INDUCTANCE AND CAPACITY 



365 



R G and L G the resistance and self-inductance of the galvanometer. 

 The resistances ab and cd are negligibly small. The condenser 

 is rapidly charged and discharged by the switch e, which is usually 

 a commutator driven at a constant and known speed. 



When contact is made at c, the condenser is discharged and so 

 remains until the tongue e touches b. Until e touches b the cur- 

 rents in the network are determined by the e.m.f. of the battery 

 and the resistances, and a steady current flows through the galva- 

 nometer as indicated by the arrow. When contact is made with 

 b, a varying current i c will flow into the condenser until it is 

 fully charged. A part of this varying current flows through M 



FIG. 2lo. Connections for Maxwell method for determining a capacity 

 in absolute measure. 



and a part flows upward through the galvanometer, tending to 

 deflect it in a direction opposite to that produced by the steady 

 current. When C is fully charged, the currents return again to 

 their steady values. By properly adjusting the arms of the 

 bridge and the number of times per second the condenser is 

 charged and discharged, the galvanometer deflection can be 

 reduced to zero, which means that the net quantity displaced 

 through the instrument in a second is zero. 



When e and b are not in contact, the currents have the steady 

 values I M , I N , Ip, I G . 



Call the quantity of electricity on the condenser when it 

 is fully charged, Q c . Then 



Q c = C(P.D.)od. 



