502 Changes produced by Magnetisation in Wires. [May 19. 



current seems to "be greater than without it, while in the ten remaining" 

 pairs the retraction is slightly greater without the current than with 

 it. It may, perhaps, be fairly concluded that the current has a real but 

 very small effect in diminishing the retraction. Now I have before 

 remarked that the degree of retraction which nickel undergoes when 

 magnetised is materially affected by comparatively small changes of 

 temperature : the retraction of the same specimen has been found to 

 be greater in a cold room than in a warm one, at least in fields up to 

 400 or 500. Probably this is to be explained by the influence of 

 heat in diminishing the magnetic susceptibility of nickel, the retrac- 

 tions being really the same for the same intensity of magnetisation. 

 Such small effect as appears to be produced by the action of the 

 current may, therefore, be accounted for simply by the rise of tem- 

 perature (2° "6) which it causes. 



Tension has a large effect upon the magnetic retraction of nickel :* 

 it is, therefore, the more remarkable that the action of a current, 

 which operates so markedly upon iron, should in nickel be practically 

 insensible. 



Exp. 8. — The results with no current obtained for a strip of rolled 

 cobalt, the length of which between the clamps was 10 cm., and the 

 cross section 1*82 sq. mm., are given in the first two columns of 

 Table V. 



Table Y. — Cobalt Strip, section 1*82 sq. mm. 





Refraction in ten-millionths of length. 



Magnetic field 









due to coil. 









C.G-.S. units. 



With no current 



"With 2 amperes 



Difference. 





through strip. 



through strip. 





34 



1 



1 







50 



2 



2-5 



-0-5 



84 



4 



5 



-1 



100 



6 



6 







119 



7-5 



8-5 



-1 



153 



11 



11-5 



-0-5 



209 



16 



16-5 



-0-5 



331 



26 



27-5 



-1-5 



Exp. 9. — A current of 2 amperes through the strip caused a heat 

 elongation of about 600 ten-millionths, indicating, if the coefficient 

 of expansion is taken as 0*0000125, a rise of temperature of 4°*8. 

 The retractions observed while this current was passing are set out in 

 the third column of the table. From an inspection of the differences 



* ' Roy. Soc. Proc.,' vol. 47, p. 469. 



