GALVANOMETER

The ballistic galvanometer does not show. steady deflection (as in current galvanometer) When in use owing to the transitory nature of the current passing through it. It oscillates with decreasing amplitude; the amplitude of the first deflection or swing or throw being proportional to the charge passing. The relation¬ship between charge and the deflection is given as 

Q=KO where, Q = charge in micro-coulombs 

Equation  holds good only if the discharge of the electricity through the galvanometer has been completed before any appreciable deflection of the moving system has taken place Hence, the moving system of such a galvanometer must have a large moment of inertia compared to the restoring moment due to the suspension, This is often achieved by the addition of weights 10 the moving svstern. Large moment of inertia means that the galvanometer; has a long 'period of vibration usually of the order of 20 It) 30 seconds,

The damping of the galvanometer should also be small so that the first deflection (swing) is large. After the first deflection (coil) has been observed, electromagnetic damping may be used to bring the movement rapidly to rest. A switch to short-circuit in the galvanometer is provided to save time in bringing the movement to rest. 

Other considerations in' the construction of ballistic galvanometers are that the moving coil should be free from magnetic material, and the suspension strip should be carefully chosen and mounted. The terminals, the' coil, and connections with the galvanometer should be of copper throughout to avoid thermometric effects. The suspension is non-conducting and the current is led into the deflection coil by delicate spirals of very thin copper strip. Construction and working of D' Personal (permanent magnet moving coil, PMMC) type galvanometer has already been discussed in detail.