3G8 
MR. C. CHREE ON THE EFFECTS OF PRESSURE 
the ordinates of h are at least double those of a. The difference is surprisingly great, 
considering the similarity of the conditions under which the corresponding experi¬ 
ments were conducted. It must, presumably, be mainly attributed to the residual 
effect of the shocks employed in completing the demagnetisation in the experiments 
on which the curve h is based. 
In weak fields, as the difference between the curves c and d on the one hand, and 
the curves a and h on the other abundantly proves, the effect of pressures applied 
during the flow of the current in increasing the residual magnetisation is simply 
enormous. It would thus appear a jynori probable that the application of shocks 
immediately before the starting of the current should have some appreciable 
tendency in the same direction. Fortunately I happened to observe the induced as 
well as the residual magnetisation for the field 2'6 C.G.S. units in the experiments on 
which curve a is based. The numerical value of the induced magnetisation was 8'3, 
which is almost exactly the value obtained by interpolation for the same field from 
Table IX. Consecpiently, ill this field, and so presumably in other weak fields, the 
difference between the curves a and h cannot be attributed, in any important degree, 
to an increase in the induced magnetisation brought about by the process of 
demagnetisation employed in the case of curve h. 
As the strongest field in Table V. is only 79 C.G.S. units, no data exist for drawing 
the curve a in higher fields. It is only, however, in fields below 45 or 50 C.G.S. 
units that there is any clear difference between the results of Tables V. and VI. We 
are thus probably justified in concluding that the process of demagnetisation has an 
appreciable effect on the amount of the residual magnetisation only in fields below 
50 C.G.S. units. 
As already stated, the results of Table V. are each the mean of two observations 
taken in close succession with the same current. Almost invariably the second 
observatioii showed a slightly larger amount of residual magnetisation than the first. 
Tins property has been noticed by several observers in iron. 
The difference between the curves h and c of fig. 10, which are based on experi¬ 
ments in which the same pi’ocess of demagnetisation was applied, shows how effective 
pressure cycles are in increasing the amount of the residual magnetisation in weak 
fields. In stronger fields than those of fig. 10 the curves gradually approach one 
another, and in fields exceeding 120 or 130 C.G.S. units they cannot with certainty 
be said to differ. Thus, in fields below 120 or 130 C.G.S. units, the application of 
pressure cycles during the flow of the current increases the amount of the residual 
magnetisation, but in stronger fields no effect can with certainty be said to exist. 
In fields below 30, or at all events 20 C.G.S. units, the difference between the 
curves c and d is, though small, perfectly clear and incontestable. The existence of 
pressure during the break of the current is the only point in which the experiments 
on which curve d is based differed from those on which curve a is based. As the effect 
of pressure “ on ” during the flow of the current is an increase of induced magnetisation 
