Secondly, the charge of a single plate or wire was found to bear nearly, though, in the first case, I believe, not quite the same proportion to two similar plates or wires of half the diameter or length which it ought to do according to computation. Thirdly, the proportion which the charges of a thin circular plate and of three cylindrical bodies of different lengths and diameters bear to that of a globe agree with computation; but it must be observed that, as the proportion of the charges of the bodies to that of the globe is determined by the theory within only very wide limits, their agreement cannot be looked upon as so great a confirmation of the theory as it would otherwise be, yet as their shapes are so very different I think that their agreement, even within those limits, may be considered as a considerable confirmation of it.

PART

[EXPERIMENTS ON COATED PLATES.]

295] This part consists chiefly of experiments made to determine the charges of plates of glass and other electric substances coated in the manner of Leyden vials. The method I used in doing this was nearly of the same nature as that by which I determined the charges of the other sort of bodies in the preceding part, but the apparatus was more compact and portable and is represented in Fig. 20, where Hh is a horizontal board lying on the ground, Ll and Ll are two upright pillars supporting the two horizontal bars Nn and Pp, both at the same height above the ground, and parallel to each other.

To these two bars are fastened four upright sticks of glass covered with sealing wax; they are represented in the figure and shaded black, but are not distinguished by letters to avoid confusion. To these sticks of glass are fastened four horizontal pieces of wire Aa, Bb, Dd, and Ee, and to Bb is fastened another wire mM supported at the further end by a stick of waxed glass.

Rr is a wooden bar reaching from the wire Ee to the pillar Ll, and along the upper edge of this bar runs a wire, one end of which is wound round the wire Ee and the other reaches to the ground and serves to make a communication between Ee and the ground. Cc and Kk are two wires fastened firmly together at k serving to electrify the plate. They are moveable upon K as a center where they communicate with the inside coating of one or more large glass jars, and the same electrometer that was used in the former experiments is fastened to the prime conductor by which the jars [Not numbered by Cavendish.]

*

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

To the ends Cand 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

P.20

Fig. 20.

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 f 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 immediately 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