The $dc$ current flowing in a circuit is measured by two ammeters, one $PMMC$ and another electrodynamometer type, connected in series. The $PMMC$ meter contains $100$ turns in the coil, the flux density in the air gap is $0.2\: Wb/m^2,$ and the area of the coil is $80\:mm^2$. The electrodynamometer ammeter has a change in mutual inductance with respect to deflection of $0.5\:mH/deg.$ The spring constants of both the meters are equal. The value of current, at which the deflections of the two meters are same, is ________
