A potentiometer wire of length 100 cm has resis tance of 10 ohms. It is connected in series with a resistance R and a cell of emf 2V and negligible in ternal resistance. The circuit is as shown below.

-If a source of emf 10 milli volts is balanced by a length of 40cm of the potentiometer wire, find the value of the external resisatnce

Explanation

Potentiometer measures emf without drawing current from the cell (null method)