ON THE MAGNETISATION OF COBALT. 
377 
In practice it is difficult if not impossible wholly to separate the effects of the 
application of pressure from those of its mere existence. When the rod is put under 
pressure before its introduction into the magnetising coil, and not introduced until 
sufficient time has elapsed for any molecular motion to die out, the magnetisation 
might at first sight be supposed to differ from that possessed by the rod when the 
pressure is applied after its introduction by an amount which exactly measures the 
effect of the actual application of the pressure. It must be remembered, however, 
that sudden magnetisation, altering as it does the rod’s length, must act in part as a 
mechanical shock and set up molecular movements, whose character and amplitude 
would naturally depend to some extent on the state of the rod as to pressure. 
In the following remarks the difference between the magnetisations of the rod 
under pressure in the two circumstances of its introduction is entirely attributed to 
the effect of the application of pressure, but the previous statements should be taken 
as a warning against accepting this as more than an approximation to the truth. 
§ 62. Certain experiments bearing on the question have been already referred to in 
§ 40 in discussing the curves of fig. 4. These experiments were limited to fields 
under 60 C.G.S. units; but within that range they showed conclusively that in a 
freshly demagnetised bar the existence of pressure applied before the rod’s intro¬ 
duction into the magnetising coil increases the susceptibility. In the weaker fields of 
fig. 4 the curves show a greater susceptibility when the application of the pressure 
has succeeded than when it has preceded the introduction of the rod into the coil, 
and the evidence for this was considered satisfactory. In fields from 30 to 60 C.G.S. 
units the reverse is the case according to the curves, but reasons were given for 
regarding the evidence on this point as insufficient. ' 
In these experiments when pressure existed during the introduction of the rod into 
the coil it had always been applied some minutes previously, so that any molecular 
movements set a-going by the process had presumably pretty well subsided. Further, 
any such movements must have been inconsiderable compared to those occasioned by 
the vigorous tapping which accompanied the demagnetisation of the rod shortly before 
its introduction in both sets of experiments. 
We are thus in all probability entitled to conclude from these experiments that in 
a freshly demagnetised rod the mere existence of pressure, apart from the consequences 
of its application, has, in fields up to at least 60 C.G.S. units, the effect of permanently 
increasing the induced magnetisation, while in the weakest fields there can be no 
doubt that the actual application of pressure has an independent effect iii the same 
direction. 
§ 63. The results of a more complete series of observations bearing on the same 
point are given in the seventh column of Table IX., under the heading 3^ “ on”~“ off.’’ 
This signifies, as already stated, the excess of magnetisation possessed by the rod 
when introduced into the coil under pressure, over that it possesses when introduced 
free from pressure. The circumstances of the rod are, except as regards the pressure, 
MDCCCXC.— A. 3 G 
