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 twist

Answer 1

Hint: Moving coil galvanometer is a sensitive device that measures the electric current very precisely and is sensitive to even small changes in the electric currents. It works on the principle of magnetic-torque. A current crying magnetic coil experiences a magnetic force when placed in an external magnetic force. A body more sensitive to the amount of force or couple should have a low couple per unit twist to translate even the smallest amount of change in force to twists.

Complete answer:
A moving coil galvanometer is an electric instrument that is used to measure the amount of current flowing in an electric circuit. It is a sensitive device that measures the electric current very precisely and is sensitive to even small changes in the electric currents.
It works on the principle of magnetic-torque. A current crying magnetic coil experiences a magnetic force when placed in an external magnetic force. The strength of torque or the angle of deflection of the coil is proportional to the amount of current in the coil. So, through this deflection, the galvanometer measures the amount of flowing in the circuit or the coil of the galvanometer.
A couple is two equal forces that act on a body in the opposite direction to produce a turning effect. A body more sensitive to the amount of force or couple should have a low couple per unit twist to translate even the smallest amount of change in force to twists.
For a galvanometer to be sensitive it has to be able to respond to even small changes in the current and thus should have a small couple per unit twist.

So, Option D is the correct one.

Note:
The current sensitivity per unit current of moving coil galvanometer is given by \[\dfrac{\theta }{I}=\dfrac{nAB}{k}\], here k is the torsional constant or the couple per unit twist. As can be seen from the formula, for the device to be sensitive the value of k should be small or the couple per unit twist should be small.