Shunt is a low resistance in parallel with the galvanometer or ammeter. It protects the galvanometer and ammeter from the strong currents.
If the current flowing in a circuit is strong, a galvanometer or ammeter can not be put directly in it because the instrument may be damaged. To overcome this difficulty, a low resistance (i.e. shunt) is connected in parallel with this instrument. Then a major portion of the current passes through this low resistance (i.e. shunt) and only a small portion passes through the instrument. Due to it, the galvanometer or ammeter remains safe.
To find the current through the galvanometer circuit, let
I = total current in the circuit
G = resistance of the galvanometer
S = resistance of the shunt,
Ig, Is = current through galvanometer and shunt respectively. It is clear that
I = Ig + Is (1)
Or, Is = I - Ig
Or, Ig = I - Is (2)
According to ohm's law,
Potential difference between points A and B = Ig G = Is S (3)
Or, Ig G = (I - Ig)S
Or, Ig (G + S) = IS (4)
Or, Ig = I (S/(G + S)
Putting the value of Ig from (2) in (3), we get
(I - Is)G = Is S
Or, IG = Is (G + S)
Or, s = I (G/(G + S) (5)
Uses of shunt
1. A shunt is used to protect the galvanometer from the strong currents.
2. A shunt is used for converting a galvanometer into an ammeter.
3. A shunt may be used for increasing the range of ammeter.
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