Number 85 (Story #4), June 19, 1992 by Phillip F. Schewe and Ben Stein ELECTRICAL CURRENTS IN THE HUMAN THORAX cannot be measured directly. Nevertheless the density of current flow can be calculated and imaged using sophisticated computer graphics and the actual measurements of potentials at the surface of the body (electrocardiograms) and---via an invasive procedure---at the outer surface of the heart (epicardial electrograms). Scientists at the University of Utah have combined these calculations with the anatomical information from MRI imaging to visualize the volume current flow through the human torso. Like magnetic field lines emanating from and returning to the poles of a dipole magnet, current flow in the upper body fans out from and then returns to the heart. These studies will help to characterize the function of normal and abnormal hearts. (Computers in Physics, May/June 1992.)