Two identical cells of emf 1.5 V each connected in parallel provide a supply to an external circuit consisting of two resistors of 7 $\Omega$ each joined in parallel. A very high resistance voltmeter reads the terminal voltage of the cells to be 1.4 V. What is the internal resistance (in $\mathrm{\Omega }$) of each cell?

Resistance of the parallel combination of two 7 Ω resistors is R = 3.5 Ω. 

Since the terminal voltage is 1.4 V, the current in the circuit is I = $\frac{1.4}{3.5} = 0.4 \mathrm{A}$ = 0.4 A. 

Therefore, current through each cell = $\frac{\mathrm{i}}{2}$= 0.2 A. 

Let r be the internal resistance of each cell. 

The voltage drop across r is 1.5 - 1.4 = 0.1 V. 

Therefore, 0.2r = 0.1 or r = 0.5 Ω.