30 THE ELECTRIC ARC 



deavored to measure the resistance of the arc by a modi- 

 fication of the Wheatstone bridge, shown in Fig. 10. Two 

 arcs L! and LI are operated by a battery B^. This cir- 

 cuit is connected to the Wheatstone bridge at B and A. 

 If all the resistances are known except those in the arcs, 

 their resistances may be computed. It is evident that 

 there is great difficulty in balancing a bridge with this 

 arrangement. It is in fact surprising that von Lang was 



FIG. 10. 



able to give any definite value whatever to the resistance 

 as thus found. He found the resistance to be 1.82 ohms 

 and concluded that the counter E.M.F. was 38.9 volts. 

 This method gave approximately the value of the resist- 

 ance as defined the third way, namely, dE/dl. 



Attempts were then made to find the resistance when 

 the change in the current was made much smaller, that is, 1 

 to get the value indicated by the expression dE/dl. Arons 

 attempted to do this by using a different modification of 

 the Wheatstone bridge. The arc with the battery produc- 



1 Wied. Ann., 30, 95; 1887. 



