What is a ballistic galvanometer and what is its principle?
A ballistic galvanometer is a galvanometer specially designed to measure the quantity of charge displaced in a current of short duration (such as in the charging or discharging of a capacitor). It is a sensitive moving-coil mirror galvanometer with a coil that has a relatively large moment of inertia and a long period of vibration as compared with a galvanometer designed for current measurement.
Principle of a ballistic galvanometer (B.G.): When current passes through a coil of a B.G. then it rotates in a magnetic field and the deflection of the coil is proportional to the quantity of charge passing through it. Hence a B.G. measures the quantity of charge rather than current.