The resistance of a potentiometer wire is $8\mathrm{\Omega }$  and its length is $8 \mathrm{m}$. A resistance box and a $2 \mathrm{V}$ battery are connected in series with it. What should be the resistance in the box, if it is desired to have a potential drop of $1 \mathrm{\mu } \mathrm{V}/\mathrm{mm}$ ?