Question

The scale of a galvanometer is divided into 150 equal divisions. The galvanometer has a current sensitivity of 10 divisions per mA and a voltage sensitivity of 2 divisions per mV. How can the galvanometer be designed to read 1V per division?

• A. $9995\Omega$
• B. $9990\Omega$
• C. $10000\Omega$
• D. $10005\Omega$

Answer 1

Answer: (A)

$9995\Omega$

It is given that a moving coil galvanometer has 150 equal divisions.
Also Current sensitivity $=10$ div/mA and Voltage sensitivity $=2$ div/mV
Now the maximum current $I_g$ that can flow through the galvanometer for full scale deflection is
$I_g=\frac{150}{10}=15$ mA
Also the maximum voltage$V_g$ that can flow through the galvanometer for full scale deflection is
$V_g=\frac{150}{2}=75$ mV
Hence resistance of galvanometer $G$, is
$G=\frac{75mV}{15mA}=5\Omega$
Thus the resistance needed in series with the galvanometer to convert it into voltmeter of range $V=150\times 1=150V$, is
$R=\frac{V}{I_g}-G$
$R=\frac{150}{15\times {10}^{-3}}-5$
$R=10000-5=9995\Omega$