Inserting an iron rod into the inductor increases its self-inductance ($L$) because iron has a high relative permeability ($\mu _r$) . The inductive reactance of the coil is defined as $X_L = \omega L$; therefore, an increase in $L$ leads to an increase in $X_L$ . In a series circuit containing a light bulb (resistor $R$) and an inductor, the total impedance is $Z = \sqrt{R^2 + X_L^2}$ . As $X_L$ increases, the total impedance $Z$ of the circuit also increases. Since the RMS current is $I = V_{rms}/Z$, the increased impedance causes the current to decrease . Because the brightness (glow) of the bulb depends on the power dissipated ($P = I^2R$), a reduction in current results in the glow of the bulb decreasing .