Question 37. A galvanometer having a coil resistance of 10 Ω shows full scale deflection for a current of 3 mA. To measure a current of 8 A, the value of the shunt required is :
(1) 3\times10^{-3} Ω
(2) 4.85\times10^{-3} Ω
(3) 3.75\times10^{-3} Ω
(4) 2.75\times10^{-3} Ω
Answer: (3)
Explanation:
The resistance of the galvanometer is G=10 Ω
The current for full scale deflection is I_{g}=3\times10^{-3} A
The total current to be measured is I=8 A
To convert a galvanometer into an ammeter, a shunt resistance is connected in parallel.
The relation between shunt resistance and galvanometer parameters is
I_{g}G=(I-I_{g})SSo, the shunt resistance is S=\frac{I_{g}G}{I-I_{g}}
Substitute the values, S=\frac{(3\times10^{-3})\times10}{8-0.003}
S=3.75\times10^{-3} Ω