A student uses the circuit diagram of a potentiometer as shown in the figure. For a steady current passing through the potentiometer wire, he gets a null point for the cell ε1 and not for ε2. Give reason for this observation and suggest how this difficulty can be resolved?

a) The student doesn't get a null point for cell ε2 because the polarity of cell ε2 is opposite to that of the primary cell ε. From the diagram, we can say that VA > VB, whereas for ε2, VJ > VA, where VJ is any point on the wire, which is not possible.
b) The function of resistance R is helpful in finding the internal resistance of the secondary cell and it also helps in the potential drop across the secondary cell circuit.
c) The sensitivity of the potentiometer can be increased by increasing the length of the potentiometer wire and by reducing the current in the circuit by using a rheostat. Both methods help in decreasing the potential gradient and thereby increasing the sensitivity.