Question

Explain role of dummy wire in searle's apparatus

Answer 1

Searle’s apparatus consists of two metallic frames suspended from a rigid support with the help of experimental wire, (whose Young’s modulus is to be determined) and compensating wire. The slotted weights can be slipped to the hanger so as to load the experimental wire to a desired value. In order to keep the compensating wire tight, a fixed weight i.e. dead weight is used. A spirit level is mounted between two frames. Its one end rests on the tip of the screw of the spherometer. When the screw of the spherometer is turned, it moves against a vertical scale graduated in mm. To find Young’s modulus, the bubble in the spirit level is brought exactly at its centre. By slipping the slotted weights remains at the centre of the spirit level. So the spherometer screw is moved upward. The distance through which the screw has to be moved, is noted which gives the extension in the wire. Let L be length and r, the radius of the wire, then Young’s modulus of the material of wire.