374 
MR. C. CHREE OR THE EFFECTS OF PRESSURE 
The fact that the importance of the removal of the first pressure relative to its 
application is greatest in weak fields must, I think, be connected with the fact that in 
weak fields, as shown by cuiwes c and d, fig. 15, a larger proportion of the residual 
magnetisation is shaken out by the removal of a pressure existing during the break of 
the current, than by the application of an equal pressure when the rod is free from 
pressure during the break of the current. In the weak fields pressure cycles, which 
exist in the case of curves c and d, cause a large permanent increase in the induced 
magnetisation, and, of this, a much larger proportion is due to the application than to 
the removal of pressure. Thus, the phenomena suggest that the residual magnetisa¬ 
tion arising from the portion of the induced magnetisation which the application 
of pressure enables the rod to acquire is more easily shaken out by the removal 
of pressure than is the remainder of the residual magnetisation. 
It may also be worth mentioning that by increasing the magnitude of the first 
pressure after the break of the current the eftect of its application was increased, but 
the effect of its removal diminished absolutely as well as relatively. 
Cyclic effect of Pressure on the Residual Magnetisation. 
§ 56. The cyclic effects of pressure cycles on the residual magnetisation during the 
experiments on which the curves h, c, d are based, are shown in the sixth columns 
of Tables YI. and YII. and the fifth column of Table VIII. The cyclic effects were 
also observed in the December experiments, and are recorded in the fifth column 
of Table II. In the experiments of Table II. the demagnetisation was incomplete, 
but otherwise the CO nditioDS were identical with those of the experiments of Table VII. 
The cyclic effect was found very difficult to measure accurately, partly on account 
of its smallness, and partly, doubtless, on account of its sensitiveness to small varia¬ 
tions in the treatment of the rod during the flow, and more especially the break of 
the current. In the weaker fields the cyclic effects were observed only in the experi¬ 
ments of Table II., which were inferior in accuracy to the later observations. 
For these reasons the numerical values attributed to the cyclic effects in the tables 
cannot claim to be more than somewhat rough approximations. Thus, while believing 
the results to represent correctly the general features of the phenomena, I have not 
embodied them in curves, as the precise forms of these might have owed too much to 
the imagination. 
§ 57. Taking first the case when pressure cycles were applied during the flow of the 
current but no pressure existed during the break, we see from Tables II. and VIL 
that the cyclic effect in the residual magnetisation is an increase or a decrease of 
magnetisation when pressure is “on,” according as the pre-existing field is below or above 
a certain critical field. So far the parallelism between the residual and induced mag¬ 
netisations seems complete. The critical field, however, for the residual magnetisation 
is according to both tables only about 30 C.G.S. units, and so only about a quarter of 
