The Wheatstone bridge works on the principle of balancing two ratios of resistances to determine an unknown resistance accurately. It gives precise results only when the resistances used in the bridge are of comparable magnitude. However, very high or very low resistances cannot be measured accurately using this method due to practical limitations.
Case 1: Very Low Resistance
When the unknown resistance is extremely low, the resistance of the connecting wires, contacts, and leads becomes comparable to the unknown resistance itself. These additional resistances introduce significant errors in measurement. As a result, the bridge cannot achieve an accurate balance, and the calculated value becomes unreliable.
Case 2: Very High Resistance
When the unknown resistance is very high, the current flowing through the bridge becomes extremely small. This makes it difficult for the galvanometer to detect any deflection accurately. In addition, leakage currents and insulation resistance may affect the reading, leading to errors in measurement.
Therefore, due to the effects of contact resistance in low values and very small current and leakage in high values, the Wheatstone bridge principle is not suitable for accurately measuring very high or very low resistances.