324 BELL SYSTEM TECHNICAL JOURNAL 



the field is kept almost intact when the field is annulled. Near the 

 beginning and near the end of the curve, the magnetization which is 

 conferred upon the metal by the field departs with the field. Ewing's 

 theory of magnetization is strengthened by this fact. 



The greatest remanence, as I have intimated, occurs with magneti- 

 cally-soft materials. Magnetic hardening tends to augment the 

 coercive force at the expense of the remanence. It does not follow 

 that in constructing a good strong permanent magnet one should 

 take a piece of well-annealed iron or permalloy. A substance of low 

 coercive force is liable to lose its magnetization not only when it is 

 exposed to a weak counteracting field, but also when it is bumped or 

 jarred. A magnet which ceases to be one when dropped on the ground 

 is not of much use in the compass or the automobile.* Great coercive 

 force is much sought after in designing permanent magnets, and the 

 alloys developed for this purpose are at the opposite pole of the 

 ferromagnetic world from the permalloys. The maximum coercive 

 force is attained after the material is magnetized to saturation ; for it 

 the name coercivity is used and should be reserved. The coercivity of 

 iron, which when the metal is very pure and well annealed may be as 

 low as 0.5 gauss, is elevated past 50 by alloying with one per cent of 

 carbon, past 60 by a few per cent of tungsten, past 80 by a few per 

 cent of molybdenum, up to 370 by amalgamating the iron with 

 mercury. For the permalloys the values drop below 0.05. These 

 figures naturally relate to samples already magnetized to saturation. 



Other \-vs.-\r{ curves 



Still other I-vs.-H curves are obtained in special ways, a few of 

 which I will mention. 



If during the measuring of an initial curve the sample is continually 

 shaken, or if after each change in magnetizing field it is traversed by a 

 damped alternating current before the magnetization is read, the 

 /-vs.-// curve rises very swiftly from the origin ; it seems as if the first 

 segment had been suppressed, the second rendered steeper than for 

 the undisturbed sample. Fantastically high values of the ratio //// 

 are sometimes obtained in this way. Such curves are sometimes 

 called "ideal curves," owing to an impression that they represent the 

 true law of magnetization undisguised by accidental (?) influences. 



If in the process of measuring a hysteresis-loop the observer stops 



* There is an additional reason for not making permanent magnets out of sub- 

 stances of low coercivity. Suppose an ellipsoid of such a substance magnetized to 

 saturation by an external field He] let He be reduced gradually to zero; the field 

 H = He — Hi = He — NI passes through zero long before He does, and when He 

 finally falls to zero the value of / has fallen far below the true remanence unless the 

 I-vs.-H curve runs nearly parallel to the axis of H. 



