The mirror image of an electromagnet produces a field with the opposite polarity. Thus the north and south poles of a magnet have the same symmetry as left and right. Prior to 1956, it was believed that this symmetry was perfect, and that a technician would be unable to distinguish the north and south poles of a magnet except by reference to left and right. In that year, T. D. Lee and C. N. Yang predicted the nonconservation of parity in the weak interaction. To the surprise of many physicists, in 1957 C. S. Wu and collaborators at the U.S. National Bureau of Standards demonstrated that under suitable conditions for polarization of nuclei, the beta decay of cobalt-60 preferentially releases electrons toward the south pole of an external magnetic field, and a somewhat higher number of gamma rays toward the north pole. As a result, the experimental apparatus does not behave comparably with its mirror image.