The advantages of a potentiometer over a voltmeter are:

• Current Draw: A potentiometer doesnt draw any current from the circuit in which it is used for the measurement, while a voltmeter draws some amount of current in case of high voltages, which results in some errors in the measurements done using a voltmeter.

• Accuracy: Potentiometers work on the principle of zero deflection method, so the possibility of error is very small. In contrast, voltmeters measure voltage by drawing some amount of current using a high resistance value, which might result in errors.

• Standardization: The standardization of a potentiometer can be done directly with a standard cell, making it highly sensitive and useful for measuring small EMFs.

• Exact Value: Potentiometers provide the exact value of potential difference across any two points in a circuit because they never draw any current from the circuit, whereas in the case of voltmeters, there is always a loss of energy due to the internal resistance of the cell.

Therefore, a potentiometer is preferred over a voltmeter for the measurement of EMF of a cell because it does not draw any current from the circuit, is highly sensitive, and provides accurate measurements.