376 Comparison of Electrical Capacities of two Condensers. 



If t be very short, we may neglect the last term compared 

 with the others, and, to the same degree of approximation with 

 respect to p p, we get as the condition of no kick, 



R / C'-RC=(^--)(R'C' + RC), 

 or 



J&='- 2 (7-?> (28 > 



Again, let k, as before, be the galvanometer-constant, and 8 

 the permanent deflexion. Then, from (25), 



E fW 



{f-j}= htui *> (29) 



g7Q7 = l~-^-tanS; (30) 



and this equation enables us to determine the capacity. 



Let us suppose the adjustment made by varying R. Then, 

 starting from a position in which the first kick is in an oppo- 

 site direction to the final deflexion, adjust R until that kick 

 is just reduced to zero, and the spot of light moves off gradu- 

 ally in the one direction, and after some oscillations comes to 

 rest. Then, if 8 is the deflexion of the galvanometer-needle, 

 the capacity is 



n C'R'J- 2Gk, .1 

 0=-^-|l-- E -tanS). 



Unless the leakage is considerable, the correction will be very 

 small. 



In measuring the capacity of many condensers, the diffi- 

 culty is increased by the phenomenon of electric absorption. 

 In fact the condenser has no true capacity ; for the charge 

 produced by a given electromotive force depends on the time 

 during which that force has acted. We may, however, take 

 the capacity as the ratio of the instantaneous charge to the 

 electromotive force producing it ; and in this case (contact 

 with the battery being maintained only for a very short time) 

 we may perhaps look on electric absorption as a kind of con- 

 duction through the substance of the condenser. We must 

 suppose that the resistance to the conduction is a function of 

 the time, which becomes indefinitely great after a not very 

 long interval, but which we may perhaps treat as sensibly 

 constant during the time for which contact is maintained ; 

 and if p 0? p r be the values of this resistance during that 



