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 X and Y be Vx −Vy, then
‌Vx−Vy‌=Ig×G=Is×S
‌‌ Or, ‌‌Ig×G‌=Is×S
∴‌S‌=‌

Ig

Is

×G .......(1)
‌‌ Again ‌‌I‌=Is+Ig
∴‌Is‌=I−Ig
Putting in equation (1)
S=‌

Ig

I−Ig

×G