It can readily be demonstrated that when a low frequency alternating voltage is applied to an iron-core coil the impedance is much higher than the direct current resistance of the coil. To understand how the high level of impedance arises it is necessary to explain how magnetic and electrical induction give rise to a current in the coil that opposes (i.e. is 180 degrees out of phase with) the current generated by the applied voltage. This paper shows how the high impedance can be explained by sequential processes of magnetic and electrical induction, and presents a mathematical derivation of the equation for the current in a coil based on Maxwell's equations. The paper introduces the concept of inductive resistance which, together with inductive reactance, gives rise to the high impedance. The paper also considers the phase angle by which the current in an iron-core coil lags the applied voltage, and concludes that, in the absence of factors causing energy losses and waveform distortions, the lag angle must be less than 45 degrees. It is shown that hysteresis increases the lag angle.