A moving coil galvanometer of resistance $20\mathrm{\Omega }$ gives a full scale deflection when a current of 1 mA is passed through it. It is to be converted into an ammeter reading 20 A on full scale. But the shunt of $0.005\mathrm{\Omega }$ only is available. What resistance should be connected in series with the galvanometer coil ? (in $\mathrm{\Omega }$).