How is galvanometer converted into an ammeter?

Explanation

To convert a galvanometer into an ammeter, a low resistance called a shunt resistance is connected in parallel with the galvanometer. A galvanometer with this modification is called an ammeter. The value of the shunt resistance S depends on the maximum current to be measured.
If I be the maximum current to be measured, `I_(g)` be the current for a full scale deflection and G the resistance of the galvanometer then,
`" "I_(g)=(IS)/(G+S) " ...(1) "I_(g)G=(I-I_(g))S`
or `" "S=(I_(g)G)/(I-I_(g)) " "I_(g)G=IS-I_(g)S`
`therefore " "I_(g)=(IS)/(G+S) " "I_(g)G+I_(g)S=IS`
`" "(G+S)I_(g)=IS`
from equation (1) `I_(g) propto I`
Since S and G are constants, the scale can be graduated to give the main current directly.