Physics
Study Material

Moving Coil Galvanometer

Principle of moving coil galvanometer is that, "a current-carrying coil when suspended in the uniform magnetic field. It experiences a torque." The coil rotates due to torque.

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Posted by Mahak Jain, 3/11/2021
Moving Coil Galvanometer

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A galvanometer is a type of ammeter which is used to measure small values of current. The galvanometer can be used to determine the presence, direction, and the strength of an electric current in a conductor.

Galvanometer can be various types, 

  • Tangent galvanometer
  • Ballistic galvanometer
  • Mirror galvanometer 
  • Astatic galvanometer
  • Moving coil Galvanometer

Most widely used Galvanometer is moving coil galvanometer (MCG). It is an electromagnetic device that can measure small values of current.  

Moving-coil galvanometers itself can be of two types,

  • Suspended coil galvanometer
  • Pivoted coil galvanometer

Principle of Moving Coil Galvanometer

A current-carrying coil when suspended in the uniform magnetic field. It experiences a torque. The coil rotates due to torque.

Torque = N×IAB

Here N : number of turns in coil 

          I : current flowing in coil

         A : cross sectional area of coil

         B : magnetic field 

The angle of deflection in galvanometer is due to the effect of the magnetic torque, is proportional to the magnitude of current in the coil.

Moving Coil Galvanometer Diagram

simple galvanometer drawing

The galvanometer consists of :

  • A coil with many turns, which is suspended in uniform radial magnetic field. 
  • A cylindrical soft iron core which makes the magnetic field radial and increases the strength of the magnetic field. 

Construction of Moving Coil Galvanometer

The moving coil galvanometer (MCG) consists of a coil with large number of turns of insulated copper wire. The coil is suspended in the uniform magnetic field by a fine phosphor bronze strip from a movable torsion head produced. 

There is a cylindrical soft iron core which makes the field radial and also increases the strength of the magnetic field. 

Construction of Moving Coil Galvanometer

Image : Radial magnetic field 

The hemispherical magnetic poles produces a radial magnetic field. The plan of the coil is parallel to the magnetic field in all the direction. There is a scale arrangement, which is to measure the deflection of the coil. Deflection in the coil is the measure of current.

Working of Moving Coil Galvanometer

A current I flow through the coil having n number of turns and a cross-sectional area A. This coil is placed in a uniform radial magnetic field B. Due to magnetic field B  the coil experiences a torque τ. 

Torque(τ) = N×IAB

Let consider PQRS a single turn of the coil,

Working of Moving Coil Galvanometer

A current I flows through the coil. In a radial magnetic field, the plane of the coil is always parallel to the magnetic field. 

Now, 

  • 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 therefore experiences force. 

Length of PQ = RS = l

So force on PQ = RS = Bil   

Now according to Fleming’s left-hand rule, this force on PQ acts outwards and on RS acts inward to the plane of the coil. This equal and opposite force acts on coil and produces torque.

Now torque τ = force (f) x perpendicular distance between the forces (b)

torque τ = Bil × b 

torque τ = IAB       ( l × b = A (area of coil)

One turn of coil experiences torque of IAB, coil consisting N turns will experience torque of NIAB.

Torque τ of coil having N turns = NIAB. 

This torque lead to deflection in the galvanometer that is directly proportional to current (I) that  flow through it.

Moving Coil Galvanometer Formula

The torque produced in the coil lead to deflection in the galvanometer. This deflection is directly proportional the current flowing through it therefore deflection is direct measures of current.

Torque(τ) = N×IAB 

Use of galvanometer

Use of Galvanometer in an Ammeter

The moving coil galvanometer is a highly sensitive instrument therefore it can be used to detect the presence of current in any given circuit. thus it can be used as ammeter.

Use of Galvanometer in an Ammeter

Conversion of a Moving Coil Galvanometer Into a Voltmeter

A galvanometer (G) is converted into voltmeter by  introduction of a resistance R of large value in series. 

Conversion of a Moving Coil Galvanometer Into a Voltmeter

RG : Resistance of the galvanometer

As galvanometer and resistant are in series so, 

V = IgR + IgG = Ig(R+G)

Thus, R + G = V / Ig

Therefore, R = V/Ig – G

This is the resistant required to convert galvanometer into voltmeter. 


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