Text: The tendency of magnetization to lag behind the magnetizing force as, for instance, in an iron core in a transformer (q.v.). When the core of such a transformer is undergoing rapid reversals of magnetism, there is often an expenditure of energy which is not useful and which is converted into heat, thus representing a loss in power. This effect is more noticeable with certain qualities of iron. In other words, when rapid reversals of magnetism take place in a poor quality of iron core it may display a certain sluggishness. This is actually the lagging of the magnetic flux behind the magnetizing force producing it. This loss of energy is assumed due to the work required to alter the position of the molecules of the iron composing the core and the less energy expended in this manner the less the hysteretic losses. In designing transformers or armature cores, hysteresis must be taken into consideration and the iron must be of high quality to hold these losses at a minimum. ( See Eddy Currents, also Core, Transformer.)

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Source: 190