336] In order to find out, if possible, how much the charge of the globe was increased hereby, I made four flat plates of a mixture of rosin and bees wax*, about 4 inches square and 22 thick, and coated each of them with circles of about 18 inches in diameter, and compared the charge of each of them separately with that of a circular plate of tin, 93 inches in diameter. I then compared the charge of two of these plates together with that of a tin circular plate 18 inches in diameter, and lastly I compared the charge of all together with that of a circle of 36 inches diameter†.

337] By a mean of the different experiments it appears that the charge of each of the rosin plates was alike, and that the charge of any one of them was to that of the circle of 9.3 inches as 10:34 to 9:3, that the charge of the circle of 18 inches was to that of two of the rosin plates together as 20-19 to 21.96, and that the charge of the circle of 36 inches was to that of all four plates as 43.75 to 42.06.

But the charge of the four plates together will not be exactly four times the charge of one plate singly, as some allowance must be made for the charge of the wire connecting their upper surfaces, and, besides that, the charge of the plates when placed close together will not be quite so great as if placed at a distance from each other+.

By trying the charge of all four rosin plates together by the machine, Fig. 20, both when placed close together and at as great a distance from each other as I could, I found their charge when close together to be to their charge when placed at a distance nearly as 41 to 41, and, from some other experiments I made, I am inclined to think that the charge of each of the wires which connected the upper coatings of the plates was to that of one plate alone as 28 to 930§.

ratio of 2 to 3. Hence the height of the center of the bodies from the floor was 42 inches, and the height of the room 4 x 42 inches, or 14 feet. This would agree with the height of the top of the circle of 18 inches being 51 inches from the floor (Art. 472).]

* These plates are non-conductors of electricity, and may be charged as Leyden vials. The manner in which I made them will be described in the following pages [Arts. 373, 514]. My reason for making them of these materials is that the charge of such a plate is much less than that of a plate of glass of the same dimensions.

+ It must be observed that, in the two last mentioned comparisons, the rosin plates were placed close together and their upper surfaces connected by a piece of wire. [Arts. 555, 558, also 443.]

+ [Art. 557.]

From these circumstances, I am inclined to think that the charge of two plates together is to that of one plate alone as 21.96 to 10-34, and that the charge of the four plates together is to that of one alone as 42:06 to 10:34, and consequently that the charges of the tin circles of 93 inches, 18 inches and 36 inches are to each other as 9:3, 20-19 and 4375*.

338] Though I do not know how to calculate how much the charge of the circles ought to be increased by the attraction of the undercharged ground, yet I think there can be little doubt but that if the charge of the plate of 18 inches is increased in any ratio whatever as that of x to x-18, the charge of the plate of 36 inches will be increased in the ratio of x to x-36, and that of the plate of 9.3 inches in the ratio of x to x-93; therefore if we suppose that the charge of the 18 inch plate is increased in the ratio of 9 to 8, or of 166 to 166 - 18, the charges of the three plates should be to each other as

which agrees very nearly with experiment, and nearer so than it would have done if we had supposed the charge of the 181⁄2 inch plate to have been increased in any other proportion which can be expressed in small numbers+.

339] I think we may conclude therefore that the charge of the 12.1 inch globe was increased by the attraction of the undercharged ground nearly in the proportion of 9 to 8, for I think there can be little doubt but that the charge of the globe must be increased thereby in nearly the same ratio as that of the 18 inch plate, and therefore we may conclude that the charge of the plate D is to the charge which the 121 inch globe would receive, if it was placed at a great distance from any over or under-charged matter, nearly in the proportion of 26.3 to 121, or, in other words, the charge of the plate D is 26-3, which is rather more than eight times greater than it ought to be if the electric fluid did not penetrate into the glass. I shall speak further as to the cause of this in [Art. 349].

340] In order to try the charge of what Epinus‡ calls a plate of air, I took two flat circular plates of brass, 8 inches in diameter [Art. 649.] + [Art. 652, and Note 24.] [Mém. Berl. 1756, p. 119.]

and thick, and placed them on the bars Nn and Pp of the machine (Fig. 20), the two plates being placed one over the other, and kept at a proper distance from each other by three small supports of sealing-wax placed between them, the supports being all of the same height, so that the plates were exactly parallel to each other. Care was also taken to place the plates perpendicularly over each other, or so that the line joining their centers should be perpendicular to their planes.

The lowermost plate communicated with the ground by the wire RS, and the uppermost communicated with Mm by the wire V, just as was done in trying the Leyden vials.

