SCIENCE AND PRACTICE. 257 



otherwise inserted between them. Let this be p. Substi- 

 tuting these values for B, in (I., 



i = .JL_ ..... (ii. 



The truth of this equation may be proved experimentally, 

 as follows : 



Evidence of the direct proportion of the intensity to the 

 electro-motive force is obtained by comparing the known 

 function of the deflections of a magnetic needle of a galvano- 

 meter due to the current in a circuit in which r and p the 

 circuit resistances remain constant while the number of 

 pairs is charged. The resistance r of a pair of plates of equal 

 surface, at the same distance, diminishes as their surface is 

 increased, and vice versa ; but the resistance of more pairs 

 joined up in series, increases proportionally to their number. 

 Therefore, we take a single pair of plates of known surface 

 and connect them in the circuit of a galvanometer, and of a 

 length of wire determined by a Rheocord, or other adjustable 

 resistance, and note the deflection, ^>. Then we double the 

 electro-motive force, E, by inserting, in the place of these , 

 two pairs of plates of each double the surface of the former , 

 by which the resistance r remains unchanged ; the wire p 

 remains also the same, but we have another deflection, ^> 1 . 

 For the intensity I, with the single pair, we have the expression 



p + r 



and by the second reading, with two pairs, 



^ T _-C.^oN_. 2E 

 4J . . . Ij _C (<Pi ) 



O -f- T 



F being the function sine, tangent, or whatever it may be 

 which connects degrees of arc with those of force. From 

 these two equations it follows, and will also be found, that 



1=21 

 The same method of experimental proof may be extended 



