HYSTERESIS (Gr. IQTEprIo-es, from vvrkp€iv, to lag behind)  , a term added to the vocabulary of physical science by J . A . Ewing, who defines it as follows: When there are two qualities M and N such that cyclic variations of N cause cyclic variations of M, then if the changes of M lag behind those of N, we may say that there is hysteresis in the relation of M to N (Phil . Trans., 1885, 176, p . 524) . The phenomenon is best known in connexion with magnetism . If an iron bar is subjected to a magnetic force which is first gradually increased to a maximum and then gradually diminished, the resulting magnetization of the bar for any given value of the magnetic force will be greater when the force -is decreasing than when it is increasing; the iron always tends to retain the magnetic condition which it has previously acquired, and changes of its magnetization consequently lag behind changes of the magnetic force . Thus there is hysteresis in the relation of magnetization to magnetic force . In consequence of hysteresis the process of magnetizing a piece of iron to a certain intensity and then restoring it to its original condition, or of effecting a double reversal of its magnetization, involves the expenditure of energy, which is dissipated as heat in the iron . Electrical generators and transformers often contain pieces of iron the magnetization of which is reversed many times in a second, and in order to economize power and to avoid undue heating it is essential that hysteresis should in such cases be as small as possible . Iron and mild steels showing remarkably little hysteresis are now specially manufactured for use in the construction of electrical machinery .