478 



Mr. S. Bidwell. On the Effect of Tension [Apr. 24, 



diagram, but also reaching its turning point sooner, and therefore 

 intersecting at least three or four of the others. Such an assump- 

 tion is confirmed by some results obtained with a nickel rod 0*3 cm. 

 in diameter, which have been given in a former communication,* and 

 are reproduced in Table V. 



Table Y.—Unstr etched Nickel Rod. 



Contractions in 

 Magnetic field ten-millionths 

 in C.G-.S. units. of length. 



65 104 



125 167 



181 199 



237 218 



293 233 



343 240 



393 242 



A curve plotted from this table will be found to begin its descent 

 between the curve for 420 kilos, and the vertical axis, and after 

 crossing the others to intersect that for 1400 kilos, at H = 355. But 

 of course experiments made with two different specimens of nickel 

 are not strictly comparable. 



In fig. 4 the results for nickel are presented in a somewhat 

 different form, the curves showing the magnetic contractions of the 

 wire under various loads in certain constant fields. The values for 

 no load are taken from Table V, and the portions of the curves which 

 depend upon the accuracy of these values are distinguished by dotted 

 lines. It will be seen, however, that these portions are not of much 

 importance. 



From these curves we at once see that in a field of 125 units, 

 increase of load always causes decrease of magnetic contraction. In 

 a field of 185, magnetic contraction increases as the load is raised 

 from nothing up to about 700 kilos, per square em., 'again decreasing 

 with greater loads. And in a field of 360 units this singular reversal 

 is exhibited in a still more marked degree, the maximum magnetic 

 contraction occurring with a load of about 950 kilos, per square cm. 



The reversal phenomena observed in connexion with the magnetic 

 contraction of nickel are strikingly analogous to those which occur 

 in the magnetisation ■ of a stretched iron wire, and which are' com- 

 monly associated with the name of Villari.f 



* 'Phil. Trans.,' A, 1888, p. 228. 



f Poggendorff's ' Annalen,' 1868. See also ' Encycl. Britann.,' 9th edit., vol. 15, p. 

 269. When an iron wire subject to a magnetising force in the direction of its length is 

 stretched by aeertai i force, the magnetisation of the wire is increased or diminished 

 according as the magnetising force is less or greater than a certain critical value 



