ON THE MAGNETISATION OF COBALT. 
363 
the induced magnetisation to be shown, but the enlarged scale of fig. 2 shows the effect 
in fields belo w 30 C.G.S. units, where it is of most importance. 
The thick line a, with the individual observations indicated by dots, is merely the 
initial portion of fig. 1 on an enlarged scale. The dotted line h, with the individual 
observations indicated by circles, gives the induced magnetisation after the application 
of the pressure cycles. The curves, a and h, are based on the same experiments, and 
the difference between their ordinates answers to the values in the column of 
Table IX. in those experiments in which the rod was originally free from pressure. 
The third curve c, with the individual observations indicated by crosses, gives the 
induced magnetisation prior to the application of pressure cycles, in these experiments 
denoted by a p in Table IX., in which the rod was under pressure when introduced 
into the coil. 
In fig. 4 the ordinates give the coefficients of magnetic induction in the three cases 
of fig. 2. The data are taken from the same table, and the letters, &c., have the same 
significations. 
The difference between the curves a and h shows the very large effect of the 
pressure cycles in increasing the magnetisation in the weaker fields. It should be 
remembered that between the corresponding pairs of observations on which these 
curves are based there intervened no break of the current, nor any change in the 
resistance, or in the position of the ammeter. Thus any error in the zero or orienta¬ 
tion of the ammeter would affect each curve alike, and leave the difference of the 
ordinates practically unaffected. Valuations, it is true, in the strength of the current 
during the application of pressure cycles, would affect curve h without affecting 
curve a. Such variations, however, could escape notice in the weaker fields only 
when very small, because the ammeter was then in its most sensitive position. 
The comparison of the curves c with the others is not so satisfactory. During the 
time of most of the experiments the ammeter was being used for other purposes, and 
so had to be set up afresh every other day. Thus, though its position was carefully 
adjusted, small differences were certain to occur. 
For these and other reasons it would be unsafe to draw any conclusions from the 
crossing of the curves b and c in fig. 4. It may, however, be regarded as perfectly 
certain that in the weaker fields the curve c lies between the curves a and h, and that 
within the limits of fig. 4 the curve c lies very distinctly above the curve a. 
The form of all these curves of fim 4, suD-crests that under all conditions as to 
pressure the coefficient of magnetic induction becomes extremely small in very weak 
fields. There may, however, be a turning point in one or all of the curves in fields 
lower than those experimented on, 
§ 41. In curve a of fig. 5 the ordinates give the total change in the magnetisation 
produced by the first pressure in those experiments of Table IX. in winch the rod was 
free from pressure when introduced into the coil. The change in magnetisation is 
measured with the pressure “ on,” and so is really the algebraic sum of the cyclic and 
3 A 2 
