Moving coil Galvanometer is a device used to detect or measure the small electric current flowing in the circuit.
Principle
Moving coil Galvanometer is based on the result that when a current carrying loop or wire is placed in a uniform magnetic field it experiences a torque.
Theory
Let
N = No. of turns
l = length of the coil
b = breath of the coil
B = Strength of magnetic field
A = Area of the coil
For equilibrium, deflecting torque must be equal to resorting torque.
Deflecting Torque = Restoring Torque ...(1)
N B I A sin = Kα ...(2)
Where K = Restoring Torque per unit twist
α = Deflection
G is called Galvanometer's constant
I = G ∝
Hence I ∝ α ..(7)
The deflectioin produced in the coil of MCG is directly proportional to the electric current.
Construction
Sensitivity of a Galvanometer
A galvanometer is said to be sensitive if small current or voltage change produces large deflection in current. There are two types of sensitivity :
(1) Current Senstivity
(2) Voltage Senstivity
Current Sensitivity—The current sensitivity of a galvanometer is defined as the deflection produced in the galvanometer per current flowing through it
Hence sensitivity of a MCG can be increased by increasing no. of turns, area of cross-section and strength of magnetic field but by decreasing restoring torque per unit twist—
Voltage Sensitivity—Voltage sensitivity of a galvanometer is defined as the deflection produced in the galvanometer per unit voltage applied to it.
Voltage Sensitivity = Current Sensitivity/R
Hence voltage sensitivity can be defined as the ratio of current sensitivity and resistance of the wire. Using eqn (11)
Voltage Sensitivity = NBA/ KR ...(15)
Hence voltage sensitivity of a MCG can be increased by increasing no. of turns, strength of magnetic field, area of cross section but by decreasing restoring torque per unit twist and resistance of the wire.
Conversion of MCG into Ammeter
An ammeter is a device that measure current in the electric circuit. For an ideal ammeter, resistance is almost zero.
A MCG can be converted into an ammeter by connecting a low resistance called ‘Shunt Resistance’ connected in parallel with MCG.
In parallel grouping of resistance, potential must be equal
Conversion of MCG into Voltmeter
A voltmeter is a device that measure potential difference between two terminals of the wire. For ideal voltmeter, it's resistance is inifinite.
A moving coil galvanometer can be converted into a voltmeter by connecting a very high resistance in series.
Advantages of MCG
(i) It's sensitivity is very high
(ii) It behaves like linear scale because electric current is directly proportional to deflection produced in the coil.
