The potentiometer wire is of length $\mathrm{1200 }\mathrm{cm}$ and it carries a current of $60 \mathrm{mA}$. For a cell of emf $5 \mathrm{V}$ and internal resistance of $20 \mathrm{\Omega }$, the null point on it is found to be at $1000 \mathrm{cm}$. The resistance of potentiometer wire is