146 



Mr. Julius Frith on 



more accurately balanced, the telephones enabled the exact 

 value of R,„ to be determined, and the following figures 

 were arrived at : — 



R, = 8 ohms. R E = '25 ohms. 



R y/ = 8 ohms. R D = "04 ohms. 



R //; =*6 ohms. P = 5"35 ohms. 



R L = 3 ohms. Q = 5 "35 ohms. 



This gives the true resistance to be *6 ohms. 

 The back E.M.F. V is given by 

 V = V - RC, 

 where V is the potential difference at the carbons, R the 

 true resistance of the arc, and C the current. 

 V' = 39 volts. 

 The carbons were solid 11mm. in diameter. The arc 

 was 2mm. long from the end of the negative carbon to the 

 edge of the crater. 



In order to check this result another method, represented 

 in Fig. 5, was adopted. 



Fig. 5. 



In this D, and D„ are two continous current shunt wound 

 dynamos, coupled in series with a resistance of 4 ohms., an 

 ammeter, and an arc lamp. 



