connected to a third body by a thin wire the redundant fluid in the plate would be to that in the globe in that proportion, provided they were placed at a very great distance both from the third body and from any other over- or undercharged matter, and that the quantity of redundant fluid in the third body was exactly the same each time.

238] The method I took in making these experiments was by comparing each of the two bodies I wanted to examine, or B and b as I shall call them, one after another with a third body, which I shall call the trial plate, in this manner. I took two Leyden vials and charged both of them from the same conductor; I then electrified B positively by the inside of one of the vials, and at the same time electrified the trial plate negatively by the coating of the other vial. Having done this I tried whether the redundant fluid in B was more or less than sufficient to saturate the redundant matter in the trial plate, by making a communication between them by a piece of wire; for if the redundant fluid in B was more than sufficient to saturate the redundant matter in the trial plate, they would both be overcharged after the communication was made between them; if, on the other hand, the redundant fluid in B was not sufficient to saturate the redundant matter in the trial plate, they would be undercharged. Having by these means found what size the trial plate must be made so that the redundant matter in it should be just sufficient to saturate the redundant fluid in B, I tried the body b in the same manner, and if I found that it required the trial plate to be of the same size in order that the redundant matter in it should be just sufficient to saturate the redundant fluid in b, I was well assured that if B and b were successively made to communicate with a third body and positively electrified they would each of them contain the same quantity of fluid, supposing the quantity of redundant fluid in the third body to be the same each time; that is, that the charge of B was equal to that of b.

Having thus given a general idea of the method I used, I proceed to describe it more particularly.

239] The trial plates I made use of consisted of two flat tin plates ABCD and abcd (Fig. 15), made to slide one upon the other, so that by making the side bc of one plate extend more or

less beyond the side BC of the other it formed a plate of a greater or less size, and which consequently contained more

or less

Fig.15

240] The apparatus used in making these experiments is represented in Fig. 14, where the parallelogram T represents the trial plate and B one of the bodies to be compared together, each supported on non-conductors. dD8 is the wire for making a communication between them, having a joint in it at D, where it is supported by a non-conductor, and where are also hung two small pith balls to show whether B and T are over- or undercharged after the communication is made between them. A and a are the two vials; Ee is a wire communicating with the inside coating of A, aCc a wire communicating with the same coating of a; and Ff and Gg are wires fastened to the outside coating of a; Rr Ss is a wire for making a communication between B and the vial A, having joints in it at R and S, where it is supported by nonconductors, and mMNn is another wire of the same kind for making a communication between T and the vial a.†

241] In order to try the experiment I proceed in this manner: the wires Dd and D8 are lifted off from the plates B and T so as not to touch them, and consequently so that there is no communication between B and T: the wires Rr and Mm are suffered to rest on B and T, and the wires Ss and Nn are lifted up so as not to touch Ee and Ff. The vials are then charged by means of the wire bt which rests on Ee and Cc, and communicates by the wire

* [See table for trial plate at Art. 468.]

+ [See plan, Fig. 17, p. 128.]

Pp with the prime conductor, a communication being made between the outside of the vial A and the ground, and the vial a being made to communicate with the ground by the wire xyz, which rests on Gg, and is suspended from the wire bt by silk strings represented in the figure by dotted lines. When the vials are sufficiently charged, the wire bt is lifted up till xy bears against the bottom of Cc, xy being still suffered to communicate with the ground as before, and the communication between the outside of the vial A and the ground being still preserved. At the same time the wires Ss and Nn are let fall upon Ee and Ff. For the sake of doing this more commodiously I make use of the silk strings represented in the figure by dotted lines and passing over the pulley H. A weight is fastened to the string at w, which is supported while the vials are charging in such manner that the wires Ss and Nn are lifted up so as not to touch Ee and Ff, and the wire bt is suffered to rest on Ee and Cc, and the wire xy on Gg; and when the vials are sufficiently charged the weight is let down, by which means Ss and Nn are suffered to fall down upon Ee and Ff, and the wire bt is lifted up till xy bears against the bottom of Cc.

242] From what has been said it appears that whilst the vials were charging, the outsides of each of them communicated with the ground, and consequently the inside of each vial is overcharged and the outside undercharged. As soon as the vials are charged the communication of each of them with the prime conductor is taken away, and at the same time the communication between the outside of the vial a and the ground is taken away, so that it is intirely insulated, and, immediately after, a communication is made between its inside and the ground, and at the same time the body B is made to communicate with the inside of the vial A, and the trial plate with the outside of the vial a; consequently the body B will be overcharged as it communicates with the overcharged part of the vial A, while the undercharged side communicates with the ground; and the trial plate will be undercharged, as it communicates with the undercharged side of the vial a, while the overcharged side communicates with the ground.

Immediately after this operation is performed the wires Rr and Mm are lifted up, so as to cut off the communications of the

bodies B and T with the vials, and, instantly after, the wires Dd and DS are let down, so as to make a communication between the body B and the trial plate. For the sake of expedition this operation was performed nearly in the same manner as the former, by means of the silk strings passing over the pullies L and 1, and represented in the figure by dotted lines. I also employed an assistant to turn the electrical machine and to manage the silk strings passing over the pulley H, while I stood ready near D to perform the last mentioned operation as soon as the wires Ss and Nn were let down, and also to see whether the pith balls separated

or not.

243] From the manner of performing the last mentioned operation it appears that the communication is not made between B and T till after their communication with the vials and all other bodies is cut off; consequently, if the quantity of redundant fluid communicated to B is more than sufficient to saturate the redundant matter in T, they will be overcharged after the communication is made between them, and the pith balls at D will separate positively, but if the redundant fluid in B is not sufficient to saturate the redundant matter in T they will be undercharged, and the pith balls will separate negatively.

244] The balls were made of pith of elder, turned round in a lathe, about one-fifth of an inch in diameter, and were suspended by the finest linen threads that could be procured, about 9 inches long.

245] In making these experiments I did not open the trial plate to such a surface that the pith balls should not separate at all on making the communication between B and T, and assume that for the size which must be given to the trial plate in order that the deficience of fluid in it should be equal to the redundance in B (or for the required surface of the trial plate, as I shall call it for shortness); but I first made the surface of the trial plate such that the deficient fluid therein should exceed the redundant in B, and that the pith balls should separate negatively, just enough for me to be sure they separated: I then diminished the surface of the trial plate till I found, on repeating the experiment, that the pith balls separated positively as much as they before separated negatively, and the mean between these I concluded to be the required surface of the trial plate.