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In specific resistance measurement of a ...

In specific resistance measurement of a wire using a meter bridge, the key k in the main circuit is kept open when we are not taking readings. The reason is

A

the emf of cell will decrease.

B

the value of resistance will change due to joule heating effect.

C

the galvanometer will stop working.

D

none of these

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To answer the question regarding why the key \( k \) in the main circuit is kept open when not taking readings in the specific resistance measurement of a wire using a meter bridge, we can follow these steps: ### Step-by-Step Solution: 1. **Understanding the Setup**: In a meter bridge experiment, we measure the resistance of a wire by balancing it against known resistances. The key \( k \) is part of the circuit that allows current to flow through the wire. 2. **Effect of Current Flow**: When the key \( k \) is closed, current flows through the wire. This current generates heat due to the resistance of the wire, a phenomenon known as the Joule heating effect. 3. **Joule Heating**: The heat generated by the current causes the temperature of the wire to rise. As the temperature increases, the resistance of the wire also changes. This is because the resistance of most materials increases with temperature. 4. **Impact on Measurements**: If the key \( k \) is kept closed for an extended period, the continuous heating can lead to a significant change in the resistance of the wire. This change can lead to inaccurate readings when measuring the specific resistance. 5. **Conclusion**: To ensure accurate measurements, the key \( k \) is kept open when not taking readings. This prevents current from flowing through the wire, thereby avoiding any change in resistance due to heating. ### Final Answer: The key \( k \) in the main circuit is kept open when not taking readings to prevent the current from flowing through the wire. This avoids the Joule heating effect, which can change the resistance of the wire due to an increase in temperature, leading to inaccurate measurements. ---
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