TORQUE is unidirectional (acts in the same direction) whatever may be the polarity of the current.The deflecting torque is given by\({T_d} = {I^2}\frac{{DL}}{{dθ }}\)Where,I is operating currentL is self-inductanceθ is deflection\({T_d}\propto {I^2}\)So that the deflection torque of the moving iron INSTRUMENT is proportional to the SQUARE of the RMS value of the operating current and change in self-inductance. Calculation:When the current through a moving iron instrument is INCREASED by 20%, then deflection torque is\({T'_d}\propto {{(1.2I)}^2}\)∴ T'd = 1.44 TdHence deflection torque is increased by 44%