Question

Which of the two meters, voltmeter or potentiometer, is preferred to measure the emf of a battery? Why?

Answer 1

Hint: Potentiometer and voltmeter are both used to measure the voltage. Main difference between their working is the potentiometer measures the emf where the voltmeter reads the terminal voltage of the circuit.

Complete step-by-step answer:
Potentiometer is preferred to measure the emf of the cell over voltmeter. This is because:
The voltmeter draws current in order to measure the voltage. So we cannot measure the actual voltage using voltmeter. But the potentiometer does not draw any current from the circuit as it is an open circuit measurement. So we can measure the actual emf of the battery.
The sensitivity of the potentiometer is higher than the voltmeter.

Additional information:
Potentiometer works by varying the position of the sliding contact across a uniform resistance given, the total input voltage is given across the length of the wire and the output voltage is measured by the voltage drop between the sliding contact and the fixed wire.
Potentiometer is a null method device. It does not draw any current from the circuit at the null point and hence no potential drop occurs in the circuit. It measures the potential difference in an open circuit.

Working of the voltmeter is based on Ohm's law. The voltmeter is connected in parallel to the branch in which we want to measure the voltage. It has a very high resistance. And hence it draws some current and measures the output voltage.
Voltmeter is a current controlled device. It draws some current from the circuit to show the deflection and the deflection torque is directly proportional to the current drawn by that.

Note: A potentiometer is called sensitive if the potential gradient is small. Potential gradient is the fall of potential per unit length. We can increase the sensitivity of the potentiometer by making the potentiometer wire long and by reducing the current through the circuit using a rheostat.
The resistance of a potentiometer is infinity and the same for the voltmeter is very high but measurable.