INDUCTANCE AND CAPACITY 375 



series resistance. R, and a shunt, S, on the galvanometer. Call 

 the deflection so obtained Z> 2 ; then 



10* ,-_ 



C = - - } microfarads. 



( jr ) 



The two deflections should be approximately equal. 



A better procedure was adopted by Fleming and Clinton, who 

 used a special differential galvanometer of the D'Arsonval type. 

 The two movable coils were rigidly attached to the same vertical 

 insulating stem, one above the other, each coil having its own 

 permanent magnet. The adjustment for obtaining an exactly 

 differential instrument was by means of a magnetic shunt. This 

 construction allows the two coils to be highly insulated, which is 

 essential in order that the results may not be vitiated by cross- 

 leakages, since the voltage employed on the condensers may be 

 considerable 100 volts or more. One coil is traversed by the 

 discharges from the condenser, while the other carries a steady 

 current derived from the battery. When the instrument stands 

 at zero, 



10* 

 = n[(R G + S)R + R G S] mlcrofarads - 



A separate experiment must be made to determine the capacity 

 of the leads. 



Thomson Method for Comparing Capacities. 1 1 If two con- 

 densers be charged with equal quantities of electricity the 

 voltages required will be inversely as the capacities. To take 

 advantage of this relation some ready means must be provided 

 for indicating when the charges are equal and for showing the 

 relative voltages applied to the condensers. The arrangement 

 shown in Fig. 222 is that necessary for carrying out the measure- 

 ments according to Thomson's method. The battery current 

 flows through Ri and R* in series. If KI and K* are both de- 

 pressed at the same time, C x is charged to a voltage IRi while 

 C P is charged to a voltage IR*. When Ki and K z are released 

 and make contact with their back stops, the condensers are con- 

 nected in series + to so that their charges tend to neutralize 

 each other or "mix." To see if the neutralization is perfect, 

 that is, to see if there were equal quantities on the two condensers, 



