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I now proceed to the experiments.

314] I procured ten square pieces of plate-glass all ground out of the same piece of glass, three of them 8 inches each way and about inch thick; three more of about the same thickness 4 inches each way, the rest were as near to of that thickness as the workman could grind them, one being 8 inches long and broad, and the other 4 inches. They were not exactly of the same thickness in all parts of the same piece, but the difference was not very great, being no where greater than of the whole. The mean thickness was found both by actually measuring their thickness in different parts by a very exact instrument and finding the mean, and also by computing it from their weight and specific gravity and the length and breadth of the piece. The mean thickness, as found by these two different ways, did not differ in any of them by more than 2 thousandths of an inch.

315] All these plates were coated on each side with circular pieces of tinfoil, the opposite coatings being on the same size and placed exactly opposite to each other. The mean thickness of the plates, which for more convenience I have distinguished by letters of the alphabet, together with the diameters of the coatings, and their computed charge, supposing the electricity not to spread on the surface of the glass, are set down in the following tablet.

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316] The sizes of the coatings were so adjusted that the computed charges of D, E, and F are all very nearly alike. Those of K, L and M were intended to be three times as great as those of

A cubic inch of water was supposed in this calculation to weigh 253 grains Troy. [See Arts. 592, 593.]

+ [Art. 482.]

the former, and consequently the diameters of their coatings nearly the same. The computed charges of A, B and C were intended to be three times as great as those of K, L and M, and consequently the diameter of their coatings about three times as great, and the computed charge of H was intended to be three times as great as that of A. By some mistake, however, the coatings of K, L and M were made rather too small, but the error is very trifling.

317] My first trials with these plates were to examine whether the charge of the three plates D, E, and F together was sensibly less when they were placed close together than when they were placed at 6 inches distance from each other, that is at as great a distance as my machine would allow of. I could not perceive any difference. This is conformable to the theory, as is shown in [Art. 185]. I chose to make the experiment with these three plates, as the difference should be more sensible with them than with any of the others.

318] Secondly. I compared together each of the plates D, E and F. I could not perceive any sensible difference in their charges*.

Thirdly. The charge of the plate K was found to exceed that of the three plates D, E and F together in the proportion of 1.016 to 1. The charge of L was not sensibly different from that of K, and that of M very little different.

Fourth. The charge of each of the plates A, B and C was to that of the three plates L, K and M together, as 0·905 to 1.

Fifth. The charge of H was equal to that of the three plates A, B and C together.

Therefore the charges of D, K, A and H were to each other as 1, 305, 8.28 and 249+.

319] It appears, therefore, that the proportion which the charge of K bears to that of D, and which H bears to that of A, is very nearly the same as that of their computed charges, but the proportion which the charge of A hears to that of K is near less than it ought to be.

part

This in all probability proceeds from the effect of the instantaneous spreading of the electricity bearing a greater proportion to

* [See Art. 489, Feb. 4, 1772.]

+ [See also Arts. 656 to 658.]

the whole in the plate K than it does in A, the diameter of whose coating is near three times as great.

320] In order to form some judgment, if possible, how great the effect of this instantaneous spreading of the electricity was, I took off the coatings from the plates A and B*, and put on others of just the same area in the form of a rectangular parallelogram (that of A was 6-414 long and 5.310 broad, and that of C 6.398 long and 5-201 broad), and compared their charges with that of the plate B, whose charge, as was before said, was just the same as those of these two plates before their coatings were altered.

321] I then took off these coatingst, and on A I put a square coating 6.388 each way with slits cut in it, as in Fig. 23, each broad, so as to divide it into 9 smaller squares, each 1863 inches

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each way. The narrow communications marked in the figure between these squares were each of an inch broad.

On CI put an oblong coating 6.377 long and 6·343 broad, with four parallel slits cut in it, as in Fig. 24, each broad, the narrow space left between these slits and the outside being broad. Having done this, I compared their charges with that of the plate B as before.

It must be observed that the area of these slit coatings was somewhat less than that of the circular or oblong ones, but their whole circumference, including the circumference of the slits, is more than three times as great as that of the circular or oblong ones, so that the surface of glass charged by means of the instantaneous spreading of the electricity was more than three times as great in these coatings as the former, and consequently the quantity of that surface may be determined thereby, supposing that, if it was not for the spreading of the electricity on the surface, the charge of a coated plate would be the same whatever shape its coating is of, provided the area of the coated surface is given.

322. In order to find whether the electricity spread to the same distance upon thin glass as thick, I also took off the coatings from the plate H, and in its room put on first a square coating 6.03 inches each way, and then an oblong one 6708 long and 6.514 broad, with four slits in it, as in Fig. 24, each broad, and ascertained the proportion which its charge with each of these coatings bore to that with the circular coating by comparing it with another plate, the proportion of whose charge to that of the circular coating I had before ascertained *.

323] It appeared from these experiments that if we suppose the electricity to spread instantaneously about 07 of an inch on the thick glass plates such as A and C, and about '09 on the thin ones, not only the charges of A, C and H with the three different coatings, but also the charges of all the plates will agree very well with the theory, as will appear by the following table; whereas, if we suppose that the electricity does not spread sensibly on the surface of the glass, the charge of the plate H with the slit coating would be greater in proportion to its charge with the circular or oblong coating than it ought to be in the ratio of 7 to 6, and the error in the plates A and C would not be much less.

* [Arts. 659-663.]

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By the observed charge in the foregoing table, I mean only the proportion which the observed charges bore to each other, not the real observed charges. [See Art. 671.]

326] From the circumstance of the light mentioned in [Art. 307], it appears plainly that the electricity does actually spread instantaneously to a small distance on the surface, and from the rings of dirt taken notice of in Art. 308 it seems likely that the distance to which it spreads is not very different from what we have here supposed; moreover, if the distance to which the electricity spreads is such as we have supposed, the charges of all these plates bear very exactly the same proportion to each other that they ought to do by theory, whereas if the distance to which the electricity spreads is different from that here assigned, and consequently the proportion of the charges of different plates to each other different from that furnished by theory, it seems very strange that their charges should all have happened to agree with computation, notwithstanding that their thickness and the size and shape

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