Conversion of galvanometer into ammeter:
Ammeter is connected in series in a circuit. So, its resistance should be such that it does not increase the equivalent resistance value of the circuit appreciably. So, to convert a galvanometer into an ammeter a very low resistance value shunt is to be connected in parallel with the galvanometer.
If potential difference between points $\mathrm{X}$ and $\mathrm{Y}$ be ${\mathrm{V}}_{\mathit{x}}$ $-{\mathit{V}}_{\mathit{y}}$, then
${\mathit{V}}_{\mathit{x}}-{\mathit{V}}_{\mathit{y}}={\mathit{I}}_{\mathit{g}}\times \mathit{G}={\mathit{I}}_{\mathit{s}}\times \mathit{S}$

$\therefore \mathit{S}=\frac{{\mathit{I}}_{\mathit{g}}}{{\mathit{I}}_{\mathit{s}}}\times \mathit{G}$ .......(1)

$\therefore {\mathit{I}}_{\mathit{s}}=\mathit{I}-{\mathit{I}}_{\mathit{g}}$
Putting in equation (1)
$\mathit{S}=\frac{{\mathit{I}}_{\mathit{g}}}{\mathit{I}-{\mathit{I}}_{\mathit{g}}}\times \mathrm{G}$