A galvanometer can be converted into a voltmeter by connecting a

Explanation

A moving coil galvanometer can be converted into voltmeter by connecting high resistance of manganin ( an alloy of copper, manganese and nickel) in series with galvanometer.

Let G and `R_(S)` be the resistance of galvanometer and of high value of series resistor respectively. Let V and `I_(g)` be the potential difference and current flowing through galvanometer.
By Ohm's law
`V=V_(g)+V_(R) `
`therefore V=I_(g)(G+R_(S)) " " ` ...(i)
`therefore G+R_(S)=(V)/(I_(g))`
`therefore R_(S)=(V)/(I_(g))-G " " ` ...(ii)
which is the value of resistance of high resistor which is connected in series with M.C.G.
Now, `n=(V)/(V_(g))`
`therefore V=V_(g)*n " " `...(iii)
Putting (iii) in equation (ii), we get
`R_(S)=(V_(g).n)/(I_(g)) -G`
`R_(S)=nG-G " " [because (v_(g))/(I_(g))=G]`
`R_(S)=G(n-1)`