4.11 T HE M OVING C OIL G ALVANOMETER

. T HE M OVING C OIL G ALVANOMETER Currents and voltages in circuits have been discussed extensively in Chapters . But how do we measure them? How do we claim that current in a circuit is . A or the voltage drop across a resistor is . V? Figure . exhibits a very useful instrument for this purpose: the moving Conversion of galvanometer into ameter and voltmeter:
  
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 coil galvanometer (MCG). It is a device whose principle can be understood on the basis of our discussion in Section . . The galvanometer consists of a coil, with many turns, free to rotate about a fixed axis (Fig. . ), in a uniform radial magnetic field. There is a cylindrical soft iron core which not only makes the field radial but also increases the strength of the magnetic field. When a current flows through the coil, a torque acts on it. This torque is given by Eq. ( . ) to be τ = NI AB where the symbols have their usual meaning. Since the field is radial by design, we have taken sin θ = in the above expression for the torque. The magnetic torque NIAB tends to rotate the coil. A spring S p provides a counter torque k φ that balances the magnetic torque NIAB; resulting in a steady angular deflection φ . In equilibrium k φ = NI AB where k is the torsional constant of the spring; i.e. the restoring torque per unit twist. The deflection φ is indicated on the scale by a pointer attached to the spring. We have NAB I k φ   =     ( . ) The quantity in brackets is a constant for a given galvanometer. The galvanometer can be used in a number of ways. It can be used as a detector to check if a current is flowing in the circuit. We have come across this usage in the Wheatstone’s bridge arrangement. In this usage the neutral position of the pointer (when