MR. C. CHRRE ON THE EFFECTS OF PRESSURE 
87r. 
strengtli of the field is raised to 30 or 40 C.G.S units, but very slow when the field is 
raised ov^er 150 C.G.S. units. 
A comparison of this curve with the curves of figs. 4 and 6 brings out clearly 
the fact that tlie residual magnetisation, which is most easily removed by pressure 
cycles, is that left on the break of those fields in which pressure cycles have the 
greatest effect on the induced magnetisation. 
The data in the last columns of Tables VII. and VIII. would lead to curves 
very closely resembling Curve II. Thus the preceding remarks apply equally to the 
experiments of types M and N. 
§ 60. Comparing Curves I. and II. of fig. 13, we see that in fields over 30 
C.G.S. units, tlie increase in the percentage of the induced magnetisation removed by 
breaking the current, and the diminution in the percentage of the residual magnetisa¬ 
tion removed by a definite number of pressure cycles, go hand in hand. 
It has been already mentioned that the application of a particularly severe first 
pressure after the break of a current lessened the eftect of the removal of the pressure, 
and it also lessened the joint eftect of succeeding pressure cycles, in wfiiich the pressure 
was of the normal intensity. 
Thus the close resemblance of Curves I. and II. in the stronger fields suggests that 
the sudden break of a current acts in some respects as a mechanical shock, and that 
the magnitude of this shock-effect continually increases as the strength of the field is 
raised. 
Separation of the Effects o f th e Application and Existence o f Pressure. 
§ 61. There are still some points connected with the fundamental character of the 
eftect on the rod’s magnetisation produced by the application of pressure, on which 
Tables IX. and X. throw some light. 
We have seen that in general the first application of pressure after the rod’s intro¬ 
duction into a given field, causes a change in the magnetisation. Further, as a rule, 
the greater portion of this change survives the removal of the pressure, so that for its 
continued existence the existence of pressure is not essential. It thus seems of 
importance to determine whether the change depends solely on the existence of 
pressure for a finite time during the flow of the current, or is due in whole or in part 
to the actual application of pressure. 
Taking Weber’s hypothesis for illustration, the change must consist either in a 
sub-permanent increase in the general mobility of the ultimate magnetic molecules, or 
in a swinging round of some of them into extreme positions, wliere they are kept hy 
molecular friction even after the removal of the pressure. The application of pressure 
produces doubtless molecular movements, during which the rotations of the ultimate 
magnets might be expected to go on more freely, so that on this theory we shoulo 
d priori be disinclined to attribute the entire effect to the mere existence of pressure. 
