Electrical Engineering ⇒ Topic : Ballistic Galvanometer

Ballistic Galvanometer

The basic construction of the ballistic galvanometer is the same as that of the moving-coil galvanometer. The ballistic galvanometer is used to measure the quantity of electricity (i.e., coulombs) which passes through it. This quantity of electricity is the result of e.m.f. induced in a search coil when the magnetic flux linking the coil changes.

The basic difference between the conventional galvanometer and the ballistic galvanometer is that while the conventional galvanometer carries a certain steady current which produces deflection, the passage of current through the ballistic galvanometer is momentary and the moving system will be at rest during the flow of current. After the quantity of electricity has passed through the moving coil of the ballistic galvanometer, it gives first deflection or 'throw' which is recorded by lamp and scale arrangement. After the first deflection or throw has been observed, the moving coil is rapidly brought to rest by eddy current damping. The ballistic galvanometer is so designed that the first deflection or throw is directly proportional to the quantity of electricity Q that has passed through it

                                                                             i.e. Q = k θ

                                                                         where k = Ballistic constant of the galvanometer

                                                                                    θ = First deflection or throw

Construction. Fig. (a) shows the simplified diagram of the ballistic galvanometer. It consists of a moving coil suspended between the poles of a permanent magnet. The coil is wound on a nonmagnetic former so that there is very little damping. A fine wire is used to lead the current into and out from the coil. The instrument is provided with eddy current damping. The first deflection or throw is recorded by lamp and scale arrangement. The farther is the scale from the ballistic galvanometer, the more is the amplification of the deflected angle which is indicated.

                                                                                               figure (a)

Theory. The theory involved in the working of the ballistic galvanometer is the same as that of the conventional moving coil galvanometer with the following constructional modifications :

• The moving system of a ballistic galvanometer is designed to have a large moment of intertia.This is achieved by attaching small weights to the moving system.
• The eddy current damping provided in a ballistic galvanometer is very small. This is achieved by winding the moving coil on a non-magnetic former.

The large moment of inertia of the moving system makes its period of oscillation quite long,usually 10 to 15 seconds in practice. The result is that whole of the charge passes through the galvanometer before its coil has had time to move from rest i.e., the moving system will be almost at rest during the passage of whole of the charge. This quick flow of current through the coil of the galvanometer produces an impulsive torque . This torque is applied while the movement is still effectively at rest. The impulse causes the coil to move from its initial position of rest but there is no longer a driving torque because the current has already ceased to flow. Therefore, the first deflection or 'throw' of the ballistic galvanometer will be directly proportional to the impulse of current or the quantity of charge passed through the coil.

It may be noted that this mode of operation is valid only if the charge passes through the coil of the ballistic galvanometer in a very short period of time. After the first deflection or 'throw', the movement will continue to oscillate for some time because of small damping torque. However, these oscillations are quickly brought to rest by the shunt switch provided in the instrument. Only the first deflection may be used for measurement purposes