346 
MR. C. CHREE ON THE EFFECTS OF PRESSURE 
particularly as indicating by tbeir variation the amounts of the changes actually 
occuri'inm 
O 
It should also be understood that in calculating the intensity of magnetisation 3 the 
rod was treated as magnetised solenoidally. In some of the experiments, e.g., those 
on residual magnetism, a direct calculation on this basis might have introduced a 
considerable error, owing to the small distance of the rod from the mao’netometer. 
The actually observed readings, however, were first reduced by direct comparison of 
the readings obtained in an independent set of experiments, in wliich the rod in a 
given magnetised condition was placed first of all in the positions it was about to 
occupy in the coui’se of the main observations, and then in a. certain standard position, 
where its distance from the magnetometer was so great that the precise position of 
the “ poles ” was of comparatively little moment. As stated in § 2 , 3 in reality 
doubtless varies from point to point of the rod, and the values here recorded indicate 
merely a sort of average. 
Di sturhing Agencies. 
§ 22 . Before discussing the experiments in detail it will be as well to refer to two 
disturbing agencies, the one of importance mainly in weak fields, the other in strong. 
The former, the residual effects of previous magnetisations, affected very considerably 
some of the earlier experiments here recorded. In weak fields so important was this 
that the rod was sometimes found, after showing a considerable induced magnetisa¬ 
tion, to possess, on breaking the circuit, residual magnetism of opposite sign. The 
application of pressure in general increased this residual magnetisation of opposite 
sign, sometimes to a very considerable extent. Or, supposing the residual magnetisa¬ 
tion to be at first of the same sign as the preceding induced, it might change sign on 
the application of pressure. These effects were produced at pleasure with the 
greatest ease when the rod was treated in accordance with the following hypothesis :— 
Calling the ends of the rod A and B, a weak current makes A, say, a north pole, 
and on breaking the current A is left with a quantity, of northern polarity. A 
smaller reverse current shakes out only so much of this residual magnetism, leaving a 
cjuantity, Ng, in A, which exists alongside of a larger quantity, S^, of southern polarity. 
The end A thus appears a south pole of sti’ength — Ng. On breaking this second 
current, A is left with a quantity S/, less than of residual southern magnetism, 
and a quantity N 3 ', probably less than N 3 , of residual northern magnetism. Wliether 
A appears a south or a north pole depends on whether S/ or N^' is the greater. On 
applying pressure, a considerably greater proportion of S/ is shaken out than of N^', 
and the polarity of the end A may thus change sign. 
Similar phenomena, proceeding doubtless from the same cause, are described bv 
Wiedemann as occurring in iron. They fit in well enough either with Weber’s 
theory or with the view that the deep seated magnetic molecules are less affected by 
weak fields than are the surface molecules. 
