A galvanometer with its coil resistance `25 Omega` requires a current of 1 mA for its full deflection. In order to construct an ammeter to read up to a current of 2A, the approximate value of the shunt resistance should be :

Options

A. `1. 25 xx 10^(-2)`
B. `2.5 xx 10^(-3)`
C. `2.5 xx 10^(-2)`
D. `1.25 xx 10^(-3)`

Answer

a

Explanation

Let I be the current to be measured and be the current in coil for full scale deflection then current distribution after connecting shunt (S) in parallel is as shown in figure.

Potential difference across shunt and galvanometer coil must be same hence we can write the following equation:
`I_(g)R_(g) = (I - I_(g))S`
In above equation `I = 2 " and " I_(g) = 10^(-3) A`, hence we can approximate `I - I_(g) approx I`.
`S = (10^(_3) xx 25)/2`
` S = 12.5 xx 10^(_3) = 1.25 xx 10^(-2) Omega`
Hence option (a) is correct.