391] (2.92 to one. OF ELECTRIFICATION. 191 Therefore the force with which the electrometer 2.88 endeavours to close when the wires are inserted is to that with which it endeavours to close without the wires as 3·9 to 1. 389] E and F are two coated Leyden vials, nearly of the same size. The outside coatings of both communicate with the ground, and the inside coating of E communicates with CD, but not that of F. 390] The way in which I tried the experiment was as follows. I first compared the electrometer C with the electrometer D without the wires, and found that when the jar E was electrified to (13 such a degree as to make D separate 12 divisions, C' separated (14 divisions, so that the same degree of electrification which divisions made D separate {13 divisions. 134 made C separate (13 12 I then put the wires into the electrometer D, and put the larger of the two vials in the place of E, and electrified E and consequently the rod CD and the two electrometers till D sepa(13 rated divisions. 12 The wire by which E was electrified was then immediately taken away and a communication made between E and F, so that the redundant fluid in E and CD and the electrometers was communicated to F. It was found that the electrometer C then separated divisions. (157 14 The experiment was then repeated in the same manner, except that the smaller vial was placed at E. It was found that if (13 E was electrified till D separated divisions, then on making a (131 communication between E and F, C separated divisions. 391] From hence we may conclude that if the vials had been exactly equal and E had been electrified till D separated (13 12 divisions, then on making a communication between E and F, C But it appears from the first mentioned part of the experiment, that the same degree of electrification which makes C separate (141 131 divisions is sufficient to make D without the wires separate 13 divisions. From whence it appears that if the jars are exactly 117 equal, and one of them is electrified till the electrometer D with (13 the wires separates divisions, and its electricity is then com (12 municated to the other vial, the electricity will be of that degree of strength which is necessary to make the same electrometer without the wires separate {11 divisions, that is, very nearly the same as before, or as it did with the wire before the communication of the electricity. But if the vials are equal, the quantity of redundant fluid in the first vial, after its electricity is communicated to the second, will be very little more than half of what it was before the communication, for the quantity of redundant fluid in the rod DC and the electrometers is trifling in comparison of that in the vial*, and consequently it appears that the distance to which the electrometer with the wires in it separates with a given quantity of redundant fluid in the vial is very nearly the same as that to which it separates without the wires when there is only half that quantity of redundant fluid in the vial. Therefore as the force with which the electrometer endeavours to close by its weight when the wires are in is to that with which it endeavours to close without the wires as 39 to 1, it appears that the force with which the balls of the electrometer are repelled with a given quantity of redundant fluid in the vial, is to that with which they are repelled when there is only half that quantity of redundant fluid in the vial as 39 to 1 (supposing the distance * [In a sentence which Cavendish has scored out in his MS. we read-] The charge of the two vials together was found to be 2168 inches. The diameter of the rod CD was at a medium about of an inch. [This would make the computed charge of the rod 9.7 inches.-ED.] 394] OF ELECTRIFICATION. 193 of the balls to be the same in both cases), that is, very nearly as the square of the quantity of redundant fluid in the vial, the difference being not more than what might very easily be owing to the error of the experiment. So that the experiment agrees very well with the theory. 392] It was found that if the communication was made between the two vials by a piece of metal, the electricity was diminished so suddenly as to set the straws a vibrating, and it was some time before they stopt, for which reason the communication was made by a piece of moist wood, which, though it communicates the electricity of one vial to the other very quickly, did not do it so instantaneously as to make the straws vibrate much. 393] The electricity of the vial was found to waste very slowly, so that it could not be sensibly diminished during the small time spent in communicating the electricity from one vial to the other and reading off the divisions, so that no sensible error could proceed from that cause. 