Correction for spreading of charge 161 



the glass bears so small a proportion to the size of the coating that the 

 electricity may be considered as spread uniformly thereon. 



312] It was before said that the electricity spreads instantaneously 

 to a certain distance on the surface of the glass, so that the surface of the 

 glass charged with electricity is in reality somewhat greater than the area 

 of the coating. Therefore, if the plate is flat, let the area of the coating 

 be increased by a quantity which bears the same proportion to the real 

 coating as the quantity of redundant fluid spread on the surface of the 

 glass beyond the extent of the coating does to that spread on the coated 

 part of the glass. That is, let the area of the coating be so much increased 

 as to allow for the instantaneous spreading of the electricity, and let a 

 circle be taken whose area equals that of the coating thus increased. I call 

 the square of the semidiameter of this circle, divided by twice the thickness 

 of the glass expressed in inches, the computed charge of the plate, because, 

 according to the above-mentioned suppositions, its charge ought to be 

 equal to that of a globe whose diameter equals that number of inches. 



313] In like manner, in what may more properly be called a Leyden 

 vial, that is, where the glass is not flat, but convex or concave, let a circle 

 be taken whose area is a mean between that of the inside and outside 

 coatings, allowance being made for the spreading of the electricity. I call 

 the square of the semidiameter of this circle, divided by twice the thickness 

 of the glass, the computed charge of the vial. In like manner, if the real 

 charge of any plate is found to be equal to that of a globe of x inches 

 in diameter, I shall call its real charge x. 



I now proceed to the experiments. 



314] I procured ten square pieces of plate-glass all ground out of the 

 same piece of glass, three of them 8 inches each way and about -fife inch 

 thick; three more of about the same thickness 4 inches each way, the 

 rest were as near to ^ of that thickness as the workman could grind them, 

 one being 8 inches long and broad, and the other 4 inches. They were 

 not exactly of the same thickness in all parts of the same piece, but the 

 difference was not very great, being no where greater than f of the whole. 

 The mean thickness was found both by actually measuring their thickness 

 in different parts by a very exact instrument and finding the mean, and 

 also by computing it from their weight and specific gravity and the length 

 and breadth of the piece*. The mean thickness, as found by these two 

 different ways, did not differ in any of them by more than 2 thousandths 

 of an inch. 



315] All these plates were coated on each side with circular pieces of 

 tinfoil, the opposite coatings being of the same size and placed exactly 

 opposite to each other. The mean thickness of the plates, which for more 



* A cubic inch of water was supposed in this calculation to weigh 253! grains 

 Troy. [See Arts. 592, 593.] 



c. p. i. ii 



