CONTEMPORARY ADVANCES IN PHYSICS 

 TABLE 



319 



■'max. 



(20° K.) 



Iron 



Nickel 



Cobalt 



Alloy FeiCo 



Permalloy Ni 78.5 per cent, Fe 21.5 per cent 



Heusler alloy Cu 75 per cent, Mn 14 per cent, Al 10 



per cent 



Magnetite 



Pyrrhotine 



1742 

 505 



518 



The dependence of the initial curve upon temperature and strain 

 is great and important; but it is expedient to reserve discussion of 

 these variations to later sections. 



The Hysteresis-Loops 



Any ferromagnetic material has an infinite variety of hysteresis- 

 loops, almost any one of which may turn up in practice; but I will 

 limit this discussion to those obtained by a particular procedure, thus: 

 Commence by demagnetizing the sample — increase H gradually to 

 any desired value, denote this by iJo — decrease H gradually to and 

 through zero, reversing it and bringing it to the equal and oppositely- 

 directed value (— Ho) — return gradually to + i7o — return to (— H^) — 

 and so over and over again, ten or twenty times at the least. The 

 point representing / as function of H, or B as function of H, traces 

 out at first an arc of the initial curve extending as far as Ho', thence- 

 forward it travels in sweeping detours passing around and around the 

 origin, successive ones becoming more and more closely alike, until 

 at last it settles into a routine of tracing the same oddly-shaped loop 

 over and over again. I have spoken of the "memory" of the magnetic 

 material ; this process recalls the consolidation of memory into habit. 

 The final habitual loop thus attained is the particular and chief 

 hysteresis-loop associated with Ho. Demagnetizing the sample afresh 

 and repeating the process with a new value of Ho, one gets another 

 loop *; and in this way a family of hysteresis-loops can be determined, 

 one for every point along the initial curve. 



So long as Ho is so low that the initial curve does not depart 

 appreciably from a straight line, the hysteresis also is inappreciable; 

 the point representing / (or B) as function of H goes back and forth 



* The demagnetization may be dispensed with, if the new value of IIo is greater 

 than the prior one. 



