^66' THfi MOLECULAR PROCESS IN MAGNETIC INDUCTION. 



something- to do witli tlie fact, well known to engineers, that nnmerons 

 repetitions of a straining action, so slight as to be safe enough in itself, 

 have a dangerous eflfect on the structni'e of iron or steel. 



Another thing on which the theory throws liglit is the phenomenon 

 of time-lag in magnetization. When a piece of iron is put into a 

 steady magnetic field, it does not take instantly all the magnetism that 

 it will take if time be allowed. There is a gradual creeping up of the 

 magnetism, which is most noticeable when the field is weak and when 

 the iron is thick. If you will watch the manner in which a group of 

 little magnets breaks up when a magnetic force is applied to it, you 

 will see that the process is one that takes time. The first molecule to 

 yield is some outlying one which is comparatively unattached — as we 

 may take the surface molecules in the piece of iron to be. It falls 

 over, and then its neighbors, weakened by the loss of its support, fol- 

 low suit, and gradually the disturbance propagates itself from molecule 

 to molecule throughout the group. In a very thin piece of iron — a fine 

 wire, for instance — there are so many surface molecules, in comparison 

 with the whole number, and consequently so many points which may 

 become origins of disturbance, that the breaking up of the molecular 

 communities is too soon over to allow much of this kind of lagging to 

 be noticed. 



,w ye 00 



200 300 400 



Temperature 



700' C 



Pig. 15. — Relation of magnetic inductive cai)acity to temperature in liard steel (Hopkin.son). 



Eftects of temperature, again, may be interpreted by help of the 

 molecular theory. When iron or nickel or cobalt is heated in a weak 

 magnetic field, its susceptibility to magnetic induction is observed to 

 increase, until a stage is reached, at a rather high temperature, 

 when the magnetic quality vanishes almost suddenly and almost com- 

 pletely. Fig. 15, from one of Hopkinson's papers, shows what is ob- 

 served as the temperature of a piece of steel is gradually raised. The 

 sudden loss of magnetic quality occurs when the metal has become 

 red hot; the magnetic quality is recovered when it cools again suffi- 

 ciently to cease to glow. Now, as regards the first efiect — the increase 

 of susceptibility with increase of temperature — I think that is a con- 

 sequence of two independent effects of heating. The structure is ex- 

 l)anded so that the molecular centers lie further apart. But the free- 

 dom with which the molecules obey the direction of any applied mag- 

 netic force is increased not by that only, but perhaps even more by 



