458 Mr. W. E. Sumpner on the Variation 



an accumulator. The currents were changed by inserting 

 more or less resistance into the battery-circuit. If the bridge 

 is balanced for steady currents, and the battery-circuit be 

 opened, the swing 6 l produced will be proportional to 

 L 2 —Kps, where L 2 is the coefficient required and K is the 

 known capacity. If one of the terminals of the condenser be 

 now disconnected from the bridge, and a second swing 6. 2 be 

 taken on breaking circuit, we have 



This method, although only comparative, has several advan- 

 tages over Maxwell's absolute method. It is quicker and 

 simpler ; there are only two quantities to observe, and the 

 readings may be taken immediately after each other. It is, 

 moreover, not so necessary to have a good ballistic galvano- 

 meter. It will generally be best to work with the relations 



p = q, p + r = 2g, K/>s = 2L. 



The best ratio, r/p, will be determined by the resistances and 

 batteries available, and by the currents it is desirable to use. 

 In the following experiments these relations were not adhered 

 to, because the galvanometer was sufficiently sensitive, and 

 because it was desirable to alter the currents flowing without 

 rendering it necessary to readjust the bridge. 



In all swing methods it is necessary to have the bridge well 

 balanced for steady currents. A fine adjustment can be con- 

 veniently obtained by sliding a bare wire of suitable thick- 

 ness round a terminal, since it is not generally necessary to 

 know the resistance of the arm of which it is a part, and it 

 can of course be placed in the arm of unknown resistance. I 

 believe the device used in the Cavendish laboratory is to shunt 

 one resistance-box by another. The balance of the bridge has 

 to be so good that the heating caused by the momentary 

 passage of the current is often sufficient to destroy it. 



The annexed table indicates the results obtained with the 

 electromagnet with the iron core. The swings were obtained 

 by breaking the battery-circuit. 



