question_answer

    A galvanometer coil has a resistance of 150 and gives full scale deflection for a current of 4 mA. To convert it to an ammeter of range 0 to 6 A

A) \[10\text{ }m\text{ }\Omega \]resistance is to be connected in parallel to the galvanometer

B) \[10\text{ }m\text{ }\Omega \]resistance is to be connected in series with the galvanometer

C)  \[\text{0}\text{.1 }\Omega \] resistance is to be connected in parallel to the galvanometer

D)  \[\text{0}\text{.1 }\Omega \]resistance is to be connected in series with the galvanometer

Correct Answer: A

Solution :

                To convert a galvanometer into an ammeter, a shunt resistance in parallel is connected to galvanometer. Since, galvanometer G and shunt S are in parallel, hence                 \[{{i}_{g}}G=(i-ig)\] \[\Rightarrow \]               \[S=\frac{{{i}_{g}}G}{i-{{i}_{g}}}\]                 \[=\frac{4\times {{10}^{-3}}\times 15}{6-4\times {{10}^{-3}}}\]                 \[=\frac{60\times {{10}^{-3}}}{5.996}\]                 \[=10\times {{10}^{-3}}\Omega \]                 \[=10\,m\,\Omega \]