Q.To convert a galvanometer into a voltmeter,

A galvanometer becomes a high-resistance voltmeter by connecting a large resistance $R_h$ in series with it, so the combined resistance

is large (an ideal voltmeter has $R_v=\infty$), and the current $I_g$ needed for full-scale deflection is reached only once the potential difference across the combination is large -- so the scale, now calibrated in volts rather than amperes, reads the potential difference across whatever circuit element the voltmeter is placed across. Being high resistance, a …