what is the principle ,working,construction of the galvanometer?
Answers
Answered by
3
Moving coil galvanometer : Principle, Construction, Pointer type moving coil galvanometer, Current sensitivity of a galvanometer
Principle: Moving coil galvanometer works on the principle that a current carrying coil placed in a magnetic field experiences a torque.
Moving coil galvanometer
Moving coil galvanometer is a device used for measuring the current in a circuit.
Principle
Moving coil galvanometer works on the principle that a current carrying coil placed in a magnetic field experiences a torque.
Construction
It consists of a rectangular coil of a large number of turns of thin insulated copper wire wound over a light metallic frame (Fig 3.26). The coil is suspended between the pole pieces of a horse-shoe magnet by a fine phosphor - bronze strip from a movable torsion head. The lower end of the coil is connected to a hair spring (HS) 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 (Fig 3.27).
A small plane mirror (m) attached to the suspension wire is used along with a lamp and scale arrangement to measure the deflection of the coil.
Let PQRS be a single turn of the coil (Fig 3.28). 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,
moment of the deflecting couple = n BIl �- b (Fig 3.29)
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,
moment of the restoring couple = Cθ
where C is the restoring couple per unit twist
At equilibrium, deflecting couple = restoring couple nBIA = Cθ

i.e I α θ. Since the deflection is directly proportional to the current flowing through the coil, the scale is linear and is calibrated to give directly the value of the current.
Pointer type moving coil galvanometer
The suspended coil galvanometers are very sensitive. They can measure current of the order of 10-8 ampere. Hence these galvanometers have to be carefully handled. So, in the laboratory, for experiments like Wheatstone’s bridge, where sensitivity is not required, pointer type galvanometers are used. In this type of galvanometer, the coil is pivoted on ball bearings. A lighter aluminium pointer attached to the coil moves over a scale
when current is passed. The restoring couple is provided by a spring.Current sensitivity of a galvanometer.
The current sensitivity of a galvanometer is defined as the deflection produced when unit current passes through the galvanometer. A galvanometer is said to be sensitive if it produces large deflection for a small current.
The current sensitivity of a galvanometer can be increased by
1. increasing the number of turns
2. increasing the magnetic induction
3. increasing the area of the coil
4. decreasing the couple per unit twist of the suspension wire. This explains why phosphor-bronze wire is used as the suspension wire which has small couple per unit twist.
Voltage sensitivity of a galvanometerThe voltage sensitivity of a galvanometer is defined as the deflection per unit voltage.
where G is the galvanometer resistance.
An interesting point to note is that, increasing the current sensitivity does not necessarily, increase the voltage sensitivity. When the number of turns (n) is doubled, current sensitivity is also doubled (equation 1). But increasing the number of turns correspondingly increases the resistance (G). Hence voltage sensitivity remains unchanged.
Principle: Moving coil galvanometer works on the principle that a current carrying coil placed in a magnetic field experiences a torque.
Moving coil galvanometer
Moving coil galvanometer is a device used for measuring the current in a circuit.
Principle
Moving coil galvanometer works on the principle that a current carrying coil placed in a magnetic field experiences a torque.
Construction
It consists of a rectangular coil of a large number of turns of thin insulated copper wire wound over a light metallic frame (Fig 3.26). The coil is suspended between the pole pieces of a horse-shoe magnet by a fine phosphor - bronze strip from a movable torsion head. The lower end of the coil is connected to a hair spring (HS) 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 (Fig 3.27).
A small plane mirror (m) attached to the suspension wire is used along with a lamp and scale arrangement to measure the deflection of the coil.
Let PQRS be a single turn of the coil (Fig 3.28). 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,
moment of the deflecting couple = n BIl �- b (Fig 3.29)
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,
moment of the restoring couple = Cθ
where C is the restoring couple per unit twist
At equilibrium, deflecting couple = restoring couple nBIA = Cθ

i.e I α θ. Since the deflection is directly proportional to the current flowing through the coil, the scale is linear and is calibrated to give directly the value of the current.
Pointer type moving coil galvanometer
The suspended coil galvanometers are very sensitive. They can measure current of the order of 10-8 ampere. Hence these galvanometers have to be carefully handled. So, in the laboratory, for experiments like Wheatstone’s bridge, where sensitivity is not required, pointer type galvanometers are used. In this type of galvanometer, the coil is pivoted on ball bearings. A lighter aluminium pointer attached to the coil moves over a scale
when current is passed. The restoring couple is provided by a spring.Current sensitivity of a galvanometer.
The current sensitivity of a galvanometer is defined as the deflection produced when unit current passes through the galvanometer. A galvanometer is said to be sensitive if it produces large deflection for a small current.
The current sensitivity of a galvanometer can be increased by
1. increasing the number of turns
2. increasing the magnetic induction
3. increasing the area of the coil
4. decreasing the couple per unit twist of the suspension wire. This explains why phosphor-bronze wire is used as the suspension wire which has small couple per unit twist.
Voltage sensitivity of a galvanometerThe voltage sensitivity of a galvanometer is defined as the deflection per unit voltage.
where G is the galvanometer resistance.
An interesting point to note is that, increasing the current sensitivity does not necessarily, increase the voltage sensitivity. When the number of turns (n) is doubled, current sensitivity is also doubled (equation 1). But increasing the number of turns correspondingly increases the resistance (G). Hence voltage sensitivity remains unchanged.
Answered by
0
Answer:
Explanation:
Electric motor, Voltmeter and Ammeter.
Similar questions