308] There is another circumstance which shows this instantaneous spreading of the electricity, namely, after having charged and discharged a coated plate of glass a great many times together without cleaning it, I have frequently seen a narrow fringed ring of dirt on the glass all round the coating, the space between the ring and the coating being clean, and in general about inch broad. This must in all probability have proceeded from some dirt being driven off from the tinfoil by the explosions, and deposited on the glass about the extremity of that space over which the electricity spreads instantaneously, and therefore seems to show that the distance to which the electricity spreads instantaneously is not very different from of an inch.

309] From some experiments which will be mentioned by and by †, I am inclined to think that the distance to which the electricity spreads instantaneously is about of an inch when the thickness of the glass is about of an inch and about 8 of an inch when its thickness is about of an inch; or more properly the quantity of redundant fluid which spreads itself on the surface of the glass is the same that it would be if the distance to which it spread was so much, and that the glass in all parts of that space was as much charged as it is in the coated part.

310] If I charged and discharged a coated plate several times running, in the dark, with intervals of not many seconds between each time, I commonly observed that the flash of light round the edges of the coating was stronger the first or second time than the succeeding ones, which seems to shew that the electricity spreads further the first or second time than the succeeding ones. Accordingly I frequently found in trying the following experiments that the pith balls would separate rather differently the first or second time of trying any coated plate than the succeeding one. Observing that I now speak of the half dozen trials which, as I said in [Art. 299], I commonly took with the same plate immediately after one another.

311] Before I proceed to the experiments it may be proper to remind the reader‡ that if a plate of glass or other non-conducting substance, either flat or concave on one side and convex on the

other, provided its thickness is very small in respect of its least radius of curvature, is coated on each side with plates of metal of any shape, of the same size and placed opposite to each other, its charge ought by the theory to be equal to that of a globe whose diameter is equal to the square of the semidiameter of a circle whose area equals that of the coating divided by twice the thickness of the glass, supposing the coated plate and globe to be placed at an infinite distance from any over or undercharged body, and to be connected to the jar by which they are electrified by canals of incompressible fluid; provided also that the electricity does not penetrate to any sensible depth into the substance of the glass, and that the thickness of the glass bears so small a proportion to the size of the coating that the electricity may be considered as spread uniformly thereon.

312] It was before said that the electricity spreads instantaneously to a certain distance on the surface of the glass, so that the surface of the glass charged with electricity is in reality somewhat greater than the area of the coating. Therefore, if the plate is flat, let the area of the coating be increased by a quantity which bears the same proportion to the real coating as the quantity of redundant fluid spread on the surface of the glass beyond the extent of the coating does to that spread on the coated part of the glass. That is, let the area of the coating be so much increased as to allow for the instantaneous spreading of the electricity, and let a circle be taken whose area equals that of the coating thus increased. I call the square of the semidiameter of this circle, divided by twice the thickness of the glass expressed in inches, the computed charge of the plate, because, according to the abovementioned suppositions, its charge ought to be equal to that of a globe whose diameter equals that number of inches.

313] In like manner, in what may more properly be called a Leyden vial, that is, where the glass is not flat, but convex or concave, let a circle be taken whose area is a mean between that of the inside and outside coatings, allowance being made for the spreading of the electricity. I call the square of the semidiameter of this circle, divided by twice the thickness of the glass, the computed charge of the vial. In like manner, if the real charge of any plate is found to be equal to that of a globe of x inches in diameter, I shall call its real charge x.

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.

316] The sizes of the coatings were so adjusted that the computed charges of D, E, and Fare 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 1016 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 1.863 inches