Consider the situation when a cell of an unknown emf is being measured using a potentiometer. We slide the jockey so as to obtain the null point. Now, is there any current in the potentiometer wire at the null point? Since we know that there is no current in the arm containing the unknown cell, its terminals have acquired equal potentials,how is it possible that there is any current in the potentiometer wire when that is in parallel to that cell.
Potential difference across AB= Potential difference across CD?
E2 is the cell in the secondary circuit while E1 is the cell in the driving circuit
Best Answer
If you connect a battery with a voltage $V$ between two points that already have a potential difference of $V$, then no current will flow out of the battery. Of course if you remove the other source of the potential difference, then the battery will start to push out the current needed to keep that voltage drop across the load.
When you say that the the cell is in parallel with the potentiometer you make a mistake: the cell is in parallel with the part of potentiometer and with the DC source (which is in series with the remaining part of the potentiometer).