ON THE MAGNETISATION OF COBALT. 
381 
The difference, however, in this case between 3 a and 3 b amounts to no less than 
97, and thus the susceptibility, when B is the north pole, is lowered by nearly as 
much as the susceptibility, when A is a north pole, is increased. The residual effect 
is, in fact, almost entirely of a polar character. This appears to be about the strength 
of field at which the polar effect is largest. 
In the fields from 357 to 128 C.G.S. units the results show too large fluctuations 
to lay any claims to great accuracy. They may, however, be regarded as satisfactorily 
proving that within these limits the polar residual effect is small, and that it has the 
same sign as in the weaker fields. 
In the field of 174 C.G.S. units no residual polar effect could be detected, and in 
stronger fields the effect changes sign. There is thus a neutral field, which if not 
identical with, is certainly very near that at which the total effect of the first pressure 
on the induced magnetisation vanishes. In weaker fields the susceptibility of the rod 
is greatest when the field is of the same sign as that existing during the pressure 
cycles, in stronger fields the susceptibility is in these circumstances least. 
In fields above this neutral field there is thus, according to the previous reasoning, 
a true polar effect. In fields again, such as 12‘5 C.G.S. units, a quasi polar effect 
unquestionably existed, because there could then be obtained the phenomenon of the 
reversal of the rod’s polarity on the break of an equal reverse current described in 
§ 22. This phenomenon could not be obtained in a field of 237 C.G.S. units. 
There is thus positive proof that a true polar effect exists in strong fields and a 
quasi polar effect in weak fields, and most probably in all but certain critical fields 
they exist simultaneously. 
§ 66. We have seen that the residual polar effect has practically the same critical 
field as the total effect of the first pressure, or First “ on” of Table X. Also both 
these effects are relatively very large in weak fields, and it might thus be supposed 
they stood in some intimate relationship to one another. In fields of 30 to 50 C.G.S. 
units, however, the residual polar effect is very small, while the First “ on ” is about 
its maximum. 
An explanation of the discrepancy is supplied by a consideration of the curves h 
and c of fig. 12. The difference between the ordinates of these curves in a field of 
8 C.G.S. units is great compared to the difference in a field of 30 C.G.S. units. Thus 
in the former field pressure cycles are much more effective in increasing the retentive¬ 
ness than in the latter. A consideration of the numerical data shows that of the 
additional magnetisation which the pressure cycles enable the rod to acquire the loss 
on the break of the current is proportionally much less in a field of 8 than in a field of 
30 C.G.S. units. Thus the quasi polar residual effect must be much smaller in the 
latter field than in the former, which doubtless accounts for the rapid falling off in the 
value of the total polar effect as the field is raised. 
In the last column of Table IX. there are a few results bearing on the same point. 
These are derived from experiments in which the rod was under pressure when 
