A rectangular coil of area  $5.0\times {10}^{-4}{m}^2$ and 60 turns is pivoted about one of its vertical sides. Then coil is in a radial horizontal field of $90G$ (radial here means the field lines are in the plane of the coil for any rotation).  The torsional constant of hair spring connected to the coil if a current of 2.0 mA produces an angular deflection of  is  ${18}^0$ is $x\times {10}^{-8}Nm/\text{degree}$