Question

A galvanometer (coil resistance $99\Omega$) is converted into a ammeter using a shunt of $1\Omega$ and connected as shown in the figure $\left(i\right)$. The ammeter reads $3A$. The same galvanometer is converted into a voltmeter by connecting a resistance of $101\Omega$ in series. This voltmeter is connected as shown in figure $\left(ii\right)$. Its reading is found to be $4/5$ of the full scale reading. Find the internal resistance $r$ of the cell (in ohms)

Answer 1

when connected as ammeter effective resistance of circuit$R=r+99//1+2=r+0.99+2$
current in circuit = $\frac{12}{R}=3\Rightarrow \frac{12}{3}=R\Rightarrow 4=r+0.99+2\Rightarrow r=1.01\Omega$