If the control springs of PMMC type instrument are purposely made of large moment of inertia, then it can be used as

Ballistic Galvanometer:

• A ballistic galvanometer is an instrument, which is used to measure or indicate current in a closed circuit.
• The galvanometer also is known as the PMMC instrument, works on the principle of a permanent magnet moving coil.
• The force is generated on the coil, due to Lorentz Force Law.
• Due to the interaction of fluxes, the pointer in the meter is deflected.
• Once the pointer is deflected, different torques are exerted on the pointer to make the pointer stop at its steady-state position.
• The different torques are, deflecting torque, control torque, and damping torque.
• In a ballistic galvanometer, the damping torque is almost zero. For that reason, it is called a ballistic galvanometer.
• It consists of two magnetic poles North and south.
• In between the two poles, a coil is placed.