A coil of area \(3\times10^{-4}\,\mathrm{m^2}\) and \(80\) turns of a moving coil galvanometer is suspended in a uniform radial magnetic field of \(20\,\mathrm{mT}\). If the resistance of the galvanometer is \(50\,\Omega\) and its voltage sensitivity is \(200\,\mathrm{rad\,V^{-1}}\), then the torsional constant of the spring of the galvanometer is (in \(10^{-8}\,\mathrm{N\,m\,rad^{-1}}\))