Question

Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A) : A Wheatstone bridge is used for precise measurement of Resistance. Reason (R) : It operates on the principle of null deflection. In the light of the above statements, choose the most appropriate answer from the options given below :

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A)
• B. Both (A) and (R) are correct but (R) is not the correct explanation of (A)
• C. (A) is correct but (R) is not correct
• D. (A) is not correct but (R) is correct

Hint:

Wheatstone bridge works on the null balance principle. At balance: \[ I_g=0 \] This makes resistance measurement extremely accurate.

Answer 1

The Correct Option is A

Concept: The Wheatstone bridge is one of the most accurate methods for measuring unknown resistance. It works on the principle of: \[ \text{Null deflection} \] At balance condition:
• no current flows through the galvanometer,
• bridge becomes balanced,
• unknown resistance can be calculated accurately. Null methods are highly precise because measurement accuracy does not depend on meter calibration.

Step 1: Understanding Assertion (A). Assertion (A) states: \[ \text{A Wheatstone bridge is used for precise measurement of resistance} \] This statement is correct. The Wheatstone bridge is widely used for accurate resistance measurement because:
• balance condition improves precision,
• galvanometer sensitivity increases accuracy,
• errors due to instrument calibration are minimized.

Step 2: Understanding Reason (R). Reason (R) states: \[ \text{It operates on the principle of null deflection} \] This is also correct. When the bridge is balanced: \[ \frac{R_1}{R_2}=\frac{R_3}{R_x} \] and current through galvanometer becomes zero. This zero-current condition is called null deflection.

Step 3: Why null deflection gives high accuracy. Null methods are extremely accurate because:
• the detector only senses zero current,
• measurement does not depend on exact galvanometer calibration,
• errors due to power source fluctuations reduce significantly. Thus null deflection is the reason why Wheatstone bridge gives precise resistance measurements.

Step 4: Selecting the correct option. Both Assertion and Reason are correct. Further, Reason correctly explains Assertion. Hence the correct option is: \[ \boxed{(1)} \]