522 



SUBMARINE CABLE LAYING AND REPAIRING. 



Now increase the fixed resistance to r^ and adjust the 

 variable resistance to a higher value (R^) to obtain a second 

 balance, the distribution then being as in Fig. 293. 



If the ratio -^=n and the E.M.F. remains constant, the cur- 



T 



rent in the first observation is n times that in the second, and 

 we have 



Ei + 5=?i(R + &), ■ 



whence the resistance of battery is 



_Rl-72-R 



ohms. 



7^-1 



Also it is evident that the E.M.F. of the battery (E) is equal 

 'R + h + r 



to 



volts, 



which can be put in the simpler form of 



E = e(l+M^) volts. 



Hence, by two simple balances of the galvavometer to zero, we 

 have both the E.M.F. and the resistance of the battery. 



Fig. 293. 



The ordinary bridge bos can be used for this test connected,, 

 as in Fig. 294. It is convenient to use the two 100 coils in 

 the bridge ratio arms for r, making it 100 for the first balance 

 and 200 for the second (that is n=2). With r = 100 the vari 

 able resistance is adjusted till there is no deflection on the 

 galvanometer (the resistance so unplugged being R ohms). 

 Then making 7'j=200 the variable resistance is again adjusted 

 to balance (the resistance unplugged being Rj ohms). Then> 

 since w=2 we have ^=E — 2R. 



