Question

The net resistance of a voltmeter should be large to ensure that
(a) it does not get overheated
(b) it does not draw excessive current
(c) it can measure large potential differences
(d) it does not appreciably change the potential difference to be measured

Answer 1

(d) it does not appreciably change the potential difference to be measured

To measure potential difference across any circuit element, the voltmeter is connected in parallel to that circuit element. Let R_eq be the equivalent resistance of the voltmeter and V be the potential difference across the voltmeter.
Then, the current through the voltmeter, i = $\frac{V}{R_{eq}}$
Hence, the deflection in the voltmeter is proportional to the current i and, hence, proportional to V. However, when the voltmeter is used in a circuit, its resistance R_eq is connected in parallel to some circuit element. This might change the overall resistance of the circuit and, hence, the current. Consequently, the potential difference to be measured is also changed. To minimise the error due to this, the equivalent resistance R_eq of the voltmeter should be large. When a large resistance is connected in parallel to a small resistance, the equivalent resistance is only slightly less than the smaller one.