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.  
(1) Both (A) & (R) are true and the (R) is the correct explanation of the (A)
(2) Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
(3) (A) is true but (R) is false
(4) 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:

Concept: Meter Bridge principle, temperature dependence of resistance.
Formula: \(R_X / R_S = l_1 / (100 - l_1)\). For metals, \(R\) increases with \(T\).
Solution: When unknown resistance \(R_X\) (metal) is heated, its resistance increases. To maintain the same null point \(l_1\), the standard resistance \(R_S\) must also increase. Assertion (A) is false as it suggests decreasing \(R_S\). Reason (R) is also false as metal resistance increases, not decreases, with temperature.

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