In an experiment of comparison of EMF using potentiometer readings were not taken as the Galvanometer showed same side deflection after checking the circuit the connections were correct and after increasing the voltage of the Eliminator by turning the norm supplying current potentiometer the readings were taken.
In the potentiometer instrument, EMF of the battery connected to the potentiometer instrument ( or eleminator output voltage supplied to potentiometer instrument ) should be greater than the emf of cell
which is required to find by experiment using galvanomenter null deflection. If this condition satisfied, then only we get null deflection in galavanometer. If this condition is not satisfied, EMF of the cell exceeds the voltage drop in the potentiometer wire and null deflection in galvanometer will not be obtained.
Even if the EMF or voltage given to poteniometer instrument is greater than emf of the cell, we should also consider the rheostat settings which is in series with the potentiometer wire. If Rheostat is set to add more resistance to circuit, more voltage drop occurs at rheostat’s resistance and less voltage drop occurs across potentiometer wire which may be less than the EMF of the cell to be measured. In that case we will not get null deflection in galvanometer.
EMF is electro-motive-force, voltage difference across the terminals of cell or battery when current is not drawn. When current is drawn from battery/cell, voltage drop occurs within the internal resistance of the cell/battery ,we get (EMF-voltage drop across internal resistance) as potential difference across the terminals of battery/cell.