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Moving Coil Galvanometer

In a moving coil galvanometer, the coil is suspended between the pole pieces of a strong horseshoe magnet (Fig. 4). The pole pieces are made cylindrical and a soft iron cylindrical core is placed within the coil without touching it. This makes the field, radial. In such a field the plane of the coil always remains parallel to the field. Therefore, θ = 90° and the deflecting torque always has the maximum value.
 

 
88717.png
Fig. 4
 
The coil deflects a restoring torque which is set up in the suspension fiber. If α is the angle of twist, the restoring torque is
 
 
where C is the torsional constant of the fiber.
 
When the coil is in equilibrium NBiA = Cα  i = Kα, where K = C/NBA is the galvanometer constant. This linear relationship between i and α makes the moving coil galvanometer useful for current measurement and detection.

Current sensitivity (Si)

The current sensitivity of a galvanometer is defined as the deflection produced in the galvanometer per unit current flowing through it.
 
87328.png

Voltage sensitivity (SV)

The voltage sensitivity of a galvanometer is defined as the deflection produced in the galvanometer per unit voltage applied to it.
 
87339.png
 
The molecular theory of magnetism was given by Weber and modified later by Ewing. According to this theory.

Conversion of galvanometer into ammeter

A galvanometer may be converted into an ammeter by connecting a low resistance (called shunt S) in parallel to the galvanometer G as shown in Fig. 5.
 
88803.png
Fig. 5
  • Equivalent resistance of the combination 87345.png.
  • G and S are parallel to each other, hence both will have equal potential difference, i.e., igG = (i – ig)S; which gives
     
    Required shunt 87351.png.
  • To pass nth part of main current (i.e., ig = i/n) through the galvanometer, required shunt 87370.png.

Conversion of galvanometer into voltmeter

A galvanometer may be converted into a voltmeter by connecting a large resistance R in series with the galvanometer as shown in Fig. 6.
 
88820.png
Fig. 6
  • Equivalent resistance of the combination = G + R
  • According to ohm’s law, V = ig (G + R); which gives required series resistance,
     
    87377.png
  • If nth part of applied voltage appeared across galvanometer (i.e., Vg = V/n) then the required series resistance, R = (n – 1)G.




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