Question

In Searle’s apparatus, when the experimental wire is loaded and unloaded, the air bubble in spirit level gets shifted
(A) Towards reference wire, while loading and towards experimental wire while unloading
(B) Towards experimental wire, while loading and towards reference wire while unloading
(C) Towards experimental wire, both the times during loading and unloading
(D) Towards reference wire, both the times during loading and unloading

Answer 1

Hint: The spirit level is used to indicate the elongation of the wire when it is loaded or unloaded. The spirit level will be kept horizontal without any external interference and the air bubble inside will be brought to the center. Any deviation from this point will be corrected by moving the screw of the spherometer.

Complete answer:
Initially, the spirit level is mounted between the two wires and one end of the spirit level rests on the tip of the screw of the spherometer. The air bubble in the spirit level is brought to the center. If the position of the air bubble changes, it is an indicator that there is an elongation in the wire. The screw of the spherometer can be adjusted such that the air bubble comes back to the center. So, there occurs a change in the position only once the wire gets elongated and this happens only when the experimental wire is loaded with weights. In the question, it is given that the experimental wire is loaded. This means that the experimental wire will get elongated, thus shifting the position of the air bubble towards the experimental wire. Likewise, when the experimental wire is unloaded, the air bubble will move away from the experimental wire(or towards the reference wire).

$\therefore $ Option (B) is the correct answer.

Note:
Searle’s apparatus is normally used to find Young’s modulus of an object. Searle’s apparatus consists of two wires: Reference wire and Experimental wire that is fixed to a rigid support. It also consists of a disk, spirit level, spherometer, weights.