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Question:

A galvanometer with its coil resistance 25Ω requires a current of 1 mA for its full deflection. In order to construct an ammeter to read up to a current of 2 A, what is the approximate value of the shunt resistance?

2.5×10⁻⁶Ω

1.25×10⁻⁷Ω

1.25×10⁻⁶Ω

2.5×10⁻⁷Ω

Solution:

formula : S= GIg/ (I-Ig)
where S= shunt resistance , (I-Ig)= current through the shunt resistance , I=Current in the circuit.
Here, galvanometer resistance ,G= 25 ohm ; Ig= 0.001 A , I= 2A
S= (25 0.001 )/( 2- 0.001)=1.25×10⁻⁷Ω
A shunt resistance of 1.25×10⁻⁷Ω must be connected to make the galvanometer in the range of 0 to 2A