How can you convert a galvanometer into an voltmeter?

Explanation

A galvanometer is converted into an ammeter by connecting a low resistance in parallel with the galvanometer . This low resistance is called shount resistance S . The scale is now calibrated in ampere and the range of ammeter depends on the values of the shunt resistance.

Let I be the current passing through the circuit as shown in Figure . When current I reaches the junction A , it divides into two components.
Let `I_(g)` be the current passing through the galvanometer of resistance `R_(g)` through a path AGE and the remaining current `(I - I_(g))` passes along the path ACDE through shount resistance S. The value of shunt resistance is so adjusted that current `I_(g)` produces full scale deflection in the galvanometer. The potential difference across galvanometer is same as the potential difference across shunt resistance.
`V_("galvanometer") = V_("shunt")`
` rArr I_(g) R_(g) = (I - I_(g))S`
`S = I_(g)/((I - I_(g)))R_(g) " or " I_(g) = S/(S+R_(g)) I rArr I_(g) propto I `
Since, the deflection in the galvanometer is proportional to the current passing through it.
`theta = 1/G I_(g) rArr theta propto I_(g) rArr theta propto I `
So, the deflection in the galvanometer measures the current I passing through the circuit ( ammeter) .