ON THE MAGNETISATION OF COBALT. 
339 
by the lowering of the Villari point by the comparatively large stresses practically 
employed. Professor Thomson also has pointed out that Maxwell’s distribution of 
stress should produce an independent system of strains in the bar, the strain in the 
direction of the lines of force being an extension. Though apparently a very small 
effect this would tend in iron to raise the magnetisation where dejd^ vanishes 
somewhat over the Villari point. 
In nickel we see from the experiments of Sir W. Thomson, Mr. Shelford 
Bidwell, and Professor Ewing, that de/d^ and S3/8e: are both negative in all 
ordinary fields. If the results obtained by Sir W. Thomson in the central portions 
of a nickel bar, in a strong field, be accepted, then there does actually exist a very 
high critical magnetisation at which 83/Se vanishes. There should accordingly be, 
not in weak fields where Professor Ewing looked for it, but in very high fields, 
a critical magnetisation where a nickel bar would cease to contract. Such a 
phenomenon cannot be said to have its existence demonstrated by Mr. Shelford 
Bidwell, but at the same time it is certainly not disproved by his experiments. 
In cobalt, according to Sir W. Thomson, 83/8e is negative in weak fields, and so 
also is dejd^ if we accept the results of Mr. Shelford Bidwell. According to the 
latter observer, however, dejd^ changes sign in fields much higher, it is true, than 
the corresponding fields for iron, but still easily obtainable, Eurther, the cobalt not 
only recovers from the very considerable shortening it has experienced, but lengthens 
considerably as the field is raised. It would thus appear impossible to assign the 
phenomenon in any essential degree to the Maxwell effect. If then Professor 
Thomson’s theoretical conclusions are sound, 83/8e should change sign from negative 
to positive in a cobalt rod for a magnetisation quite within the reach of experiment. 
Experimental Verification of Theory in Cohalt. 
§ 15. The question whether or not a Villari point of the above kind exists in 
cobalt was the primary object of the following experiments, and a decided answer in 
the affirmative was obtained. Under the moderate stress employed, 83/8e was 
negative in weak fields and positive in strong, and an unmistakeable Villari point 
appeared in a field of about 120 C.Gi.S. units. 
This, it will be noticed, is a much lower field than those which Mr. Shelford 
Bidwell obtained for the vanishing point of dejd'^ in either of the specimens he 
employed, and the Maxwell effect would here tend to lower these latter fields. In 
my specimen, however, as the field was raised from 120 to 300 C.G.S. units the 
magnetisation increased by less than 20 per cent. The accordance of theory and 
experiment would thus, in reality, appear to be closer in cobalt than in any recorded 
experiments on iron. 
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