Explain the parinciple and working of a moving coil galvanometer .

Explanation

Moving coil galvanometer is a device which is used to indicate the flow of current in an electrical circuit.
Principle When a current carrying loop is placed in a uniform magnetic field it experiences a torque .
Construction : A moving coil galvanometer consists of a rectangular coil PQRS of insulated thin copper wire. The coil contains a large number of turns wound over a light metallic frame . A cylindrical soft - iron core is placed symmetrically inside the coil as shown in Figure.
The rectangular coil is suspended freely between two pole pieces of a horse - shoe magnet .
The upper end of the rectangular coil is attached to one end of fine strip of phosphor broze W and the lower end of the coil is connected to a hair spring S which is also made up of phosphor bronze . In a fine suspension strip W, a small plane mirror is sttached in order to measure the deflection of the coil with the help of lamp and scale arrangement . The other end of the mirror is connected to a torsion head T . In order to pass electric current through the galvanometer , the suspension strip W and the spring S are connected to terminals .

Working : Consider a single trun of the rectangular coil PQRS whose length be l and breadth b. `PQ = RS = l and QR = SP = b ` . Let I be the electric current flowing through the rectangular coil PQRS as shown in Figure.
The horse - shoe magnet has hemi - spherical magnetic poles which produces a radial magnetic field . Due to this radial field , the sides QR and SP are always parallel to the B - field ( magnetic field) and experience no force. The sides PQ and RS are always parallel to the B - field and experience force and due to this, torque is produced.
For single turn, the deflection couple as shown in Figure is
` tau = bF = bBIl = (lb) BI = ABI `
since, area of the coil `A = lb ` For coil with N turns, we get
`tau = ` NABI ......(1)
Due to this deflecting torque, the coil gets twisted and restoring torque ( also known as restoring couple ) is developed . Hence the magnitude of restoring couple is proportional to the amount of twist `theta ` . Thus
`tau = K theta` .....(2)
where K is the restoring couple per unit twist or torsional constnat of the spring .
At equilibrium, the deflection couple is equal to the restoring couple. Therefore by comparing equation (1) and (2) , we get
`NABI - K theta `
`rArr I = K/(NAB) theta` .....(3)
(or) `I = G theta`
where , `G = K/(NAB) ` is called galvanometer constant or current reduction factor of the galvanometer .