1904.] Thermoelectric Power produced by Magnetisation. 419 



may generally without sensible error be taken as B 2 /87r, instead of 

 (B 2 - H 2 )/8tt or 2ttI 2 + HI.* 



In the year 1895 it was pointed out by More,f who was apparently 

 ignorant of what I had written on the subject, though he was certainly 

 familiar with most of my work, that the change of length attending 

 magnetisation must be due to several causes, among which are the 

 mechanical stresses created in the rod. " The first of these mechanical 

 stresses is the tractive force of the magnet, and is measured by B 2 /87r." 

 This force, he remarks, tends always to contract the rod, and for high 

 intensities becomes one of the most important factors in the observed 

 changes of length. In plotting his curves (in which abscissae are I 

 instead of H) he makes "correction for the contraction due to the 

 B 2 /8xr force. This correction is obtained from the formula 



« 10 r _ 



where M is the modulus of elasticity. The effect of this correction is 

 to make the elongation much greater for a given intensity. The 

 maximum value of the elongation is more than twice as great as 

 the observed maximum, and the greatest intensity employed, 1300 

 C.G.S., produces an elongation and not a contraction as observed." 



The publication of More's paper led to a discussion! in which several 

 well known physicists took part • but the views expressed were by no 

 means concordant,, and I believe that at the present time it is not 

 agreed whether there is in fact any such mechanical stress ; whether, 

 supposing one to exist, it is compressive or tensile, and whether it is 

 "Maxwell's stress" or some other. The compressive stress which I 

 contemplated was, of course, quite unconnected with Maxwell's theory 

 of stress in the electro-magnetic field, and the expression employed for 

 its value was based upon principles which were well known long 

 before the date of Maxwell's work.§ 



Several papers have been more recently published) [ in which the 



* ' Phil. Mag.,' vol. 29, p. 440, 1895. The difference due to the term H.'j8* is, 

 when H = 500, - 05 per cent. ; when H = 900, 0'16 per cent. ; and when 

 H = 1400, 0-35 per cent. 



f ' Phil. Mag.,' vol. 40, p. 345, 1895. 



X ' Nature,' vol. 53, pp. 269, 316, 365, 462, 533. 



§ The magnetic force inside a narrow transverse gap in a longitudinally 

 magnetised bar is P> = H + 47iT. Supposing one portion of the bar to be fixed, the 

 force acting upon the face of the other portion is less than B by 2nT, the part due 

 to the face itself; thus the attractive force per unit area = (B — 2ttT) x I = 

 27rI 2 + HI. The stress between any two portions of a magnetised bar, divided by 

 an imaginary transverse plane, is sustained by the interrnolecular springs, whatever 

 their physical nature may be, to which the elasticity of the metal is due. 



|| Klingenberg, 'Rostock Univ. Thesis,' Berlin, 1897; Brackett, ' Phye. Bev.,' 

 vol. 5, p. 257, 1897 ; Rhoads, loc. cit. 



