Q.Why is a potentiometer preferred over a voltmeter for measuring emf?

A potentiometer's balance-point (null) method avoids drawing current from the source under test, unlike a voltmeter.

Advantages of a potentiometer over a voltmeter:

1. No current drawn at balance — at the null point, no current flows through the cell/galvanometer branch being measured, so the potentiometer measures the true emf, whereas a voltmeter, having finite (though large) resistance, always draws some current and so reads only the terminal potential difference, which is less than the true emf.
2. Higher accuracy and sensitivity — because it is a null-method instrument (comparison, not deflection), a potentiometer is far more precise; its sensitivity can be increased by increasing the length of the potentiometer wire. …

Potentiometer: null method, no current drawn, exact. Voltmeter: deflection method, draws current, some error.

A potentiometer compares an unknown emf against a calibrated potential drop by finding a balance (null-deflection) point at which no current is drawn from the source being measured; being effectively an infinite-resistance instrument at balance, it measures emf exactly.

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A potentiometer measures potential difference by a null (zero-current) method, unlike a voltmeter which always draws some current.

Advantages of a potentiometer over a voltmeter:

1. A potentiometer works on the principle of a null method — at balance, no current is drawn from the source/cell whose emf or p.d. is being measured, so it measures the true emf, not just the terminal voltage.
2. Because of this, a potentiometer has, in effect, infinite resistance at balance, whereas even the best voltmeter has finite resistance and draws some current, introducing an error (it measures only terminal p.d., which is less than emf when current flows).
3. A potentiometer is more sensitive and accurate, since its accuracy is limited only by the accuracy of length measurement (using a standard cell as reference), whereas a voltmeter's accuracy is limited by its scale/needle resolution. …