A $21$ cm long cylindrical metal rod slides inside a tube filled with oil $($Newtonian fluid$).$ The inner diameter of the tube is $6\mathrm{.}5$ cm and the clearance is $0\mathrm{.}08$ mm $.$ The mass of the bar is $1\mathrm{.}1$ kg when $1.$ du immersed in the oil. What is the viscosity of the oil, if the steady$-$state velocity of the rod is $0\mathrm{.}12$