# A resistance of 75 Ohms is connected in shunt of a galvanometer, having an internal resistance of 25 Ohms, to convert it into an ammeter.

A resistance of 75 Ohms is connected in shunt of a galvanometer, having an internal resistance of 25 Ohms, to convert it into an ammeter. What is the value of current (in A) flowing through the galvanometer, if the total current in the circuit is 5 A?

#### SOLUTION

Extension of Ammeter Range

The current range of a DC moving coil ammeter is extended by connecting a shunt resistance Rs (low resistance) across the coil, the circuit as shown in Figure

I = Total current = 5A
Im = full-scale deflection current of ammeter = ?
Ish = shunt current
Rm = resistance of the ammeter = 25Ω
Rsh = shunt resistance = 75Ω

The full-scale deflection current Im is given as

$\begin{array}{l}{I_m} = \dfrac{{I \times {R_{sh}}}}{{{R_m} + {R_{sh}}}}\\\\{I_m} = \dfrac{{5 \times 75}}{{(25 + 75)}}\\\\{I_m} = 3.75A\end{array}$

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