HYS'TERE'SIS (Yco-Lat.. from Gk. paaLs, deficiency, from barepEip, bystercin. to be behind, from Varepos, hysteros, latter; connected with Skt. ad, AS. fit. 0110. fiz, Ger. errs. Eng. out). The name given a phenomenon in the magnetization of magnetic substances, which was first observed by. Warburg.•in 1851. and later, in 1885, independently by Ewing, to whom the name is due. It is found that if a rod of iron—or any magnetic substance—is placed in a mag netizing helix, and the electric current through this gradually increased to a certain value. and then slowly decreased. the magnetic properties of the rod do not follow the same course when the magnetizing current is decreasing as they did when it was increasing. All the properties due to the magnetization—the induction, the change in length, the change in elasticity, etc.—lag behind the magnetizing force. That is. if these properties have certain values for a given wag. netizing force when the current is increasing, they will not return to the same values. when, as the current is decreasing, the same magnetiz ing force is reached; but this force must be still further decreased, or even reversed. before these
same values are again obtained. The amount of hysteresis varies greatly with different qualities of iron and steel, and with different substances. Its value is most important from a commercial standpoint, because each time the magnetization of a piece of iron is reversed—as happens at each alternation of all alternating electric current— there is a definite amount of energy lost in heat ing the hon. and this amount depends on the hysteresis. There is another kind of hysteresis which is quite different. If a given magnetizing current is applied suddenly to a rod of iron, it may not attain its full magnetism instantly; and the fact that time is required to reach this is said to be due to 'viscous' hysteresis or 'mag netic lag.' For a full of the impor tance of hysteresis, and of the molecular theory, which explains it. eonsult Ewing, Magnetic In duction (London, 1s'91). Sec ntGNETISM.