Moving coil galvanometer- construction and working

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Moving coil galvanometer 

Introduction:

In this article I would like to provide you information about the moving coil galvanometer .It is a highly sensitive instrument which can be used to detect the presence of current in any type circuit arrangement. If we connect a galvanometer  in a Wheatstone's bridge circuit, the pointer of the galvanometer will show null deflection, which indicates that no current flows through the device.

The system consists of a soft iron core which is responsible for increasing the strength of the field . The device was discovered in the year 1820 by the well known scientist "Hans Oersted ".

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Moving coil galvanometer 

Definition:

Moving coil galvanometer  is an instrument used to measure and detect the small amount of  electric currents  or we can say electric charge .

Principle :

It works on the principle that when a current carrying conductor is placed in an external magnetic field , it experiences a torque, and the value of torque which is experienced by the current carrying conductor can be changed by changing the current in the conductor . The value of torque can be calculated from the expression which is given as :

                                       τ = nIBAcosθ

Construction :

The arrangement consists of a coil ABCD which is having a large number of turns of a copper wire. The coil is wounded on a non metallic frame which is having a central soft iron core. The coil is suspended from torsion head which is denoted by H by means of a phosphor bronze strip between the pole pieces North and south of the strong magnet . The lower end of the coil is connected to one end of the hair spring which is denoted by S'.  The other end of this hair spring is connected to terminal T₂. M is a plane mirror which is attached to the phosphor bronze strip which is used to note the reading by the lamp and scale method. The torsion head is connected to the screw T₁.

Moving coil galvanometer 

 

Radial magnetic field : 

The magnetic pieces are given concave spherical shape which helps the magnetic field lines to pass through the radius of the core and have the magnetic field called as the radial of magnetic field. In the core it should be noted that,  θ = 0⁰

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Moving coil galvanometer

Theory :

Suppose in the magnetic field B  ̄ ͐,  the coil ABCD is suspended freely. let us consider the length of coil equal to "i " and breadth "B" .Let the number of turns in the coil be "n" and "i " be the current flowing through the coil. Also,  let 'B' be the magnetic field produced by the pole pieces North and south. 

Now,  torque acting on coil ABCD, carrying current I, placed in magnetic field B is given by :

                                               ⇒ τ = nIBAcosθ

In the case of radial field,  θ = 0⁰

           
                                                 ⇒  τ = nIBA.cos0⁰

                                                  ⇒      τ = nIBA

Due to this torque, the coil gets deflected. Hence it is also named as the deflecting torque. 

                                          ⇒    Deflecting torque =  nIBA............    (1)

Due to hair spring, another type of torque named as restoring torque also acts on the coil, mathematically. 

                                           ⇒ Restoring torque = K. θ.................(2) 

where,   K = torque per unit deflection. 

             

       
             θ = Deflection produced in the coil. 

At Equilibrium :

Deflection torque = Restoring torque 

                         
                                        ⇒    NIBA = K. θ

                                        ⇒      I = K/nBA . θ

Put   K/nBA = G = Galvanometer constant. 

                                         ⇒   I =  G. θ

                                         ⇒   I ∝ θ

Thus current flowing through the galvanometer is directly proportional to deflection produced in the coil. 

Current Sensitivity :

It is defined as the amount of defection produced in the galvanometer , when a unit current flows through it. It is denoted by I..... s.    .......

If θ be the deflection produced in the galvanometer when current I passes through it, then 

    ⇒       Is = θ/ I = nBA/k 

The  S. I unit of Is Radian per ampere. 

Voltage sensitivity :

It is defined as the deflection produced in the galvanometer, when a unit voltage is applied across the galvanometer. It is denoted by Vs. 

If V be the voltage applied across the terminals of galvanometer and θ be the deflection produced in the galvanometer, then 

 ⇒    Vs = θ/v = nBA/KR

The unit of voltage sensitivity is radian per volt. 

Conditions for sensitive Galvanometer :

A galvanometer is said to sensitive , if the deflection shown by it is very large even when a small amount of current is passed through it. 

From theory of Galvanometer, 

we know that :

      ⇒      I = K/nBA . θ

       ⇒     θ = nBA/k  . I 

Thus it is clear from the equation that value of θ will be large if n is large, B is large,  A is large and K is small. 

a)  The value of B can be increased with the help of a strong horse shoe magnet. 

b)  Increasing the value of n beyond s certain limit can make the galvanometer bulky. 

c)  we cannot increase the value of A beyond a certain limit, because it makes the galvanometer unmanageable and bulky. 

d)  we can decrease the value of K by using phosphor bronze strip because the value of K for phosphor bronze strip is very small.

"Advantages"

Following are the advantages of a moving coil galvanometer :

1) They are having high sensitivity. Not easily affected by stray magnetic fields.

2) The sensitivity of moving coil galvanometer can be increased. 

3) The instrument is having a linear scale. 

4) The instrument can be used for A. C measurement if a rectifier is used. 

5) The instrument uses high value of magnetic field , the deflection is almost un-influenced by external magnetic fields. 

Conclusion:

The topic discussed above provides you information related to the moving coil galvanometer. It also provides you information about the principle on which it works including construction, theory and advantages of galvanometer . By the end of the report the information about moving coil galvanometer is clear.