484] Two of the old ground glass plates H, I, K, L of Nuremberg glasst, were coated with oblong squares to serve for trial plates to the plates I, K, L, M of Nairne, but the observations were found to be so irregular that nothing could be made of them, owing, as was supposed, to the spreading of the electricity on the surface of the glass. To prevent this, all the four plates H, I, K and L were coated with oblong squares, and cased in cement composed of 2 parts rosin, 1 of bees' wax, and 3 of brick dustt. In making it the bees' wax was first melted and imperfectly dephlegmated, the rosin was then added and melted with as little heat as possible, and then the brick dust, previously heated so as to be very dry, was added. By this means the cement is more safe and sticky than if more heat is used in making it. In some of the mixtures also 8 a small part of the rosin, never exceeding 1th of the whole, was exchanged for as much pitch, which was added after the rest was melted and mixed. The plates E, F, G, and I, K, L of Nairne were cased in the same cement, about 165 thick. A plate of the same cement was also cast by pouring it out on a tin plate. This was coated with circles about 2.2 in diameter. 485] The spreading of the electricity on the surface of the trial plates seemed not to be prevented by casing them in cement, for putting the plate L of Nairne on the positive side, and the trial plate H on the negative, then if the apparatus was let down and drawn up again immediately, the pith balls separated about half an inch negatively, but if the apparatus was suffered to rest at the bottom about half a minute, and then drawn up immediately, they separated considerably more than 1 inch, and if it was suffered to rest at the bottom but a very short time, and then kept mid way for minute, and then drawn up, the balls at first separated positively but closed very soon, and after a long time separated negatively. If a sliding plate containing about part of the electricity in the plate L was put on the positive side as an additional plate, and the apparatus was let down and drawn up immediately, the balls separated about 1 diameter negatively, but if it rested at the bottom a minute and was then drawn up immediately they separated about 1 inch negatively. 486] In order to see how fast the electricity spread on the surface of the glass, the heavy paper cylinders* were placed in the usual place, and the light ones on the wire Bb, the wires Gg and Ff were detached from Cc and rested at bottom, and a coated plate on the positive side. The wire Cc was suffered to rest on Aa and Bb while the jars were charging, and the wire V drawn up so as not to rest on the coated platet. When the heavy cylinders, and a fortiori the light ones separated, the wire was let down on the plate and the wire Ce immediately drawn up, and the time elapsed till the closing of the light cylinders counted, which was as follows: 487] Another way was taken to try the same thing, namely, the wire Ff was taken off and Gg placed so as to lye below the wire Dd and to be drawn up against it by a string. The coated plate to be tried was placed on the negative side, the wire & touching its bottom coating. The jars were then charged, the wire Cc resting all the while on Aa and Bb, and Gg drawn against Dd, and ẞ drawn up so as not to rest on the plate. When the jars were sufficiently charged ẞ was let down on the plate, and the wire Gg dropt immediately after, so as to take away the communication between Dd and the ground, so that the pith balls which were hung to D shewed whether much electricity passed round to under side of plate or spread on the surface. When the pith balls communicated with the ground they separated about of an inch negatively by the repulsion of the wires on them. Plate of closed almost instantly, separated again in about 10". cement 488] Three sliding coated plates were made, each covered all over with tinfoil on one side, with a slip of tinfoil 18 by 9 in. on the other. Two flat pieces of brass were also prepared, one 1.8 by 9 and the other 1.8 square. The tinfoil was divided in breadth into 6 equal parts, and the breadth of the coated surface is expressed in those divisions or in 6th parts of the breadth*. The first plate was one of exper. rosin 345 thick. The 3rd, a bit of the large piece of whitish plate glass. as much electricity as the double plate A. = 24) contains = 12) The 3rd sliding plate when its breadth = 103 contains as much elec tricity as plate F of Nairne. electricity in plate D, E, F, or G of Nairne. 7 inches of additional wire answers to 1 inch of computed power in plates of Nairne. 2 trial plates were made for the plates I, K, L and M of Nairne out of 2 of the ground plates first got from Nairne. The dimensious of the coating of the small one was about 3.3 by 3·1, and that of the large one 37 by 3-4, the thickness of glass unknown. Two trial plates were also made for the plates A, B and C of Nairne out of the old ground plates E and F. F, the smallest, was 5'7 square, and E was 6.3 by 6 nearly. Two trial plates were made of crown glass for the plate H of Nairne, the small one 5.7 by 51, the other 6 by 5.9. 489] Tuesday, Feb. 4 [1772]. Th. 47°. S. H. 17. C. H.-5. Trial of plates D, E, F and G of Nairne, and of the 2 double plates, the plates E, F and G being cased with cement: tried by means of additional wiret. 490] Feb. 4 continued. Comparison of the three plates E, F and G together with the plates I, K, L and M, tried by the two above-mentioned trial plates and the 1st and 2nd sliding plate*. 491] Trials of the same kind as those in [Art. 487]. Large trial plate for the experiments of this page-balls closed in 13′′ 492] Wed. Feb. 5 [1772]. Th. 49°. S. H. 17. C. H. 41. Comparison of I, K, and L together, with A, B and C by means of the trial plates E and F, and of A, B and C together, with H by means of the two crown glass trial plates*. |