176 Measurement of the Internal Resistance of Cells. 



br R 1 + E 2 — R 3 i 



•'• bT^ = 2"~ — ' sa 3 r > ec l lials a > 



then the required resistance of the cell, 

 &=# + _ + fL + & c< 



If, as in the case of an accumulator, x is small compared 

 with r } then 



6=^+ — 



When an accumulator is discharging, taking b = x gives us a 

 a value for b about 2 per cent, too low. 



If the P.D. at terminals of the cell under test is greater 

 than the E.M.F. of a single balancing cell (as is the case 



Fig. 2 a. 



during charge) then the circuit must be modified as shown 

 (fig. 2 a). 



If a Wheatstone's bridge be employed to measure B, ly R 2 , 

 and R 3 , there will be no necessity to employ any special 

 instrument in the testing for equality of potential of the 

 points A, B, and C. For all that need be done is to remove 

 an infinity plug on the bridge, close the galvanometer circuit, 

 but not that of the testing battery, and adjust raj, ra 2 , n v n 2 

 until the galvanometer remains at zero. 



