842 
MR. C. CHREE ON THE EFFECTS OF PRESSURE 
cation of pressure cycles during the flow of the current. In stronger fields no certain 
effect could be attributed to the pressure cycles. The effect, as may be seen from 
fig. 10, was greatest in very weak fields. 
The existence of pressure during the break of the current proved to have a con¬ 
siderable influence on the intensity of the residual magnetisation. As might have 
been anticipated it increased the residual magnetisation in weak fields, but the effect 
changed its sign in fields far below the Villari point, and showed no signs of 
vanishing even in a field of 400 C.G.S. units. 
O 
The ratios of the residual magnetisation to the strength of the pre-existing field 
and to the intensity of the induced magnetisation under various conditions as to 
pressure are shown in figs. 11 and 12 {Plate 16). From these it appears that in the 
absence of all pressure the intensity of the residual magnetisation is extremely small 
in weak fields. In this case the phenomena closely resemble those noticed by many 
observers in iron. The only essential difference apparently is that the fields at which 
certain phenomena appear in cobalt are much higher than the fields at which the 
corresponding phenomena appear in ordinary iron. This is in exact agreement with 
the views already expressed as to the relative positions of the Wendepunkt. 
The application of pressure cycles during the flow of the current increased immensely 
the intensity of the residual magnetisation in weak fields. The increase, in fact, due 
to this cause in the residual magnetisation in the weakest experimental fields, was 
even greater proportionally than in the induced, so that the ordinates of curves c and 
d of fig. 12 continually increase as the strength of the field is reduced. 
The effects of the application or removal of pressure in shaking out the residual 
magnetisation were also observed, and are shown in Curve II., fig. 13, and in figs. 14 
and 15. From the first of these curves it appears that the percentage of residual 
magnetisation shaken out by a series of pressure cycles continually diminished as the 
strength of the field was raised. 
When the rod was free from pressure during the break of the current, the percentage 
of the residual magnetisation, shaken out by the first application of pressure, was in 
weak fields decidedly diminished by the previous application of pressure cycles duriug 
the flow of the current. In fields over 70 or 80 C.G.S. units, however, the effect of 
previous pressure cycles was extremely small. In weak fields the removal of a pressure 
that had existed during the break of the current was fully as effective in shaking out 
I'esidual magnetisation as was the application of a pressure when the rod, during the 
break of the current, remained free from pressure. In fields over 30 or 40 C.G.S. units, 
however, the removal of pressure became decidedly less effective than the application. 
As has been stated in §§ 8 and 11 , Villari and Professor Ewing found that cyclic 
applications of stress were followed by cyclic changes in the residual magnetisation of 
the same character as those produced in the induced magnetisation. As it seemed 
important to test the generality of these conclusions, the cyclic effects of pressure on 
