# To determine the internal resistance of a primary cell

- Initially in secondary circuit, key
*K*â€™ remains open and balancing length (*l*_{1}) is obtained (Fig. 12). Since cell*E*is in open circuit, so its emf balances on length*l*_{1}, i.e.,*E*=*xl*_{1}(i) - Now key
*K*â€™ is closed; so cell*E*comes in closed circuit. If the process of balancing is repeated again then potential difference*V*balances on length*l*_{2}, i.e.,*V*=*xl*_{2}(ii)

**Fig. 12**

- By using internal resistance formula, we get

# Comparison of emfs of two cells

Let

*l*_{1}and*l*_{2}be the balancing lengths with the cells*E*_{1}and*E*_{2}, respectively, then*E*_{1}=*xl*_{1}and*E*_{2}=*xl*_{2}â‡’**Fig. 13**

Let

*E*_{1}>*E*_{2}and both are connected in series. Let the balancing length is*l*_{1}when cells assist each other and it is*l*_{2}when they oppose each other as shown in Fig. 14. Then**Fig. 14**

â‡’ or