416 



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



this paper speak of a metal A as being positive to B when the thermo- 

 current passes from A to B through the cold junction, or from B to A 

 through the hot. The thermoelectric power of bismuth with respect 

 to lead is thus negative, while that of antimony is positive. 



Previous Researches. 



The results of my own experiments compel me to dissent from some 

 of the conclusions which have been reached by other workers. The 

 discrepancies noticed are, I think, in most cases due to the fact already 

 referred to, that the thermoelectric behaviour of weakly magnetised 

 with respect to strongly magnetised iron and impure nickel may be 

 very different from that of the unmagnetised with respect to the 

 magnetised metal. 



The earliest observations of the effects of magnetisation upon thermo- 

 electric power are those of Professor W. Thomson (Lord Kelvin), 

 who in 1856* announced that magnetisation rendered iron and steel 

 positive to the unmagnetised metals (the thermo- current passing from 

 unmagnetised to magnetised through hot), while in nickel the opposite 

 effect was produced. The latter statement, though it has never, I 

 believe, been disputed, is beyond doubt erroneous. Iron, steel, and 

 nickel, when free from mechanical stress, all become more positive 

 when magnetised. 



The effects of tension and magnetisation upon the thermoelectric 

 quality of iron have been investigated by Ewing,f who found that 

 " the presence of load diminishes the general thermoelectric effect of 

 magnetisation, and finally reverses it when the load is great." 

 Although this happens to be a correct statement of the facts, it is, 

 I venture to think, exceedingly doubtful whether a genuine reversal 

 was actually attained in a field of only 17 units, the strongest applied 

 by Ewing, even with a load on the 'wire of 4000 kilogrammes per 

 sq. cm. From the description of the apparatus (p. 368) it appears 

 that the piece of wire regarded as unmagnetised was continuous with 

 the magnetised portion, the hot junction separating the magnetised 

 from the unmagnetised iron being just outside the magnetising coil. 



Chassagnyl was the first to announce that the increase of thermo- 

 electric power due to magnetisation reaches a maximum in a moderate 

 field and diminishes in stronger fields. In his experiment the electro- 

 motive force was greatest in a field of 55 units, falling to about half its 

 maximum value in a field of 200. 



In a similar experiment by Houllevigue§ a maximum was indicated 

 at H = 42, and at H = 352 the increase of thermoelectric force due 



* Bakerian Lecture, ' Phil. Trans.,* 1856, p. 722. 



f ' Phil. Trans.,' 1886, p. 361. 



% 5 C. Pv,.,' vol. 116, p. 977, 1893. 



§ 'Journ. de Physique,' vol. 5, p. 53, 1896. 



