Question

Phosphor-Bronze wire is used for suspension in a moving coil galvanometer because it has.

• A. High conductivity
• B. High resistivity
• C. Large couple per unit twist
• D. Small couple per unit twise

Answer 1

The correct option is D Small couple per unit twise

Moving coil galvanometer, made on the basis of torque of magnetic field on small circuit.
If torque is T for 'twist constant' c and deflection angle $x$ then $T=cx$.
Also, $T=INBA$$B=$ magnetic intensity, $I=$ flow of current in the coil,
$N=$ number of loops, $A=$ Area of the coil.
$INBA=cx$
For unit flow, $x=NBA/c$
So in order to get large deflection, $c$ i.e. twist per unit deflection has to be small.
Phosphor-Bronze wire is used for suspension in a moving coil galvanometer because it has small restoring torque per unit twist and high tensile strength.