Conventional systems for the measurement of magnetic circular dichroism (MCD) employ a dc magnet in the sample space of a circular dichroism spectropolarimeter, where the state of polarization of the light beam is modulated between left and right circular. The photometric system of the instrument measures the difference between sample absorbances for the two states of polarization of the light beam. The method described in this paper uses a fixed state of polarization of the light beam, left or right circular, an alternating or pulsed magnetic field, and otherwise the same photometric system as the conventional method. The two methods are shown to be mathematically and physically equivalent. The proposed method offers two fundamental advantages: (1) Only the magnetically induced circular dichroism is measured, even in the presence of natural optical activity, and (2) the method is capable of extension to spectral regions where polarization modulation is difficult to achieve. Experimental apparatus using the two methods are described and their relative merits are discussed.