C is suspended, as in the figure, the electrometer described in Art. 249, and at the other end D is suspended a similar electrometer, only the straws reached to the bottom of the cork balls A and B, but not beyond them, and were left open so as to put in pieces of wire, and thereby increase their weight and the force with which they endeavoured to close. The lower ends of these wires when used were just even with the bottom of the cork balls, and were kept in that situation by wax, the wax being cut off even with the bottom of the corks, so as to leave no roughnesses to carry off the electricity. In like manner, when the wires were not used, the ends of the straws were closed up with wax.

388] The proportion which the force with which the balls of this electrometer endeavoured to close when the wires were inserted bore to that with which they endeavoured to close without the wires was thus found. The weight of the straw

A

B

with its

(7.6

ball and centre pin but without its wire was found to be 6.65

grains, and the distance of its center of gravity from the center of suspension was

(5:36
5.285

inches, as was found by balancing it on the

edge of a knife. Consequently the force with which this straw when put in its place, endeavours to descend towards the perpendicular, supposing it to be removed to a given distance from it, was

length was

(1.23
1·00

inches, so that as the distance of the bottom of

the cork balls from the center of suspension was 111 inches, the distance of its center of gravity from the center of suspension was 9.87

{10.17 inches, and therefore the excess of the force with which the

ball endeavours to descend towards the perpendicular when the wire is inserted above that with which it endeavours to descend without [the wire] is to the force with which it endeavours to (12:03 × 9.87 (7.6 × 5:36 descend without the wire as 10 × 10.1

to

(6·65 × 5·285'

or as

(2.92 to one. Therefore the force with which the electrometer 12.88

endeavours to close when the wires are inserted is to that with which it endeavours to close without the wires as 39 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 such a degree as to make D separate divisions, C separated

134

(13

12

(14 divisions, so that the same degree of electrification which divisions made D separate (144 divisions.

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

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 S157

divisions.

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 12

E was electrified till D separated divisions, then on making a

communication between E and F, C separated

(131

121

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 divisions is sufficient to make D without the wires separate

(141

131

(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

the wires separates (13 divisions, and its electricity is then com

(13

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.]

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 ATTEMPTS TO IMITATE THE EFFECTS OF THE TORPEDO BY ELECTRICITY. BY THE HON. HENRY CAVENDISH, F.R.S.*

395] Although the proofs brought by Mr Walsh†, 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.]