ON THE MAGNETISATION OF COBALT. 
341 
subsequent applications of longitudinal stress, referred to in §7 as occurring in iron, is 
equally characteristic of cobalt, so long as the magnetisa.tion is not much in excess of 
that of the critical or Villari point. In the present experiments the stress applied 
was a pressure, and so Se being always negative, and the magnetisation in the weaker 
fields greatest when the rod was under pressure, S3/Se was, as already stated, negative 
in fields below the critical. 
The effect of the first application of pressure may, as explained in § 7, be regarded 
as the sum of the cyclic effect and of a non-cyclic or shock-effect. In all fields below 
200 C.G.S, units the shock-effect gave an unmistakeable increase of magnetisation. 
In all stronger fields, up to at least 725 C.G.S. units, the magnitude of the shock- 
effect was so small that its sign even was a doubtful matter. Thus, in cobalt, the 
shock-effect, so long at least as it is an appreciable quantity, gives an increase of 
magnetisation. This consequence of the shock-effect, even in fields much above the 
Villari point, is known from Villari’s experiments, recorded in § 8, to be also 
characteristic of iron. 
The absolute measures, both of the shock-effect of the first pressure and of the 
cyclic effect, possessed maxima in tlie neighbourhood of the Wendepunkt. Relative, 
however, to the magnetisation existing prior to pressure, both these effects increased 
continually in importance as the strength of the field was reduced within the actual 
limits of the experiments. 
The total effect of the first application of pressure had a critical field of about 
160 C.G.S. units; which, it will be noticed, is decidedly higher than the critical field 
for the cyclic effect. As stated in § 11, in discussing Professor Ewing’s experiments, 
the existence of a higher critical field for the first application of stress than for the 
cyclic application, is equally characteristic of iron. 
As it is important to know how far, if at all, the nature and magnitude of the 
cyclic effect of pressure depend on the circumstances attending the process of 
magnetisation, the rod was sometimes subjected to pressure before and during 
its introduction into the magnetising spiral. It was found that there was no 
material alteration in the cyclic effect. The mode of variation of the effects of the 
first and of the cyclic application of pressure with the strength of the field is 
exhibited in figs. 5-8 (Plate 15). 
Phenomena Observed in the Residual Magnetisation. 
§ 18. Comparing figs. 1 and 9, it will be seen that the residual magnetisation 
approaches “ saturation ” in much lower fields than does the induced. 
The intensity of the residual, as of the induced, magnetisation was found to depend 
on the treatment of the rod during the flow and the break of the current. When 
the rod was free from pressure at the instant the current was broken, the residual 
magnetisation in all fields below 120 or 130 C.G.S. units was increased by the appli- 
