a galvanometer coil has a resistance of 12 ohm and it required 3 micro ampere current for full scale deflection design a circuit to convert it into voltmeter of range 0 to 18 volt
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Answer:
The Moving Coil Galvanometer
Have you ever wondered how the utility company knows how much power you use each month? In short, it uses an electric meter. The galvanometer is an instrument used to determine the presence, direction, and strength of an electric current in a conductor.
When an electric current is passing through the conductor, the magnetic needle tends to turn at right angles to the conductor so that its direction is parallel to the lines of induction around the conductor and its north pole points in the direction in which these lines of induction flow. A galvanometer is a type of ammeter. It is an instrument for detecting and measuring electric .
Magnetism and Moving Charge
Magnetic moment
Moving Coil Galvanometer
Moving coil galvanometer is an electromagnetic device that can measure small values of current. It consists of permanent horseshoe magnets, coil, soft iron core, pivoted spring, non-metallic frame, scale, and pointer.
Browse more Topics under Moving Charges And Magnetism
Ampere’s Circuital Law
Magnetic Field Due to a Current Element, Biot-Savart Law
Magnetic Force and Magnetic Field
Motion in Combined Electric and Magnetic Field
The Solenoid and the Toroid
Torque on Current Loop, Magnetic Dipole
Principle
Torque acts on a current carrying coil suspended in the uniform magnetic field. Due to this, the coil rotates. Hence, the deflection in the coil of a moving coil galvanometer is directly proportional to the current flowing in the coil.
Galvanometer
The Moving Coil Galvanometer
Construction
It consists of a rectangular coil of a large number of turns of thinly insulated copper wire wound over a light metallic frame. The coil is suspended between the pole pieces of a horseshoe magnet by a fine phosphor – bronze strip from a movable torsion head. The lower end of the coil is connected to a hairspring of phosphor bronze having only a few turns.
The other end of the spring is connected to a binding screw. A soft iron cylinder is placed symmetrically inside the coil. The hemispherical magnetic poles produce a radial magnetic field in which the plane of the coil is parallel to the magnetic field in all its positions. A small plane mirror attached to the suspension wire is used along with a lamp and scale arrangement to measure the deflection of the coil.
Working
Let PQRS be a single turn of the coil. A current I flows through the coil. In a radial magnetic field, the plane of the coil is always parallel to the magnetic field. Hence the sides QR and SP are always parallel to the field. So, they do not experience any force. The sides PQ and RS are always perpendicular to the field.
PQ = RS = l, length of the coil and PS = QR = b, breadth of the coil. Force on PQ, F = BI (PQ) = BIl. According to Fleming’s left-hand rule, this force is normal to the plane of the coil and acts outwards.
Force on RS, F = BI (RS) = BIl. This force is normal to the plane of the coil and acts inwards. These two equal, oppositely directed parallel forces having different lines of action constitute a couple and deflect the coil. If there are n turns in the coil, the moment of the deflecting couple = n BIl – b
Hence the moment of the deflecting couple = nBIA
When the coil deflects, the suspension wire is twisted. On account of elasticity, a restoring couple is set up in the wire. This couple is proportional to the twist. If θ is the angular twist, then, the moment of the restoring couple = Cθ, where C is the restoring couple per unit twist. At equilibrium, deflecting couple = restoring couple nBIA = Cθ
Hence we can write, nBIA = Cθ
I = (C / nBA) × θ where C is the torsional constant of the spring; i.e. the restoring torque per unit twist. The deflection θ is indicated on the scale by a pointer attached to the spring.