question_answer

A galvanometer of \[100\Omega \]resistance gives full scale deflection with 0.01 A current. How much resistance should be connected to convert it into an ammeter of range IDA?

A) 0.1\[\Omega \] in parallel            

B) 0.1\[\Omega \] in series

C) 0.2\[\Omega \] in parallel            

D) 0.2\[\Omega \] in series

Correct Answer: A

Solution :

Key Idea: An ammeter is made by connecting a low resistance in parallel with a moving coil galvanometer. A galvanometer as such cannot be used as an ammeter, because it has appreciable  resistance and it can measure only a limited current. In practice, an ammeter is made by connecting a low resistance in parallel with G. The potential difference across G and S is same \[\therefore \]    \[{{i}_{g}}\times G=(i-{{i}_{g}})\times S\] \[\Rightarrow \]    \[S=\left( \frac{i}{i-{{i}_{g}}} \right)G\] Putting the numerical values, we have \[S=\left( \frac{0.01}{10-0.01} \right)\times 100=0.1\Omega \]