394] I tried the experiment before in the same manner, and with the same electrometers, except that the straws were not gilt, but only moistened with salt. It then seemed as if the force with which the balls of the electrometer were repelled with a given quantity of redundant fluid in the vial was to that with which they were repelled with only half that quantity in the vial as 4 to g. As I suspected that this small difference from the theory was owing to the straws not conducting sufficiently readily, I gilt the straws, when, as was before shewn, the experiment agreed very well with theory. It must be observed that if the straws do not conduct sufficiently readily, the balls of the electrometer will not be so strongly electrified and will not separate so much as they ought to do, and in all probability the difference will be greater in the stronger degree of electricity, in which the electricity wastes much faster, than it is in the weaker, and will therefore diminish the degree of separation more in the stronger degree of electricity than in the weaker, and will therefore make the force with which the balls repel with the stronger degree of electricity appear to be less in proportion to that with which they repel with the weaker degree than it ought to be. AN ACCOUNT OF SOME THE EFFECTS OF THE TORPEDO BY ELECTRI- 395] Although the proofs brought by Mr Walsht, that the phenomena of the torpedo are produced by electricity, are such as leave little room for doubt; yet it must be confessed, that there are some circumstances, which at first sight seem scarcely to be reconciled with this supposition. I propose, therefore, to examine whether these circumstances are really incompatible with such an opinion; and to give an account of some attempts to imitate the effects of this animal by electricity. 396] It appears from Mr Walsh's experiments, that the torpedo is not constantly electrical, but hath a power of throwing at pleasure a great quantity of electric fluid from one surface of those parts which he calls the electrical organs to the other; that is, from the upper surface to the lower, or from the lower to the upper, the experiments do not determine which; by which means a shock is produced in the body of a person who makes any part of the circuit which the fluid takes in its motion to restore the equilibrium. 397] One of the principal difficulties attending the supposition, that these phenomena are produced by electricity, is, that a shock may be perceived when the fish is held under water; and * From the Philosophical Transactions for 1776, Vol. LXVI. Part 1. pp. 196—225. Read Jan. 18, 1775. [Philosophical Transactions, 1773, pp. 461-477. Of the Electric Property of the Torpedo. In a letter from John Walsh, Esq., F.R.S., to Benjamin Franklin, Esq., LL.D., F.R.S., &c. Read July 1, 1773.] 398] RESISTANCE OF IRON, WATER, ETC. TO ELECTRICITY. 195 in other circumstances, where the electric fluid hath a much readier passage than through the person's body. To explain this, it must be considered, that when a jar is electrified, and any number of different circuits are made between its positive and negative side, some electricity will necessarily pass along each ; but a greater quantity will pass through those in which it meets with less resistance, than those in which it meets with more. For instance, let a person take some yards of very fine wire, holding one end in each hand, and let him discharge the jar by touching the outside with one end of the wire, and the inside with the other; he will feel a shock, provided the jar is charged high enough; but less than if he had discharged it without holding the wire in his hands; which shews, that part of the electricity passes through his body, and part through the wire. Some electricians indeed seem to have supposed that the electric fluid passes only along the shortest and readiest circuit; but besides that such a supposition would be quite contrary to what is observed in all other fluids, it does not agree with experience. What seems to have led to this mistake is, that in discharging a jar by a wire held in both hands, as in the above-mentioned experiment, the person will feel no shock, unless either the wire is very long and slender, or the jar is very large and highly charged. The reason of which is, that metals conduct surprisingly better than the human body, or any other substance I am acquainted with; and consequently, unless the wire is very long and slender, the quantity of electricity which will pass through the person's body will bear so small a proportion to the whole, as not to give any sensible shock, unless the jar is very large and highly charged. 398] It appears from some experiments*, of which I propose shortly to lay an account before this Society, that iron wire conducts about 400 million times better than rain or distilled water; that is, the electricity meets with no more resistance in passing through a piece of iron wire 400,000,000 inches long, than through a column of water of the same diameter only one inch long. Sea water, or a solution of one part of salt in 30 of water, conducts 100 times, and a saturated solution of sea salt about 720 times better than rain water. [Arts. 576, 577, 684, 687.] |