Shunt:
If in any galvanometer current is flowing more than its limit, then the heat generated by the current is so much that it can burn the galvanometer. Apart from this the relative deflection generated due to the current flowing can break the pointer or twist it.
To overcome these difficulties, one attaches a small resistance called shunt, in parallel with the galvanometer coil, so that most of the current passes throught the shunt. Generally, shunt is made up of copper wire; its resistance relative to the galvanometer is very small. Hence by using shunt, the current flowing in the galvanometer can be controlled. Figure shows a galvanometer using shunt.
By the use of shunt most of the part of electric current passes through the shunt. Hence, the current flowing through the galvanometer is that much by which it is safe.
If in the figure I is the main current and the necessary current in the galvanometer for maximum deflection is Ig then current passing through the shunt S is I – Ig
Since, shunt and galvanometer are attached in parallel hence, there would be same potential difference.
∴ IgG = IsS (G is the resistance of the coil of the galvanometer)
It is clear from equation (10) the value of Ig is \([\frac{S}{S+G}]\) part of main current. On the basis of equation (10) expected range galvanometer can be made.
