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Explain the parinciple and working of a moving coil galvanometer . |
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Answer» Solution :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 PQRSwhose length be l and breadth b. `PQ = RS = l and QR = SP = b ` . Let I be the electric current flowing through therectangular coil PQRS as shown in Figure. The horse - shoe magnet has hemi - spherical magnetic poles which produces a RADIAL magnetic field . Due to this radialfield , the sides QR and SPare always parallel to the B - field ( magnetic field) and experience no force. The sides PQ andRSare 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 ` `rArrI = K/(NAB) theta`.....(3) (or) `I = G theta` where , `G = K/(NAB) `is called galvanometer constant or current reduction factor of the galvanometer . |
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