414 



Dr. S. Bidwell. On the Changes of [Apr. 11, 



compression due to magnetisation,* the change of thermoelectric power 

 appears to be proportional to the change of length. Change of thermo- 

 electric power expressed in microvolts is nearly (and perhaps under 

 perfect experimental conditions would be exactly) numerically equal to 

 the " corrected " change of length in ten-millionths multiplied by a 

 factor which is constant for the same specimen in the same physical 

 condition, but differs for different specimens, and for different physical 

 conditions of the same specimen. For my sample of pure iron when 

 in a free state, the factor giving the best agreement is 183 x 10~ 5 ; for 

 a sample of good commercial iron it is 63*6 x 10~ 5 ; and for the pure 

 iron stretched by a load of 1620 kilogrammes per sq. cm. its value is 

 112 x 10 -5 . The curves expressing the relations of thermoelectric 

 power and of change of length to magnetising force are not indeed 

 exactly coincident, as may be seen by reference to figs. 4 and 5 ; but 

 since identical specimens of the metal were not used in the two sets of 

 experiments, while the conditions were necessarily somewhat different, 

 the divergencies cannot but be regarded as very small; sometimes, 

 indeed, they hardly exceed the limits of experimental error. The 

 thermoelectric and the elongation curves for iron appear to be similarly 

 influenced by the physical condition of the metal ; I have shownf that 

 the elongation curve is lower for annealed than for unannealed iron, 

 and that tensile stress also lowers the curve ; the same is the case with 

 the curves of thermoelectric power. The strength of the magnetic 

 field at which, as indicated by the corrected curve, there would be no 

 change of length under tensile stress, appears to be just the same as 

 that at which the thermoelectric force becomes zero ; when this strength 

 is exceeded, retraction occurs instead of elongation, and a simultaneous 

 reversal occurs in the direction of the thermoelectric force. Unlike 

 previous experimenters I find that when the iron is free from tensile 

 stress the direction of the thermoelectric force is never reversed by 

 magnetisation, even in fields up to 1600 C.G.S. units; neither does 

 the curve of change of length when " corrected " for mechanical stress 

 ever cross the horizontal axis. 



Nickel — Partly on account of the smaller magnetic susceptibility of 

 nickel, and partly in consequence of the relatively great changes of 

 length which that metal undergoes when longitudinally magnetised, 

 the correction for mechanical stress is almost negligible, averaging 

 less than 3 per cent, for fields up to 1200. Here, too, the forms of the 

 curves for change of length and change of thermoelectric power in 

 relation to H are strikingly alike (see fig. 7), the correspondence being 

 even closer than in the case of iron. For a specimen of pure nickel 

 the increase of thermoelectric power in microvolts, due to magnetisa- 



* It is a disputed point whether there actually is any such compression. The 

 subject is discussed later. 



f < Roy. Soc. Proc.,' vol. 55 ; p. 228, 1894 ; vol. 47, p. 469, 1890. 



