sliding plate, when its coating is of a given size, bears to that of B. This I found by finding what sized coating must be given to the sliding plate that its charge should be equal to that of another plate, the proportion of whose charge to that of B I was acquainted with. It is plain that, if it is necessary to give one inch less surface to the coating of the sliding plate in trying B than in trying b when the trial plate L is made use of, it will be necessary to make the same difference in the surface of the sliding plate when the trial plate is made use of, so that I might have saved the trouble of making two trial plates. However, for the sake of more accuracy, I always chose to make two trial plates and to take the mean of the results obtained by means of each trial plate for the true result. 299] One reason why this method of trying the experiment is more exact than the former, or that by means of a sliding plate only, is that in the former method I was liable to some error from inaccuracy in judging how much of the tinfoil coating of the trial plate was left uncovered by the sliding brass plate, whereas in this method, as the charge of the sliding plate is but small in respect of that of B, it was not necessary to be accurate in estimating its surface. But I believe the principal reason is that an error which will be taken notice of by and by, and which proceeds from the spreading of the electricity on the surface of the glass, is greater in a sliding plate than in one coated in the usual manner. In general I think it required scarcely so great an increase of the charge of the trial plate to make a sensible alteration in the degree of separation of the pith balls in the following experiments as in the preceding, and therefore it should seem as if these experiments were capable of rather more exactness than the former, but this was not the case, as the different trials were found not to agree together with quite so much exactness in these experiments as the preceding. For this reason, and also because they were attended with less trouble, I repeated the experiments oftener, as I not only compared each plate with the trial plate for more times together as I did in the preceding experiments, but in general I repeated the experiment on several different days. 300] The circumstance which gave me the most trouble in these experiments was the spreading of the electricity on the sur face of the glass. To understand this, let A Bab, Fig. 21, be a flat plate of coated glass, cd and CD being the two coatings, and let CD be positively electrified, and let cd communicate with the ground. It is plain that the electric fluid will flow gradually from CD and spread itself all round on the surface of the glass, and nearly the same quantity of fluid will flow from the opposite side of the glass into cd, so that those parts of the glass which are not coated gradually become charged, those parts becoming so soonest which are nearest the edge of the glass. On discharging the plate the uncoated part of the glass gradually discharges itself, as on the side AB the fluid will flow gradually from the uncoated part of the glass into CD, and on the opposite side it will flow into the uncoated part of the glass from cd. 301] There is a great deal of difference in this respect between different kinds of glass, as on some kinds it spreads many times faster than on others. The glass on which it spreads the fastest of any I have tried is a thin kind of plate-glass, of a greenish colour, much like that of crown-glass, and which I have been told is brought from Nuremberg*. On the English plate-glass it does not spread near so fast, but there is a great deal of difference in that respect between different pieces. On the crown-glass it spreads not so fast as on the Nuremberg, but I think faster than on the generality of English plate-glass. On white glass I think it spreads as slowly as any. 302] The way in which I compared the velocity with which it spread on different plates was as followst. I took away the wire Ff (Fig. 20) and placed the plate which I wanted to try where the plates L or I used to be placed, the lower coating communicating as usual with Dd by the wire 8, but the wire ẞ being drawn up by a silk string so as not to touch the upper coating. The wire Cc is suffered to rest on Aa and the jars electrified. * [Art. 497.] + [See Arts. 485, 486, 487. Also 494 to 499.] When they are sufficiently charged B is let down on the upper coating, which instantly charges the plate to be tried, and immediately the wire Gg is lifted up from Dd, but not high enough to touch Aa. Consequently, immediately after the plate is charged, the communication between Dd and the ground is taken away, and consequently as fast as any fluid flows from the uncoated part of the under surface of the glass to the lower coating, some fluid will flow into Dd and overcharge it, and consequently make the pith balls separate. 303] In order to prevent, if possible, the ill effects proceeding from this spreading of the electricity, I took some coated plates of glass, and covered all the uncoated part with cement to the thickness of or an inch, as in Fig. 22, which represents a section Fig. 22. passing through the middle of the plate perpendicular to its plane, and in which the glass plate and coatings are represented by the same letters as before, and the dotted lines represent the cement*. Thinking that it would be impossible for the electricity to spread between the cement and the glass, in which case this method must have been perfectly effectual, as it would be necessary for the electricity to spread itself not only on the perpendicular surface ef, but also to some distance on the horizontal surface fg, before the quantity of redundant fluid lodged on the surface of the cement could bear any sensible proportion to that in the coating CD. 304] The result was that in dry weather the electricity seemed to spread as fast on those plates which were covered with cement as on the others, but in damp weather not so fast, the difference between dry and damp weather being less in those plates which were covered with cement than the others; and besides that there seemed as much difference between the swiftness with which it spread on the surface of the Nuremberg and English plates [Art. 484.] after they were covered with cement as before, which shows plainly that the electricity spread between the cement and the glass, and not on the surface or through the substance of the cement. It could not be owing, I think, to its passing through the substance of the glass, for if it was, there would hardly be much difference in the uncoated plates between damp and dry weather, whereas, in reality, there was a very great one. I also tried what effect varnishing the glass plates would have, but I did not find that it did better, if as well, as covering them with cement. 305] As there seemed, therefore, to be very little advantage in covering the plates with cement or varnishing them, and as it was attended with a good deal of trouble, I did not make use of those methods, but trusted only to letting the wires down and up pretty quick, so as to allow very little time for the electricity to spread on the surface of the plates, and this I have reason to think was sufficiently effectual, as I never found much difference in the divergence of the pith balls, whether the wires were let down and up almost as quick as I could, or whether they were suffered to rest a second or two at bottom. 306] As the wire Cc is suffered to rest so short a time on Aa and Bb, it is plain that the lower coatings of the trial plate and plate to be tried must have a very free communication with the ground and the outside coating of the jars, or else there would not be time for them to receive their full charge. I accordingly took care that the wires which made the communication should be clean and should touch each other in as broad a surface as I could conveniently. As for the method I took to have a ready communication with the ground, it is described in [Art. 258]. 307] Besides this gradual spreading of the electricity on the surface of the glass, there is another sort which is of much worse consequence, as I know no method of guarding against it, namely, the electricity always spreads instantaneously on the surface of the glass to a small distance from the edge of the coating, on the same principle as it flies through the air in the form of a spark. This is visible in a dark room, as one may see a faint light on the surface of the glass all round the edges of the coating, especially if the glass is thin, for if it is thick it is not so visible*. * [See Art. 532, Feb. 1, 1773.] |