ON THE MAGNETISATION OP COBALT. 
333 
the accuracy of Mr. Shelford Bidwell’s conclusions, at least for the actual cobalt 
rods of his experiments. 
Effects of Longitudinal Stress on the Magnetisation of Rods and Wires. 
§ 7. A preliminary general idea of these effects may be obtained from a consideration 
of the phenomena presented by a soft iron wire stretched beyond its original limit of 
perfect elasticity, and then subjected, in a weak magnetic field, to a stress cycle 
consisting of the application and removal of a definite tension for which it is perfectly 
elastic. 
The first application of the tension causes a large increase in the magnetisation of 
the wire, and on the removal of the tension this increase nearly all remains, or may 
even be added to. The next one or two tension cycles cause a distinct progressive 
increase in the magnetisation. Presently, answering to the tension cycles, there 
appears a nearly cyclic change in the magnetisation, in which the maximum magneti¬ 
sation appears when the tension is ‘‘on.” Also the magnitude of the cyclic effect 
remains, at least approximately, constant for, at all events, a large number of tension 
cycles. 
Subtracting the effect of the final cyclic change of magnetisation from the change 
accompanying the first application of tension, we get what may be fairly regarded as 
that portion of the effect of the first tension which is of a permanent and non-cyclic 
character. This quantity I have here termed the “ shock-eftect.” If the magnetising 
current be broken, the first application of tension or of pressure has a large shock- 
effect, which invariably consists in a diminution of the residual magnetisation. The 
effects of beating,"^ or other mechanical agitation of a rod, are in many respects very 
similar. 
The experiments on the effects of stress on magnetism which are of most importance 
for our present purpose are those of ViLLARi,t Sir W. Thomson, I Mr. Shida,§ and 
Professor Ewing. |j All these observers have, at least in practice, recognised the dis¬ 
tinction between the effect on a bar’s magnetisation of the first few applications of a 
given longitudinal stress and its subsequent applications. 
§ 8. ViLLARi unfortunately gives no clue to the strength of his magnetising fields, 
save the number and relative size of the Daniell or Bunsen elements he employed. 
He discovered that the non-cyclic effects of the first tension and the cyclic effects of 
the later tension cycles in bars—or rather stout wires—-of iron in weak magnetic 
fields are of the character indicated in the last paragraph. In stronger fields he found 
* See Wiedemann’s ‘ Elektricitat,’ vol. 3, p. 666, &c. 
t Poggendoeff’s ‘ Annalen,’ vol. 126, 1865, pp. 87-122. 
t ‘ Phil. Trans.,’ 1879, pp. 55-85; or ‘Mathematical and Physical Papers,’ vol. 2. 
§ ‘ Roy. Soc. Proc.,’ vol. 35, 1883, pp. 404-454. 
II ‘Phil. Trans.,’ 1885, pp. 523-640; 1888, A., pp. .325-337. 
