1 64 Experiments on Coated Plates 



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 dis- 

 tance 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 6-708 long and 6-514 broad, with four slits in it, as in Fig. 24, 

 each -fa 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.] 



