question_answer

A galvanometer coil has a resistance of 15\[\Omega \] and gives full scale deflection for a current of 4 mA. To convert it to an ammeter of range 0 to 6A

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

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

C) 0.1\[\Omega \]resistance is to be connected in D parallel to the galvanometer

D) 0.1\[\Omega \] in 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 5 are in parallel, hence \[{{i}_{g}}G=(i-{{i}_{g}})\] \[\Rightarrow \]               \[S=\frac{{{i}_{g}}G}{i-{{i}_{g}}}\]                 \[=\frac{4\times {{10}^{-3}}\times 15}{6-4\times {{10}^{-3}}}\]                 \[=\frac{60\times {{10}^{-3}}}{5.996}\]                 \[=10\times {{10}^{-3}}\Omega \]                 \[=10m\Omega \]