i 86 Theory of multiple dielectric plates 



But by comparing the charges of the plates of crown glass with those 

 of the two other pieces of crown glass in the table, there does not seem to 

 be any difference which can be depended on with certainty. 



The experiment indeed would have been more satisfactory if the piece 

 of ground glass and the pieces with which it was compared had been all 

 made out of the same pot. But as it would have been difficult procuring 

 such pieces, and as I have found very little difference in the specific gravity 

 of different pieces of crown glass, and as I am informed it is all made at 

 the same glass house, I did not take that precaution. 



379] Let two or more flat plates of different non-conducting substances, 

 as AabB, BbcC and CcdD, (Fig. 29) be placed close 

 together and coated in the manner of a single plate 

 with the coatings Ee and Ff. Let the charge of the 

 plate AabB, supposing it placed by itself and coated 

 in the usual manner, be equal to that of a plate of 

 glass whose thickness is A and whose coatings are of 

 the same size as those of AabB. 



In like manner let the charge of BbcC be equal to 

 that of a plate of the same glass whose thickness is 

 equal to B, and let that of CcdD equal that of one 

 whose thickness is C. 



Then whichever of the three ways of accounting 

 for the excess of the real charge of glass plates above 

 the computed we prefer, it is a necessary consequence 

 of our theory that the charge of this compound plate 

 AadD should be equal to that of a single plate of glass whose thickness 

 equals A + B + C, and whose coatings are of the same size as Ee and Ff. 



380] In like manner if two or more plates of the same kind of glass 

 are placed together and coated as above, the charge of this compound 

 plate should be equal to that of a single plate of the same glass whose 

 thickness is equal to that of all the plates together. This appears from the 

 following experiments to be the case, for 



1st. I took the three plates of glass A, B and C*, and laid them on 

 one another, having first taken off their old coatings and coated the outside 

 surfaces as in Fig. 29 with circles of tinfoil 6-6 inches in diameter. The 

 charge of this compound plate was found to be to that of the three plates 

 D, E and F together as -944 to i. The sum of the thicknesses of A, B 

 and C together is -6309, and the computed charge of a plate of that 

 thickness with coatings 6-6 in. diameter is to that of D, E and F together, 

 allowing in the same manner as in [Art. 328] for the instantaneous spreading 

 of the electricity, as -94 to one. So that the charge of this compound plate 

 is exactly the same that it ought to be according to the foregoing rule. 



* [Arts. 534, 544, 546, 677.] 



