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 6 A

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 parallel to the galvanometer

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