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The zero-potential contour produced on the wall of a homogeneous conducting circular cylinder by a current dipole is investigated for both centric and eccentric dipoles located midway between the end-caps of the cylinder. Properties and characteristics of the zero-potential contour are discussed. The perpendicular-plane construction, employed in conjunction with the null-contour on the human subject to determine the orientation of a centric heart dipole, is clarified conceptually and the errors involved in this construction are determined quantitatively. It is shown that the heart dipole angle is obtainable from the perpendicular-plane construction if the latter result is multiplied by a correction factor of approximately 1.6, for dipoles tilted at angles from 0Â° to 45Â° with respect to the cylinder axis. The application of this result to electrocardiography is restricted by the assumptions that the human torso may be represented by a homogeneous conducting circular cylinder, and that the heart may be represented by a dipole located on the cylinder axis midway between the end-caps of the cylinder.