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



The cobalt used was a narrow strip measuring 100 mm. by 2 - 6 mm. 

 by 0*7 mm. ; its cross section being, therefore, 1*82 sq. mm. It was 

 not possible to obtain this metal in the form of a wire ; and for the 

 piece of thin rolled sheet from which the strip above-described was 

 formed, I am indebted to the kindness of Messrs. Henry "Wiggin and 

 Co., of Birmingham, who had it specially prepared for me. The 

 loads employed for the cobalt strip were equivalent to 772, 344, and 

 75 kilos, per sq. cm. The first of these was represented by an 

 actual weight of 31 lbs.,* which was as great as the apparatus seemed 

 capable of bearing without risk of injury. 



In all the experiments the loads were successively applied in 

 decreasing order of magnitude, and before every single observation 

 the wire or strip was demagnetised by reversals, without, of course, 

 being removed from the coil.f 



The results obtained for iron are given in Table I, and also shown 

 in the curves in fig. 2. 



Those for nickel are given in Tables II and III, and in figs. 3 and 

 4. In fig. 3 the curve for 700 kilos, is represented by a dotted line 

 for the sake of distinctness. Table III and fig. 4 are constructed 

 from data obtained from a complete set of curves, like those in fig. 3 ; 

 they show the magnetic contractions that would occur under increas- 

 ing loads in constant magnetic fields of 125, 185, and 360 C.G.S. 

 units respectively. These magnetic contractions would, of course, be 

 superposed upon elongations of a purely mechanical nature, due to 

 the tensional stress. 



The results for cobalt are contained in Table IV and fig. 5. In 

 the figure the contractions corresponding to the various loads are indi- 

 cated by different kinds of marks, and a single curve has been drawn 

 as smoothly as possible through the whole of them. 



In all the tables and figures magnetic fields (which are those due 

 to the coil alone) are given in C.G.S. units, and increments and 

 decrements of length are expressed in ten-millionths of the length 

 (10 cm., or about 4 inches) of the experimental wire or strip. In 

 figs. 2 and 5, therefore, the height of each little square corresponds 

 to 1/10,000 mm., or 1/250,000 inch, and in figs. 3 and 4 to 

 1/2,000 mm. or 1/50,000 inch. 



* 14 kilos. 



+ The apparatus used for this purpose is described in ' Phil. Trans.,' vol. 179 

 (1888), A, p. 206. 



