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(a) With the help of a diagram, explainthe principle and working of a moving coil galvanometer. (b) What is the imporance of a magnetic field and how is it produced ? (c) Why is it that while using a moving coil galvanometer as a voltmeter as a voltmeter a high resistance in series is required whereas in an ammeter a shunt is used ? |
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Answer» Solution :(a) A labelled diagram of a pivoted type moving coil galanometer has been shown in fig. Principle , If a current coil is freely pivoted in a uniform magnetic field, hence on passing current I thorugh it a deflecting torque acts on the coil which is given by `tau = NAIB` where N = total number of turns in the coil, A = area of coil , B = magnetic field. The spring `S_p` attached to the coil provides the counter torque and in equilibrium state balances the deflecting torque. If `phi` be the steady angular deflection then counter torque is `kphi`, where K = torsional constant of the spring. In equilibrium state `NAIB = k phi implies I = k/(NAB) cdot phi` Thus , deflection is directly proportional to the current. (b) In a radial magnetic field the plane of the rotating coil always lies in the direction of magnetic field. Thus, torque on the coil remains constant `(tau = NAIB)` and does not change with angular deflection angle `phi`. Radial magnetic field is produced by making the POLE piece of HORSE shoe magnet concave and by introducing a cylindrical soft iron core in the middle of GALVANOMETER coil. (c) A voltmeter mus draw a very small current so as to given correct voltage measurement. Therefore, a high resistance is connected in series with the galvanometer. An ammeter should have negligible resistance so that its presence does not alter the circuit current. Therefore, a small shunt resistance is connected in parallel to the galvanometer so as to reduce its resistance. 
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