Electromagnetic relays deteriorate in service. Part of this deterioration is due to ageing of the iron employed in the magnetic circuit, and, in particular, to the use of annealing atmospheres, such as cracked ammonia, which contain nitrogen. If such atmospheres are unavoidable, then, for most practical purposes, ageing can be eliminated by the use of irons containing about 0.1 % of aluminium. Silicon, in the iron initially or available in the furnace lining, increases nitrogen ageing. The best properties are obtained by annealing in a furnace lined with alumina, in an atmosphere of wet hydrogen and with an iron free from additions. The properties obtained by treatments at high temperatures are not always unique: they can sometimes be obtained by more prolonged treatment at a lower temperature. Nitrogen-aged piece-parts can be salvaged and restored to a condition better than new by reannealing in hydrogen.The magnetic circuit of a relay must be able to lift and to release its springset load when needed. The first requirement depends on the normal magnetization curve, and the second on the coercive force. Nevertheless, relays are manufactured without sufficient consideration of the properties of the iron necessary for the performance of specific functions. An experimental technique has been devised for the retrospective determination of the requisite properties. The significance of the normal magnetization curve is ascertained by studying the alteration in load-lifting ability as the (field-dependent) permeability is systematically degraded, starting with a ‘better-than-necessary’ material. This is done by reducing the cross-section of the magnetic circuit in such a way that the configuration of leakage paths is not altered. The relation between release m.m.f. and coercive force is approached by first considering the change to be expected in the hysteresis loop for a uniform ring when a gap is introduced, and then examining the relationship between this changed loop and the release of a uniform and of a non-uniform relay.