Question

What is moving coil galvanometer? Write down its principle, working

with the help of neat and clean diagram? Also explain sensitivity of

galvanometer and how we can increase it?


5marks​

Answer 1

A galvanometer is an electromechanical instrument which is used for the detection of electric currents through electric circuits. It is a sensitive instrument and cannot be used for the measurement of heavy currents. It works on the principle of conversion of electrical energy into mechanical energy when a current flows in a magnetic field it experiences a magnetic torque and thus rotates through an angle proportional to the current flowing through it.

By making uniform radial magnetic field magnetic field lines are perpendicular to the magnetic moment of a galvanometer.

Soft Iron core makes the electromechanical field as radial which also increase the magnetic field.

Current Sensitivity- It is deflection produced per unit current flowing in the galvanometer.

Voltage Sensitivity- It is the minimum change in a voltage that produces an observable change in the indication or output of galvanometer.

Since Voltage sensitivity decreases with increase in resistance of the coil and the effect of an increase in the number of turns is nullified in case of voltage sensitivity so there is no change of voltage sensitivity, whenever there is a change in current sensitivity

Answer 2

$\bf\green{Moving\:\:coil\:\: galvanometer}$

α mσvíng cσíl gαlvαnσmєtєr íѕ αn ínѕtrumєnt whích íѕ uѕєd tσ mєαѕurє єlєctríc currєntѕ. ít íѕ α ѕєnѕítívє єlєctrσmαgnєtíc dєvícє whích cαn mєαѕurє lσw currєntѕ єvєn σf thє σrdєr σf α fєw mícrσαmpєrєѕ.

Movíng cσíl gαlvαnσmєtєr príncíplє

α currєnt-cαrrчíng cσíl whєn plαcєd ín αn єхtєrnαl mαgnєtíc fíєld єхpєríєncєѕ mαgnєtíc tσrquє. thє αnglє thrσugh whích thє cσíl íѕ dєflєctєd duє tσ thє єffєct σf thє mαgnєtíc tσrquє íѕ prσpσrtíσnαl tσ thє mαgnítudє σf currєnt ín thє cσíl.

Wσrkíng σf mσvíng cσíl gαlvαnσmєtєr

Working of Moving Coil Galvanometer

Let a current I flow through the rectangular coil of n number of turns and a cross-sectional area A. When this coil is placed in a uniform radial magnetic field B, the coil experiences a torque τ.

Let us first consider a single turn ABCD of the rectangular coil having a length l and breadth b. This is suspended in a magnetic field of strength B such that the plane of the coil is parallel to the magnetic field. Since the sides AB and DC are parallel to the direction of the magnetic field, they do not experience any effective force due to the magnetic field. The sides AD and BC being perpendicular to the direction of field experience an effective force F given by F = BIl

Using Fleming’s left-hand rule we can determine that the forces on AD and BC are in opposite direction to each other. When equal and opposite forces F called couple acts on the coil, it produces a torque. This torque causes the coil to deflect.

We know that torque τ = force x perpendicular distance between the forces

τ = F × b

Substituting the value of F we already know,

Torque τ acting on single-loop ABCD of the coil = BIl × b

Where lx b is the area A of the coil,

Hence the torque acting on n turns of the coil is given by

τ = nIAB

The magnetic torque thus produced causes the coil to rotate, and the phosphor bronze strip twists. In turn, the spring S attached to the coil produces a counter torque or restoring torque kθ which results in a steady angular deflection.

Under equilibrium condition:

kθ = nIAB

Here k is called the torsional constant of the spring (restoring couple per unit twist). The deflection or twist θ 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. Hence it is understood that the deflection that occurs the galvanometer is directly proportional to the current that flows through it.