Question

A galvanometer with internal resistance $100\Omega$ and full-scale current 1 mA is used to realize a dc voltmeter with a full scale range of 1 V. The full scale range of this voltmeter can be extended to 10 V by connecting an external resistance of value

• A. $9k\Omega$
• B. $9.9k\Omega$
• C. $10k\Omega$
• D. $11k\Omega$

Answer 1

The correct option is A $9k\Omega$

When galvanometer measures full scale voltage of 1 V, its resistance is,

$R=\frac{1V}{I_{fs}}=\frac{1V}{1mV}=1k\Omega$

Now, to measure full scale voltage of 10 V, its resistance is,

$R=\frac{10}{I_{fs}}=\frac{10}{1mA}=10k\Omega$

$\Rightarrow$ External resistance required $=10-1=9k\Omega$