We used thermally stimulated depolarization current and potentiometer measurements to study the fundamental electrical properties, such as the surface potential, of polarized bioceramic hydroxyapatite. We demonstrate that, under certain conditions, the surface potential attributable to several overlapping electrets is given by the algebraic sum of the surface potentials of individual electrets. This can occur when the electrets are arranged in series and the components, found by resolving the polarization vector, determine the surface potential on the electrets. In addition, we prove that the surface potential is proportional to the surface charge density attributable to the electrets, and we further prove that the electrostatic shielding is caused by surrounding the electret with a conductor or a dielectric. Potentiometer measurements, obtained for different levels of shaved electret thickness, revealed that the electric field in the electret was not uniform and was stronger nearer the inner surface of the electret, as compared to the field in the bulk of the electret. (c) 2013 Elsevier Ltd. All rights reserved.