Theory of Magnetic Detection of the Heart's Electrical Activity

The currents set up in the chest by the electromotive forces of the heart produce magnetic fields at the chest surface which have a peak intensity of about one microgauss. To detect these fields, an optimized coil assembly has been constructed which yields an rms noise level of about 10−8 G in a 1- to 40-cycle band. The design of the coil assembly and the interpretation of its output are based on an analysis employing an unusual form of the reciprocity theorem. The theory shows that magnetic detection is fundamentally different from its electric counterpart and may reveal new clinical information. It also shows how immunity to magnetic interference can be achieved without resorting to bulky and expensive magnetic shields.