I then found its charge, or the quantity of redundant fluid in the uppermost plate, in the usual manner, by comparing it with the plate D, and found it to be to that of D as*...

341] As I was desirous of trying larger plates than these, and was unwilling to be at the trouble of getting brass plates made, I took two pieces of plate-glass† 111⁄2 inches square, and coated each of them on one side with a circular plate of tinfoil 11.5 inches in diameter, and placed them on the machine as I did the brass plates in the former experiment, with the tinfoil coatings turned towards each other, and kept at the proper distance by supports of sealing-wax as before, care being taken that the tinfoil coatings should be perpendicularly over each other.

For the more easy making a communication between the circular coating of the lower plate and the ground, and between that of the upper plate and the wire Mm, I stuck a piece of tinfoil on the back of each plate, communicating by a narrow slip of the same metal with the circular coatings on the other side.

I then tried the charge as before, the lower plate communicating with the ground and the upper with the wire Mm.

As glass does not conduct electricity, it is plain that the quantity of electric fluid in the pieces of tinfoil will be just the same that it would be if the glass was taken away, and the pieces of tinfoil kept at the same distance as before.

• The memoranda I took of that experiment are lost, but to the best of my remembrance the result agreed very well with the following experiment.

+ [Art. 517.]

The distance of the two circular coatings of tinfoil was measured by the same instrument with which I measured the thickness of the plates of glass, and may be depended on to the 1000th or at least to the 500th part of an inch *.

342] In this manner I made the experiment with the plates at four different distances, namely 910, 420, 288 and 256, and when I had made a sufficient number of trials with the plates at each distance, I took off these circular coatings and put on smaller, namely of 6.35 inches diameter, and tried the experiment as before with the plates at 259 inches distance. The result of the experiments is given in the following table:

It is plain that some allowance ought to be made in these trials for the spreading of the electricity on the surface of the glass. In the above table I have supposed it to spread 05 of an inch, but the effect is so small that it is of very little signification whether that allowance is made or not.

344] In my former paper [Art. 134] I expressed a doubt whether the air contained between the two plates in this experiment is overcharged on one side and undercharged on the other, as is the case with the plate of glass in the Leyden vial, or whether the redundant and deficient fluid is lodged only in the plates, and that the air between them serves only to prevent the electricity from running from one plate to the other, but the following experiment shows that the latter opinion is true.

I placed the two brass plates on the machine (Fig. 20), and tried their charge as before, except that, after having charged the plates, I immediately lifted up the upper plate by a silk string so as to separate it two or three inches from the lower one, and let it * [See Art. 459, "Bird's instrument," and "dividing machine," Art. 517. Also 594, 595.]

[See Arts. 669, 519.]

[Arts. 511, 516, Dec. 18, 26, 1772.]

down again in its place before I found its charge by making the communication between Bb and Dd and between Aa and Ee.

The way I did this was that as soon as I had let down the wire Cc on Aa and Bb, and thereby charged the plates, I lifted it up again half way so as to take away the communication between Cc and the upper plate &c., but did not lift it quite up, so as to make the communication between Bb and Dd, and between Aa and Ee, till after I had separated the upper plate from the lower, and put it back in its place.

I could not perceive any sensible difference in the charge, whether I lifted up the upper plate in the above-mentioned manner, or whether I tried its charge without lifting it up.

345] It is plain that in lifting up the upper plate from the lower and letting it down again, the greatest part of the air contained between the two plates must be dissipated and mixed with the other air of the room, so that if the air contained between the two plates was overcharged on one side and undercharged on the other, the charge must have been very much diminished by lifting up the upper plate and letting it down again, whereas, as I said before, it was not sensibly diminished.

I think we may conclude, therefore, that redundant and deficient fluid is lodged only in the plates, and that the air between them serves only to prevent the electricity from running from one plate to the other.

346] As this is the case, the charge of these plates ought, according to the theory, to be equal to that of a globe whose diameter equals the square of the radius of the plate or circular coating divided by twice their distance, that is, to their computed charge, provided the electricity is spread uniformly on the surface of the plates, and therefore in reality the numbers in the last column but one ought to be rather greater than in the last but two, and moreover the less the distance of the plates is in proportion to the diameter of the coating, the less should be the proportion in which those numbers differed, and if the distance is infinitely small in proportion to the diameter, the proportion in which those numbers differ, should also be infinitely small.

347] This will appear by inspecting the table to be the case, only it seems from the manner in which the numbers decrease,