(a) Priciple and working: When current (I) is passed in the coil, torque τ acts on the coil, given by
τ = NIABsinθ
where θ is the angle between the normal to plane of coil and the magnetic field of strength B, N is the number of turns in a coil.
When the magnetic field is radial, as in the case of cylindrical pole pieces and soft iron core, then in every position of coil the plane of the coil, is parallel to the magnetic field lines, so that θ = 90° and sin 90° =1
Deflecting torque, τ = NIAB
If C is the torsional rigidity of the wire and θ is the twist of suspension strip, then restoring torque =Cθ
For equilibrium, deflecting torque = restoring torque
deflection of coil is directly proportional to current flowing in the coil and hence we can construct a linear scale.
(b) Importance (or function) of uniform radial magnetic field: In radial magnetic field sin θ =1, so torque is τ = NIAB. This makes the deflection (θ) proportional to current. In other words, the radial magnetic field makes the scale linear. To produce radial magnetic field pole pieces of permanent magnet are made cylindrical and a soft iron core is placed between them. The soft iron core helps in making the field radial and reduce energy losses produced due to eddy currents.
(c) A voltmeter is used to measure p.d. across a resistance in an electrical circuit. It is connected in parallel across the resistance. If a voltmeter has very high resistance, it will not affect the resistance of circuit, hence reading will be true. That is why while using a moving coil galvanometer on a voltmeter, a high resistance in series is required. An ammeter is used to measure current in circuit, hence it is connected in series with the circuit. If an ammeter has very low resistance it will not affect the circuit - resistance and so reading will be true. That is why while using a moving coil galvanometer as an ammeter, a shunt (small resistance in parallel) is used.
