A moving coil galvanometer is an instrument which is used to measure electric currents. It is a sensitive electromagnetic device, which can measure low currents even of the order of a few microamperes.
The moving coil galvanometer has a rectangular coil that has many turns and it is usually made of thinly insulated or fine copper wire that is wounded on a metallic frame. The coil is free to rotate about a fixed axis. A phosphor-bronze strip is connected to a movable torsion head. This is used to suspend the coil in a uniform magnetic field.
In order to improve the strength of the magnetic field and to make the field radial a soft iron core is placed inside the coil The lower part of the coil is attached to a phosphor-bronze spring having a small number of turns. Binding screws are connected at the other end of the spring.
A steady angular deflection is produced by the spring to produce a counter torque which balances the magnetic torque. A plane mirror which is attached to the suspension wire, along with a lamp and scale arrangement is used to measure the deflection of the coil. Zero-point of the scale is at the centre.
The deflection or twist θ of the moving coil galvanometer is measured as the value indicated on a scale by a pointer which is connected to the suspension wire.
θ= ( nAB / k)I
Therefore, θ ∝ I
The quantity nAB / k is a constant for a given galvanometer.
Q1: The deflection θ is related to the electric current I in a galvanometer by the relation
- I ∝ θ
- I ∝ tan θ
- I ∝ sin θ
- I ∝ cos θ
Answer: (a) I ∝ θ
Q2: A moving coil galvanometer carries a current I and the magnetic field B is radial. The coil has N number of turns and an effective area A. The torque acting on the coil of a moving coil galvanometer is given by
Answer: (c) NABI
Q3: The sensitivity of a moving coil galvanometer can be increased by decreasing
- The number of turns in the coil
- The area of the coil
- The magnetic field
- The couple per unit twist of the suspension
Answer: (d) The couple per unit twist of the suspension
Q4: In ballistic galvanometer, the frame on which the coil is wound is non-metallic. It is
- To avoid the production of induced e.m.f
- To avoid the production of eddy currents
- To increase the production of eddy currents
- To increase the production of induced e.m.f
Answer: (b) To avoid the production of eddy currents
Q5: The reason the coil is bound over the metallic frame in moving coil galvanometer is
- The metallic frame helps in oscillation
- The metallic frame helps in making steady deflection without any oscillation
- The metallic frame increases the magnetic field
- None of the above
Answer: (b) The metallic frame helps in making steady deflection without any oscillation
Q6: The deflection in moving coil galvanometer is
- Inversely proportional to the area of the coil
- Directly proportional to the torsional constant
- inversely proportional to the current flowing
- Directly proportional to the number of turns of the coil
Answer: (d) Directly proportional to the number of turns of the coil
Q7: The pole pieces of the magnet used in a pivoted coil galvanometer are
- Plane surfaces of a horse-shoe magnet
- Cylindrical surfaces of a bar magnet
- Plane surfaces of a bar magnet
- Cylindrical surfaces of a horse-shoe magnet
Answer: (d) Cylindrical surfaces of a horse-shoe magnet
Q8: A current-carrying rectangular coil placed in a uniform magnetic field. In which orientation will the coil rotate?
- In any orientation
- The magnetic field is parallel to the plane of the coil
- The magnetic field is at 450 with the plane of the coil
- The magnetic field is perpendicular to the plane
Answer: (d) The magnetic field is perpendicular to the plane
Q9: What is the shape of a magnet in moving coil galvanometer to make the radial magnetic field?
- Convex cylindrical magnet
- Horse-shoe magnet
- Concave cylindrical magnet
Answer: (c) Concave cylindrical magnet
Q10: Phospher- bronze wire is used in suspension because it has
- A large couple per unit twist
- A small couple per unit twist
- Low conductivity
- High Sensitivity
Answer: (b) A small couple per unit twist