Moving coil galvanometer is a device which is used to detect and measure small electric currents present in an electric circuit.
It is based on the principle that, “When a current carrying coil is placed in magnetic field, it experiences torque.”
A moving coil galvanometer consists of a rectangular coil having large number of turns winded in a non-metallic frame with a soft iron as a central ore. The coil is suspended between two concave poles of two permanent magnets with the help of phosphor-bronze strip. The strip also acts as a path of current to the coil because it is connected to the terminal ‘t1‘ of the galvanometer. Other end of the coil is connected to a light spring which exerts small restoring couple on the coil and then it is finally connected to terminal ‘t2‘. A concave mirror is attached on the strip to note the deflection of the coil by using a lamp and scale arrangement.
When current is passed to the coil through the terminals ‘t1‘ and ‘t2‘, the coil gets deflected due to torque on it. This deflecting torque on the coil is given by, τd = BINAcosθ
Since the poles of the magnet are concave, the magnetic field is radial, i.e. θ = 0o. Therefore,
τd = BINA
As the coil gets deflected, the suspension wire gets twisted and restoring torque is developed on it. Let, k be the restoring torque per unit angle of twist for the suspension wire, then the total restoring torque for an angle θ is
τr = kθ
For the equilibrium of the coil,
Deflecting torque (τd) = Restoring torque (τr)
or, BINA = kθ
or, I = kθ⁄BNA
∴ = Gθ, where G = k⁄BNA is a constant for galvanometer.
Hence, in moving coil galvanometer, deflection is directly proportional to current.