Presents a reprint from the IEEE Global History Network's STARS articles.

Muscle and nerve cells are electrically active and give rise to currents and potentials in the surrounding tissues, which are conductors of electricity. The study of the relation of these bioelectric sources to currents they produce in the surrounding tissue is called the volume conductor problem. The basic physics of the volume conductor were worked out by Helmholtz. The Helmholtz paper is still extensively cited. Just who was Helmholtz and exactly what did he contribute so many years ago? What events and circumstances molded the life of such an individual? An attempt is made to answer these questions.

Almost every aspect of our lives has been affected by new pharmaceuticals, new surgical techniques, new imaging techniques, and new understandings of disease. The story of the transformation of our view of our inside selves, and its consequences, is well known to the general public. What is perhaps less well known is the role of electrical engineering in this unprecedented revolution, and the authors reveal some of this "inside story". As in almost every field of human technological endeavor, the application of electrical engineering to medicine in the twentieth century was profoundly affected by the development of electronics, its stepchild the computer, and related mathematical advances. Biomedical engineers have been involved in the invention of instrumentation, the development of sensory aids, prostheses, and other therapeutic devices, and the applications of physics and systems theory to physiological systems, including the use of mathematical models. There are three areas of biomedical engineering, however, that uniquely involve the transmission and/or reception of electromagnetic radiation into or out of the human body itself in order to diagnose and/or treat illness-pure "electromagnetic engineering". It is on these areas, therefore, that the authors focus in telling the story of the "bioelectrical century". We refer to electrophysiology, electrotherapy, and imaging.

The biophysical basis for understanding the electrocardiogram is set forth. Bioelectric sources arise from electrical activity in the heart at the cellular level. The relation of these sources, which can be formally represented as impressed currents, to potentials involves solution of the volume conductor problem. This solution is based on Green's theorem. Sources are related to the transmembrane action potential through a bidomain model of heart muscle. Microscopic and macroscopic aspects of the bidomain model are developed. Various transformations of the source are considered, including multipoles, multiple dipoles, and replacement of the volume distribution with distributions on the heart surface. Time integrals of the waveform are related to excitation time and action potential duration. The theoretical results form the basis of a computer model of the electrocardiogram that relates skin potentials to the spatial and temporal distribution of action potentials in the heart.>

Electrical activity associated with excitation and recovery of heart muscle is manifest as a time-varying potential distribution on the surface of the body. The surface electrocardiogram may be represented by a multipole expansion. The multipole terms can be derived from the surface potentials, and in turn are related to the cardiac sources. Studies indicate that a substantial portion of the information is contained in the dipole plus quadrupole terms of the multipole expansion. An investigation of one normal subject has revealed that surface isopotential maps reconstructed from the dipole-quadrupole are similar to maps constructed from recordings at several hundred electrodes. Preliminary results indicate that accurate dipole and quadrupole components may be obtained from a lead system utilizing as few as 17 electrodes. A theory for possible refinement of the lead system to correct for body size is presented.