Meter Bridge Null Point and Temperature – Rankers Physics

Measuring Devices ( Galvanometer, Voltmeter and Ammeter & Meter Bridge ): Practice Problem & Solution

Assertion (A): In a Meter Bridge experiment, null point for an unknown resistance is measured. Now, the unknown resistance is put inside an enclosure maintained at a higher temperature. The null point can be obtained at the same point as before by decreasing the value of the standard resistance. Reason (R): Resistance of a metal decreases with increase in temperature.  
Both (A) & (R) are true and the (R) is the correct explanation of the (A)
Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
(A) is true but (R) is false
Both (A) and (R) are false

Solution Explained:

To solve this problem, we apply the core principles of Measuring Devices ( Galvanometer, Voltmeter and Ammeter & Meter Bridge ). Understanding the underlying formula is key to arriving at the correct answer below:

Assertion (A) is false. If the unknown resistance (presumably metallic) is heated, its resistance increases. To maintain the null point at the same position in a Meter Bridge (\(R_{\text{unknown}} / R_{\text{standard}} = L_1 / L_2\)), the standard resistance must also be *increased* proportionally, not decreased. Reason (R) is false. For metals, resistance generally *increases* with an increase in temperature, not decreases.

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