(a) Current sensitivity of galvanometer:
Current sensitivity of galvanometer $=\mathit{\theta }∕\mathit{I}=\mathit{N}\mathit{B}\mathit{A}∕\mathit{c}$
So, to increase the current sensitivity:
(i) Permanent magnet should be strong so that $\mathit{B}$ is high.
(ii) $\mathrm{N}$ and $\mathrm{A}$ should be high.
(iii) Value of $\mathrm{c}$ must be low.
(b) To convert into a voltmeter of range $\left(0-\mathit{V}\right)$,
$\mathit{V}={\mathit{I}}_{\mathit{g}}\left({\mathit{R}}_{\mathit{g}}+{\mathit{R}}_1\right)$ .....(i)
To convert into a voltmeter of range $\left(0-2\mathrm{V}\right)$,
$2\mathit{V}={\mathit{I}}_{\mathit{g}}\left({\mathit{R}}_{\mathit{g}}+{\mathit{R}}_2\right)$ .....(ii)
Dividing equation (i) by (ii)
$\frac{\mathit{V}}{2\mathit{V}}=\left({\mathit{R}}_{\mathit{g}}+{\mathit{R}}_1\right)∕\left({\mathit{R}}_{\mathit{g}}+{\mathit{R}}_2\right)$

$\therefore {\mathit{R}}_{\mathit{g}}={\mathit{R}}_2-2{\mathit{R}}_1$