324 ELEMENTARY LESSONS ON [CHAP. vi. 



When this condition of things is attained the 

 battery resistance is balanced by those of the 

 other three arms. 



362. Measurement of Capacity of a Con- 

 denser. The capacity of a condenser may be measured 

 by comparing it with the capacity of a standard con- 

 denser such as the -^ microfarad condenser shown in 

 Fig. 1 06, in one of the following ways : 



(a) Charge the condenser of unknown capacity to a 

 certain potential ; then make it share its charge with the 

 condenser of known capacity, and measure the potential 

 to which the charge sinks ; then calculate the original 

 capacity, which will bear the same ratio to the joint 

 capacity of the two as the final potential bears to the 

 original potential. 



(b) Charge each condenser to equal differences of 

 potential, and then discharge each successively through 

 a ballistic galvanometer (Art. 204), when the sine of half 

 the angle of the first swing of the needle will be propor- 

 tional in each case to the charge, and therefore to the 

 capacity. 



(c) Charge the two condensers simultaneously from 

 one pole of the same battery, interposing high resistances 

 in each branch, and adjusted so that the potential rises 

 at an equal rate in both ; then the capacities are inversely 

 proportional to the resistances through which they are 

 respectively being charged. 



(d) Another method, requiring no standard condenser, 

 is as follows : Allow the condenser, whose capacity is to 

 be measured, to discharge itself slowly through a wire of 

 very high resistance. The time taken by the potential 

 to fall to any given fraction of its original value is pro- 

 portional to the resistance, to the capacity, and -to the 

 logarithm of the given fraction. 



363. Resistance Expressed as a Velocity. It will be 

 seen, on reference to the table of "Dimensions" of electro- 

 magnetic units (Art. 324), that the dimensions of resistance are 



