Moving coil galvanometer: A galvanometer is used to detect current in a circuit.
Construction: It consists of a rectangular coil wound on a non-conducting metallic frame and is suspended by phosphor bronze strip between the pole-pieces (N and S) of a strong permanent magnet.
A soft iron core in cylindrical form is placed between the coil.
One end of coil is attached to suspension wire which also serves as one terminal (T1) of galvanometer. The other end of coil is connected to a loosely coiled strip, which serves as the other terminal (T2). The other end of the suspension is attached to a torsion head which can be rotated to set the coil in zero position. A mirror (M) is fixed on the phosphor bronze strip by means of which the deflection of the coil is measured by the lamp and scale arrangement. The levelling screws are also provided at the base of the instrument.
The pole pieces of the permanent magnet are cylindrical so that the magnetic field is radial at any position of the coil.
Principle and working: When current (I) is passed in the coil, torque τ acts on the coil, given by
τ = NIAB sin θ
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.
A current carrying coil, in the presence of a magnetic field, experiences a torque, which produces proportionate deflection. i.e., Deflection, θ ∝ τ (Torque)
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. The coil experiences a uniform coupler.
Deflecting torque, τ = NIAB
If C is the torsional rigidity of the wire and is the twist of suspension strip, then restoring torque = C θ
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.
mportance (or function) of uniform radial magnetic field: Torque for current carrying coil in a magnetic field is τ = NIAB sin θ
In radial magnetic field sinθ = 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.
The cylindrical, soft iron core makes the field radial and increases the strength of the magnetic field, i.e., the magnitude of the torque.
Sensitivity of galvanometer:
Current sensitivity: It is defined as the deflection of coil per unit current flowing in it.
Voltage sensitivity: It is defined as the deflection of coil per unit potential difference across its ends
Where Rg is resistance of galvanometer.
Clearly for greater sensitivity number of turns N, area A and magnetic field strength B should be large and torsional rigidity C of suspension should be small.
Dividing (ii) by (i)
Clearly the voltage sensitivity depends on current sensitivity and the resistance of galvanometer. If we increase current sensitivity then it is not certain that voltage sensitivity will be increased.
Thus, the increase of current sensitivity does not imply the increase of voltage sensitivity.
