Since an ammeter is connected in series to measure the current, its resistance should be as small as possible. Hence, a very low shunt resistance (R2) is put in parallel to the galvanometer so that the Req =
RgR2
Rg+R2
= R2 where R2 is the resistance of the galvanometer. Since a voltmeter is put in parallel to measure the voltage, its resistance should be as high as possible. Hence, a high resistance (R1) is put in series with the galvanometer so that Req = Rg+R1 = R1