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Moving Coil Galvanometer-Notes

A moving coil galvanometer is a device used to detect small electric currents and can be modified to measure current and potential difference. It operates on the principle that a current-carrying coil in a magnetic field experiences torque, leading to deflection proportional to the current. The sensitivity of the galvanometer can be defined in terms of current sensitivity (deflection per unit current) and voltage sensitivity (deflection per unit voltage).

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1K views4 pages

Moving Coil Galvanometer-Notes

A moving coil galvanometer is a device used to detect small electric currents and can be modified to measure current and potential difference. It operates on the principle that a current-carrying coil in a magnetic field experiences torque, leading to deflection proportional to the current. The sensitivity of the galvanometer can be defined in terms of current sensitivity (deflection per unit current) and voltage sensitivity (deflection per unit voltage).

Uploaded by

iitjeenotes2025
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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Moving coil galvanometer

It is a device used to detect small current flowing in an electric circuit.

With suitable modifications ,a moving coil galvanometer can be used to


measure current and potential difference.

Principle

Its working is based on the fact that when a current carrying coil is
placed in a magnetic field,it experiences a torque.

Construction

 It consisit of a coil PQRS with many turns free to rotate about a


fixed axis between the poles of a permanent magnet NS as shown
in fig: the coil is wound on a non-mettalic frame and has a central
soft iron core.
 The coil is suspended from the torsion head with a phosphor
bronze strip. The other end of the coil is attached to a hair spring.
 A small concave mirror M is attached to the suspension wire . the
whole arrangement is enclosed ina non-magnetic case to avoid
disturbance due to air.
 The galvanometer can be leveled with the help of leveling
screws,so that the coil can rotate freely without touching the iron
core or the poles of the magnet.
 To connect the galvanometer in the circuit ,the binding screws T 1
and T2 are provided at the back of the case of the galvanometer.
Working/theory
Suppose the coil is suspended freely in the m.field.
Let,
and b be the length and breadth of the coil respectively.
A be the area of the coil
N-number of turns in the coil.
B-strength of the m.field.
I-current passing through the coil in the direction PQRS.
θ –angle at which normal drawn on the plane of the coil makes
with the direction of m.field.
The rectangular coil carrying current when placed in the m.field
experiences a torque whose magnitude is given by,
τ = 𝑁𝐼𝐴B sin θ
= 𝑁𝐼𝐴B ( the m.field is radial)
This torque tends to rotate the coil in the magnetic field and
produce a twist in the suspension wire and hair spring. As a result
of it, a restoring torque develops which tends to bring the coil
back to its original position.
Let ∅ be the twist produced in the phosphor bronze strip due to
the rotation of the coil and k be the restoring torque per unit
twist of strip (torsional constant of the spring).
Then the total restoring torque = K∅
In equilibrium, the restoring torque = deflecting torque
𝐾∅ = 𝑁𝐼𝐴B
𝐼= )∅
I=G∅……….(1)
Where G= ),known as galvanometer constant.
From (1), I ∅
It means that ,the deflection produced is proportional to the
current flowing through the galvanometer.
Sensitivity of Galvanometer
A galvanometer is said to be more sensitive if it shows large
deflection even when a small current is passed through it.
(i) Current Sensitivity
The current sensitivity of a galvanometer is defined as the
deflection produced in the galvanometer per unit current flowing
through it.
∅
It is given by Is =
Is=
Unit of current sensitivity is rad/A or div/A
(ii) Voltage Sensitivity
Voltage sensitivity of a galvanometer is defined as the deflection
produced in the galvanometer per unit voltage applied to it.
∅
It is given by Vs = =
∅
= , Where R is resistance of galvanometer
Vs=
Unit of voltage sensitivity is rad/V or div/V

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