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www.GOALias.blogspot.com164Physicscoil galvanometer (MCG). It is a device whose principle can be understoodon the basis of our discussion in Section 4.10.The galvanometer consists of a coil, with many turns, free to rotateabout a fixed axis (Fig. 4.24), in a uniform radial magnetic field. There isa cylindrical soft iron core which not only makes the field radial but alsoincreases the strength of the magnetic field. When a current flows throughthe coil, a torque acts on it. This torque is given by Eq. (4.26) to beτ = NI ABwhere the symbols have their usual meaning. Since the field is radial bydesign, we have taken sin θ = 1 in the above expression for the torque.The magnetic torque NIAB tends to rotate the coil. A spring S pprovides acounter torque kφ that balances the magnetic torque NIAB; resulting in asteady angular deflection φ. In equilibriumkφ = NI ABwhere k is the torsional constant of the spring; i.e. the restoring torqueper unit twist. The deflection φ is indicated on the scale by a pointerattached to the spring. We have⎛NAB⎞φ = ⎜ I⎝⎟k ⎠FIGURE 4.24 The moving coilgalvanometer. Its elements aredescribed in the text. Depending onthe requirement, this device can beused as a current detector or formeasuring the value of the current(ammeter) or voltage (voltmeter).(4.38)The quantity in brackets is a constant for a givengalvanometer.The galvanometer can be used in a number of ways.It can be used as a detector to check if a current isflowing in the circuit. We have come across this usagein the Wheatstone’s bridge arrangement. In this usagethe neutral position of the pointer (when no current isflowing through the galvanometer) is in the middle ofthe scale and not at the left end as shown in Fig.4.24.Depending on the direction of the current, the pointerdeflection is either to the right or the left.The galvanometer cannot as such be used as anammeter to measure the value of the current in a givencircuit. This is for two reasons: (i) Galvanometer is avery sensitive device, it gives a full-scale deflection fora current of the order of µA. (ii) For measuringcurrents, the galvanometer has to be connected inseries, and as it has a large resistance, this will changethe value of the current in the circuit. To overcomethese difficulties, one attaches a small resistance r s,called shunt resistance, in parallel withthe galvanometer coil; so that most of the currentpasses through the shunt. The resistance of thisarrangement is,R Gr s/ (R G+ r s) ; r sif R G>> r sIf r shas small value, in relation to the resistance ofthe rest of the circuit R c, the effect of introducing themeasuring instrument is also small and negligible. This