in the first circuit below, the potentiometer is at balance point and we know that Va = Vc; and Vj = Vd. In this situation, no current flows through the secondary circuit

If we take this same differences in potential and convert it into another circuit like the 2nd one at the bottom, there will be current flowing through the circuit.

So my question is why is it that for a potentiometer, the resistance wire AJ is treated like a cell carrying the same EMF but in the opposite direction and hence there is zero current at null point. What is so different about the 2 cases?