(a) Draw a labelled diagram of a moving coil galvanometer. Describe its
principle and working.
(b) Why is it necessary to introduce a cylindrical soft iron core inside the
coil of a galvanometer.
(c) Increasing current sensitivity may not necessarily increase its voltage
sensitivity. Explain giving reason
Answers
Answer:
a) Moving coil galvanometer:
A galvanometer is used to detect current in circuit.
Construction: It consists of a rectangular coil wound on a non-conducting metallic frame and is suspended by phosphor bronze strip between the pole-pieces (N and S) of a strong permanent magnet.
A soft iron core in cylindrical form is placed between the coil.
One end of coil is attached to suspension wire which also serves as one terminal (T
1
) of galvanometer. The other end of coil is connected to a loosely coiled strip. Which serves as the other terminal (T
2
). The other end of the suspension is attached to a torsion head which canbe rotated to set the coil in zero position. A mirror (M) is fixed on the phosphor bronze strip by means of which the deflection of the coil is measured by the lamp and scale arrangement. The levelling screws are also provided at the base of the instrument.
The pole pieces of the permanent magnet are cylindrical so that the magnetic field is radial at any position of the coil.
Principle and working: When current (I) is passed in the coil, torque T acts on the coil, given by
T=NIABsinθ
Where θ is the angle between the normal to plane of coil and the magnetic field or strength B,N is the number of turns in a coil.
A current carrying coil, in the presence of a magnetic field, experience a torque, which
i.e, Deflection, θ∝T ( Torque)
When the magnetic field is radial, as in the case of cylindrical pole and soft iron core, then in every position of coil in the plane of coil is parallel to the magnetic field lines, so then in every position of coil the plane of the coil, is parallel to the magnetic field lines, so that θ=90
o
, and sin90
o
=1. The coil experiences a uniform coupler.
b) answer
The cylindrical soft iron core, when placed inside the coil of a galvanometer, makes the magnetic field stronger and radial in the space between it and pole pieces, such that whatever the position of the rotation of the coil is the magnetic field is always parallel to its plane
c) answer
Sensitivity of the galvanometer:Current sensitivity : It is defined as the deflection of coil per unit current flowing in it.Sensitivity, S
1
=(
I
θ
)=
C
NAB
...(i)
Voltage sensitivity: It is defined as the deflection of coil per unit potential difference across its ends
i.e., S
v
=
V
θ
=
R
g
C
NAB
, ...(ii)
Where R
g
is resistance of galvanometer.
Clearly for greater sensitivity number of turns N, area A and magnetic field strength B should be large and torsional rigidity C of suspension should be small.
Dividing (II) by (I)
S
I
S
v
=
G
1
⇒S
V
=
G
1
S
I
Clearly the voltage sensitivity depends on current sensitivity and the resistance of galvanometer. If we increase current sensitivity then it is not certain that voltage sensitivity will be increased. Thus, the increase of current sensitivity does not imply the increases of voltage sensitivity
