Moving coil galvanometer
Today we are going to discuss about moving coil galvanometer which is one of the topic of class 12 physics chapter four – moving charges and magnetism.
1. What is moving coil
Moving coil galvanometer is a device or instrument which is used to measure or detect the small value of current flowing through a circuit.
2. Principle of moving coil
Moving coil galvanometer is based on the fact that whenever a current carrying loop is plate inside uniform magnetic field it experiences the torque which provides turning or rotating effect to the coil.
# Radial magnetic field
These are those magnetic field lines which acts in such a manner that magnetic field is perpendicular to every point on the surface of coil and acts along the radius of the coil.
3. Construction of moving coil
Above shown diagram is of moving coil galvanometer.
The Galvanometer arrangement shown in diagram here N and S are two concave shaped field magnets such that the magnetic field is perpendicular to the surface of coil ABCD and magnetic field is radial in nature.
Working of moving coil galvanometer
Connect the above arrangement across a circuit the current of the circuit flows in the galvanometer coil ABCD, now as coil ABCD is placed inside uniform radial magnetic field, it experiences torque.
∴ τ = M × B
τ = MBsinθ
Now, as magnetic field is always radial in nature.
So, θ = 90°
τ = MBsin 90
τ = nIAB (1)
τ = nIAB —— eq.1
Now, this torque rotates the coil due to which the twist produced in galvanometer wire as, a result of it a restoring torque is produced in the coil.
If Φ = total twist
Then, restoring torque is directly proportional to the twist produced in wire.
τ(r) Is directly proportional to the Φ.
τ(r) = KΦ [ K is spring or force constant ] — eq.2
Now from eq. 1 and eq. 2 [ at equilibrium ]
τ = τ(r)
niAB = KΦ
I = KΦ / nAB
[ k/nAB = G = galvanometer constant ]
I = GΦ
4. Use of moving coil galvanometer
Moving coil galvanometer is used to measure very small value of current flowing through a circuit.
5. Current and voltage sensitivity
Now, we will discuss current and voltage sensitivity of moving coil galvanometer.
1. Current sensitivity [ Is ]
The current sensitivity of moving coil galvanometer is defined as deflection per unit current the galvanometer.
Is = Q / I
τ = τ(r)
nIAb = KΦ
nAB / K = Q / I
Is = nAB / K
2.Voltage sensitivity [ Vs ]
The voltage sensitivity of moving coil galvanometer is defined as deflection produced in galvanometer coil per unit voltage.
Vs = Q / V
Vs = Q / IR [ V = IR ]
R = resistance of galvanometer
Vs = Q/ IR [ Q/I = current sensitivity ]
Vs = Is / R
Vs = nAB / KR.
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All the best.