Question

Explain the role of dummy wire in Searle's apparatus

Answer 1

Seale’s apparatus is used to determine the Young’s modulus of the material that is used in a wire and the thermal conductivity. The dummy wire used in the setup helps in determining two types of errors. They are the errors due to the:

*Bending of the support

*Thermal expansion or contraction

Answer 2

Searle's apparatus is used for finding young's modulus of any metal.
   you can see in attachment,
                here in Searle's apparatus, dummy wire is shown.
actually dummy wire is used for the elimination of error.
1. Error due to bending of the support :- if single wire is used, and if the support from which the wire suspended bends , then the measured extension will be much greater than the actual extension in the wire, this error is eliminated by dummy wire . as both the wire {dummy wire and experimental wire } are suspended from the same support, if bending of the support occurs,both wires will be lowered to the same extent and hence, there will no shift in the position of the bubble in spirit level. 

2. Error due to thermal expansion or contraction :- as you know, if temperature will be change(increase)  then, thermal expansion occur in wire.due to this, wire will be elongate. means measured length of wire will be greater than actual length of wire. this error is eliminated by Dummy wire. as both dummy wire and experimental wire are of the same material and same original length,due to this change in length will be same for both the wires. thus, there will no shift in the position of the bubble in spirit level.