Measures with condensers 263 



541] Crown and H with slits compared with white cylinder; also on the exci- 

 tation of electricity by separating a brass plate from a glass one. 



Wed. Feb. 17 [1773]. Th. 55. N. 21. [6 observations, Art. 660.] 

 Fr. Feb. 19 [1773]. Th. 53^. N. i8J. 



A plate of glass nj inches square, coated with tinfoil 8 inc. in diameter, 

 was supported on waxed glass. A brass plate 8 inc. in diameter was supported 

 over it by silk strings in such manner as to lye on the plate perpendicularly 

 over the tinfoil, and to be drawn up till it touched a piece of wire supported 

 on waxed glass with the middle sized cork balls suspended from it. This was 

 done in order to see how much of the charge of the plate was contained in the 

 coating. 



It was found that though the plate was not electrified, yet on lifting up the 

 brass plate the balls separated some inches if the tinfoil communicated with 

 the ground, but if it did not communicate, the balls, as well as I remember, 

 separated considerably less. Some bits of thin silk thread were placed between 

 the glass and brass plate. 



In the afternoon. Th. 54. N. 17^. 



The experiment repeated with bits of card between the glass plate and 

 brass. 



. , .. fcommun. ) . , fi . 



When tinfoil { ,., > with ground, balls sep. about {, inch, 



(.did not commun.J (.f 



When there was nothing between the glass and brass plate, they sep. 1-4 inc. 

 whether the tinfoil communicated with the ground or not. 



In all these cases the brass plate was negative. 



The glass plate was found to be pos. if the tinfoil did not communicate with 

 the ground, but I could not perceive it to be at all electrified if it did com- 

 municate. 



The next morning the experiment was repeated, but the balls separated 

 much less than before. The temper, of the air was much the same. 



542] It was tried whether when three tin plates i foot square were placed 

 near to and parallel to each other, the line joining their centers being perpendi- 

 cular to their planes, the middle plate would receive much electricity on electri- 

 fying the plates*. 



The experiment was tried with the same apparatus and nearly in the same 

 manner as the experiment with the globe f, except that the two outer plates 

 were suspended by two sticks of waxed glass turning on hinges. The wire too 

 by which the plates were electrified was made so as to touch all three plates 

 at the same time. Four bits of sealing wax were stuck to the middle plate, 

 two on each side, to prevent the outer plates coming too near. 



* [Exp. vin, Art. 288 and Note 23.] t [Art. 218.] 



