ON THE MAGNETISATION OF COBALT. 
379 
Let us suppose the induced magnetisation increased bj a series of pressure cycles 
applied during the flow of a current, which makes A the north pole. The efiect is 
non-polar if the rod be brought into the condition of an unstrained rod of a different 
material possessed of a higher susceptibility. The effect is polar if the susceptibility, 
while increased in fields making A a north pole, is equally diminished in fields of 
opposite sign—supposing for the moment that an opposite field could be introduced 
without altering the rod’s temporary properties. If the susceptibility be simply 
altered by different amounts in fields of opposite sign the effect may be regarded as a 
combination of a polar and a non-polar effect. 
It is probably impossible to arrive at a complete knowledge of the character of the 
effect, because the introduction of an opposite field implies a break and make of the 
magnetised current affecting the temporary properties of the rod. It is only in so far 
as the effects of pressure cycles are residual that their polar or non-polar character 
admits of direct investigation. 
If, after the application of pressure cycles in a certain field, an increase of 
susceptibility, not otherwise accounted for, be found in fields both of the same and of 
opposite polarity, the existence of a non-polar effect would seem to be established. 
The same conclusion follows if there be a decrease of susceptibility irrespective of the 
sign of the field. If the susceptibility be found less in fields of the same name as 
that existing during the pressure cycles, than in fields of opposite name, it is difficult 
to think of any probable explanation other than a true polar effect. 
If, however, the susceptibility be greater in fields of the same sign as that existing 
during the pressure cycles than in fields of opposite sign, the phenomenon may be 
explained without assuming a true polar effect. 
The reasons for these statements will probably be most clearly presented by 
embodying them in language literally applicable only to Webek’s theory of mag¬ 
netism. According to this theory, the only way in wdiich we can imagine the 
application or removal of pressui’e to produce a permanent change in the magnetisa¬ 
tion is either by endowing the ultimate magnetic molecules with an increased or 
diminished mobility, or by enabling them to swung round into extreme positions, 
where they stick. The change in the molecular mobility is non-polar if the molecules 
turn as freely in fields of one sign as in fields of the opposite sign, otherwise the 
change is polar. 
Now when the susceptibility is found increased in fields both of the same and of 
opposite sign to that existing dinung the pressure cycles, or when it is diminished 
irrespective of the sign of the field, or, lastly, when it is less in a field of the same 
sign as that during which the pressures were applied than in a field of opposite 
sign, it is clear that the phenomena cannot be attributed to the magnetic molecules 
having stuck in positions into which they swung while under the directive influence 
of the original field. Thus, in these cases, the phenomena must be ascribed to some 
change in the mobility of the magnetic molecules, and so there must be a true polar 
3 c 2 
