrified, either positively or negatively, and an electrified body is brought towards the brass cap; the electricity of that body will be of the same kind with that of the electrometer, if the corks increase their divergency; but it will be of the contrary kind, if the corks approach one another. When this instrument is to be used to try the electricity of the fogs, the air, the clouds, &c. the observer is to do nothing more than to unscrew it from its case, and, holding it by the bottom AB, to present it to the open air a little above his head, so that he may conveniently see the corks P, which will immediately diverge if there is any electricity; the kind of which, that is, whether positive or negative, may be ascertained by bringing an excited piece Mr. CAVALLO's Account of some piece of sealing wax, or other electric, towards the brass cap EF. It is, perhaps, unnecessary to remark, that this observation must be made in an open place, as the roads out of town, the fields, the top of a house, &c. I have often made use of this electrometer in the roads between Islington and London, and by means of it I have confirmed the observation of Mr. RONAYNE, who first discovered the electricity of the fogs, as is testified by a paper of his published in the Philosophical Transactions, and who has remarked, that a fog is very rarely not electrified; and that in clear frosty weather the air is constantly electrified. Promiscuous Experiments. I. Having had frequent occasion to observe how difficult it is to deprive sealing wax of its electricity entirely, after that it has been well excited, I had the curiosity to try whether water could effect it. In order to that, I tied a stick of sealing wax to a silk string about a yard long, and after having excited it very powerfully with flannel, I plunged it in a tin vessel full of water, and immediately drawing drawing it out, brought a very accurate electrometer near it, and observed, that at first it shewed no sign of electricity; but in about half a minute's time it manifested a small but very sensible degree of negative electricity. A glass tube, treated in the same manner, was deprived of all its electricity by the water. II. I have often remarked, that after having excited a glass tube with the amalgamated rubber in the usual manner, the part of it which had been under my hand was negative. This minus state was still more conspicuous when I grasped with my hand the part next above, viz. part of that which had been excited positively by rubbing. In the same manner, when I excite a stick of sealing wax, by rubbing it with flannel, I often find, that the part of it which I hold in my hand is in a contrary state of electricity, viz. positive. III. Being desirous of trying the conducting power of the effluvia of burning bodies in a manner more satisfactory than had hitherto been done, I contrived an instrument for that purpose, which is represented in fig. 5. (a). The handle of it \(AB\) is a glass tube, into the extremity \(B\) of which a wire \(EI\), and a smaller glass tube \(BC\), are ce- (a) This instrument is just double the size of the figure. mented by means of sealing wax. From the extremity of this small tube another wire FG proceeds, which, as well as the wire EI, is bent at the top, so that the extremities of both wires EF may be about one tenth of an inch from one another. GH is a small wire fastened to the wire FG, and to the extremity of the handle, so that when the instrument is held in one's hand, this wire touches the hand. K is a small cork-ball electrometer, which, when the instrument is to be used, is affixed to the pin D, which proceeds from the wire IE. When experiments are to be tried with this instrument, the electrometer K must be affixed to the pin D, which proceeds from the wire IE, and must be electrified so that the cork balls may diverge as far as possible: this done, the extremities EF of the wires are brought within the effluvia that are to be tried, which, if they are of a good conducting nature, will complete the communication between the two wires EF, and discharge the electrometer of its electricity, otherwise the electrometer will remain electrified for a considerable time. The experiments which I have made with this instrument are neither numerous nor so often repeated as to be depended upon, excepting one only, which, perhaps, it will be not useless to mention. I found that the fumes arising arising by the action of a lens, from the amalgam of tin-foil and mercury, conducted so badly, that the electrometer lost its electricity in a time very little less than it would have done without any fumes whatever. Little St. Martin's Lane, Nov. 4, 1779.