On the Production of Transient Electrie Currents. 123 



ascribed. It also appears, as was shown first by M. E. Cohn, and 

 afterwards independently by myself in a paper which I had the 

 honour recently to lay before the Royal Society,* in the changes of 

 thermoelectric quality which occur in iron when it is subjected to 

 cyclic changes of stress. In the instances now referred to this lag- 

 ging seems to be permanent as regards time, so that if it is to be 

 ascribed to molecular friction, the friction to which it is due must 

 resemble the friction of solids rather than the viscosity of liquids. 

 To avoid much circumlocution it is convenient to give this lagging 

 action a name, and accordingly I have called it Hysteresis (from 

 vaiepeu, to come after, used either of place or time). This name may 

 be properly applied not only in cases where, as here, the persistence 

 of previous state appears to be permanent as respects time, but also in 

 those cases where (as in the relation of strain to stress in a viscous 

 material) the amount of lagging depends on the rate of change of the 

 conditioning quality, and would disappear if that were indefinitely 

 slow. 



To define the new term more precisely, let there be given two 

 qualities of matter, M and N, of which M is a function of N ; then if 

 when N" is changed cyclically the corresponding changes of M lag 

 behind the changes of N, we may say that there is "hysteresis " in 

 the relation of M to N. The value of M at any particular point of 

 the operation depends not only on the actual value of N but on all 

 the preceding changes of that quantity, and by properly manipu- 

 lating those changes any value of M within more or less wide limits 

 may be found associated with a given value of 1ST. 



In all the instances of " hysteresis " mentioned above this further 

 characteristic is present, that the range through which M varies 

 becomes gradually diminished when a cyclic variation of N is repeated 

 several times, and it would seem that only after an indefinitely large 

 number of repetitions of the cycle of N do the changes of M also 

 become exactly cyclic. f 



§ 12. In the paper referred to above I showed that tlae action here 

 called hysteresis, when it exhibited itself in the relation of thermo- 

 electric quality to stress in a soft-iron wire, could be nearly, if not 

 wmolly, destroyed by mechanically agitating the wire during or after 



* " Proceedings," vol. 32, p. 399. 



f Objection may, perhaps, be taken to the coining of a new word on the ground 

 that tbe term " retentiveness," already in use, expresses sufficiently nearly tlie same 

 idea. In pbysics, however, retentiveness is limited by custom to denote the power 

 of retaining magnetism when the magnetising force is removed, whereas one of the 

 instances in which " hysteresis" has been noticed has, at least apparently, no con- 

 nexion with magnetism. And unless the word retentiveness be used in a sense 

 much less restricted than is now customary, it will not cover even those cases of 

 " hysteresis " which occur in magnetic phenomena. 



