are electrified, in order that they may be charged to the same degree each time.

To the ends C and c of the wire Cc is fastened a silk string, as represented in the figure, passing over the pulley S, with a counterpoise w at the other end which serves to lift Cc from off the wires Aa and Bb, or to let it down upon them at pleasure. Gg is a wire the end G of which is bent into a ring, through which

M.

passes the wire Ee, so that Gg turns upon Ee as a center. Ff is a wire turning in the same manner as Dd. The ends g and ƒ of these wires are fastened by silk strings to C and c as represented in the figure, in such manner that when Cc rests on the wires Aa and Bb, Gg and Ff rest on Dd and Ee, but on lifting up Cc, Gg and Ff are also lifted off from Dd and Ee.

The counterpoise w is so heavy as to overcome the weight of Cc, and to lift it up till the wires Gg and Ff bear against Aa and Bb, which prevents Cc from rising any higher.

[Note. This Figure was found among the MS. It is not numbered, nor does any part of the MS. seem to refer to it, but it is inserted here to show some of the details of a piece of apparatus similar to that described in the text.]

296] In making the experiment one of the plates whose charges we want to compare together, or the plate B as we will call it, is laid on the bars Nn and Pp, between the sticks of glass and end N, the upper coating thereof being made to communicate with Bb and Mm by a wire V resting on Mm, and the lower coating is made to communicate with the ground by a springing wire S fastened to Rr, and by its elasticity bearing against the lower coating of the plate.

Another coated plate is laid on the same bars between the sticks of glass and n by way of trial plate, the upper coating of which communicates with Aa by the wire B, and the lower coating communicates with Dd by the springing wire 8. A pair of pith balls also, such as were used in the former experiments, were suspended from D as represented in the figure.

In trying the experiments, the jars, and consequently the wire Cc, are charged, the wire Cc being all that time lifted up as high as it will go by means of the counterpoise. When the jars are charged to the proper degree as shown by the electrometer, the wire Cc is let down on the wires Aa and Bb by lifting up the counterpoise. This instantly charges both the coated plates, for when Cc rests on Aa and Bb, and consequently Ff and Gg rest on Ee and Dd, the lower coatings of both plates communicate with the ground, and their upper coatings with Cc.

Immediately after this the counterpoise is let go, by which means Cc is lifted up, and Gg and Ff along with it, till the two last mentioned wires bear against Aa and Bb, so that immediately after the coated plates are charged, the communication between them and the wire Cc, by which they were electrified, is taken away, and at the same time the communication between the lower coating of the trial plate and the ground is taken away, and iminediately after that a communication is made between the upper coating of the plate B and the lower coating of the trial plate, and also a communication is made between the upper surface of the trial plate and the ground, so that the upper coating of the trial plate and the lower coating of the plate B both communicate with the ground, and the upper coating of B and the lower coating of the trial plate communicate with each other and the wire Dd.

Consequently, if the quantity of redundant fluid communicated to the wires Bb and Mm and the upper side of the plate B together is equal to the deficient fluid on the under side of the trial plate, they and the wire Dd will be neither over nor undercharged after the operation is completed; but if the redundant fluid in them exceeds the deficient fluid on the lower side of the trial plate, Dd will be overcharged, and the pith balls will separate positively. On the other hand, if it is less than the deficient fluid, the pith balls will separate negatively.

297] The trial plate consisted of a flat plate of glass, or other electric substance, the lower surface of which was coated all over with tinfoil, but on the upper side there was only a small coating of tinfoil. I had also flat plates of brass of different sizes which I could lay on the upper surface, and slip backwards and forwards, and thereby increase or diminish the size of the upper coating at pleasure, for the area of the upper coating is equal to the area of

the plate of brass added to that of so much of the tinfoil as is left uncovered by the brass *.

By this means I could increase or diminish the quantity of deficient fluid on the lower side of the trial plate at pleasure, for I could alter the size of the upper coating at pleasure, and the quantity of deficient fluid on the under side of the plate is not much greater than it would be if the lower coating was no greater than the upper, and consequently depends on the size of that upper coating.

As it is necessary that the trial plate should be insulated, it was not laid immediately on the bars Nn and Pp, but was supported by sticks of waxed glass fastened to those bars.

Having by these means found what size it was necessary to give to the upper coating of the trial plate in order that the pith balls should separate positively just sensibly, and what size it was necessary to give to it that they might separate as much negatively, I removed the plate B and placed the plate or plates which I intended to compare with it (or the plate b as I shall call it) in its room and repeated the experiment in just the same manner as before. Then, if I found that the size which it was necessary to give to the upper coating of the trial plate in order to exhibit the same phenomena was the same as before, I concluded that the charge of the plate b was the same as that of B. If, on the other hand, I found that it was necessary to make the area of the upper coating of the trial plate greater or less than before in any ratio, I concluded that the charge of b was greater or less than that of B in the same ratio, for the quantity of deficient fluid on the lower side of the trial plate will be pretty nearly in proportion to the area of the upper coating.

N.B. In the following experiments it was always contrived so that the charges of the plates to be compared together should be pretty nearly alike, so that if the quantity of deficient fluid on the lower surface of the trial plate was not exactly in this proportion, it would make very little error in the proportion of the charges.

298] The method above described is that which I made use of in my first experiment, but I afterwards made use of another

* N.B. In order to estimate how much of the tinfoil was left uncovered, I drew parallel lines upon it at small equal intervals from each other, and took notice which of these lines the edge of the brass plate stood at. [Arts. 442, 488.]

method a little different from this, and which I found more exact, though rather more complicated, namely, for each set of plates that I wanted to compare together I prepared two trial plates, which I shall call L and 1, not coated as that above described, but in the usual way, namely, with the coatings of the same size on both sides*.

The first of these plates, or L, was of such a size that when used as a trial plate with the plate B or b on the other side, the quantity of deficient fluid in it was rather more than ought to be in order that the pith balls should just separate negatively, and the second plate was rather greater than it ought to be in order that they should just separate positively.

I also prepared a sliding plate of the same kind as the trial plate used in the former method, but whose charge was many times less than that of the plate B or b. This sliding plate I placed along with the plate B or b on the side N, and on the other side I placed the trial plate L and found what size it was necessary to give to the coating of the sliding plate in order that the balls should just separate negatively. I then removed the plate B and put b in its room, and found what sized coating it was necessary to give to the sliding plate in order that the balls should separate the same as before. Having done this, I removed the trial plate L and put in its room, and tried each of the plates B and bas before, finding what coating it was necessary to give to the sliding plate that the balls might just separate positively.

Having done this, if I found that it required the coating of the sliding plate to be of the same size in order to exhibit the same phenomena in trying the plate B as in trying b, it is plain that the charges of B and b must be both alike, but if I found that it was necessary to give less surface, one square inch for instance, to the coating of the sliding plate in trying B than in trying b, then it is plain that the charge of B exceeds that of b by a quantity equal to that of the charge of the sliding plate when its surface is one square inch, supposing, as is very nearly the case, that the charge of the sliding plate is in proportion to the surface of its upper coating.

In this way of trying the experiment, it is plain that, in order to determine the proportion which the charges of B and b bear to each other, we must first know what proportion the charge of the

[Art. 457.]