1 68 Specific effect of the glass dielectric 



seemed to spread about ^ of an inch on it, that is, rather less than on 

 the plate H, though its thickness was, of the two, rather less. But whether 

 this difference is real, or owing to the error of the experiment, I cannot tell. 



331] There seems no reason, from the foregoing experiments, to think 

 that the charge of any of these plates is sensibly greater than it would be 

 if the electricity was disposed uniformly on their coated surfaces, as their 

 charges agree very well together without such a supposition. If we suppose 

 that the charges of any of them are sensibly greater than they would be if 

 the fluid was disposed uniformly, it will be necessary to suppose that there 

 is a still greater difference between the distance to which the electricity 

 spreads on the surface of thin plates and that of thick ones than what we 

 have assigned. But I shall speak more on this subject at the end of 

 Art. [365]. 



332] But though it appears from the foregoing experiments that the 

 charges of plates of glass of different thicknesses with coatings of different 

 shapes and sizes bear the same proportion to each other that they ought 

 to do by theory, yet their charge is many times greater in proportion to 

 that of a globe than it ought to be on a supposition that the electricity 

 does not penetrate to any sensible depth into the substance of the glass, 

 as will appear by the following experiment. 



333] I n order to compare the charge of the plate D with the globe 

 of i2 T \j inches used in the former part, I made two plates coated as a 

 Leyden vial, the charge of each of which was about \ that of D, each 

 consisting of two plates of glass cemented together and coated on their 

 outside surfaces with circular pieces of tinfoil about if inch in diameter*. 



I then compared the charge of each of these double plates with that 

 of the globe in the same manner that I compared together the charges 

 of different bodies in the former part, the only difference being that, in 

 trying either of these double plates, I made a communication between 

 the lower coating of the plate and the ground, the wires Mm and Dd 

 (Fig. 14) being contrived so that they were sure to fall on the upper 

 coating f. 



By this means the charge of each of these double plates was found to 

 be just equal to that of the globe. The charge of the plate D was then 

 compared with that of the two double plates together, and was found to 

 be less than that in the proportion of 263 to 272, and consequently the 

 charge of the plate D is to that of the globe as 26-3 to 13-6. 



334] Before we go further it will be proper to consider what effect 

 the three circumstances taken notice of in Art. 277 will have in altering 



* If they had been made of a single piece of glass, the coatings must have been 

 so small as would have been inconvenient unless the glass had been of a greater 

 thickness than could have been easily procured. [Arts. 446, 451, 649, 653, 654.] 



t [Arts. 455, 456, 478.] 



