3T<» C. Barus — Expression in Thermo-electrics. 



may be brought forward in accounting for the thermal effects 

 of the expansion of the metals themselves. 



Again since in virtue of the character of e the same state of 

 things must supervene at any section between parts of the 

 game wire at different temperatures, a mechanism for the 

 Thomson effect is also apparent and differs from that of the 

 Peltier effect only in degree. 



If the current passing through a thermoelectric junction be 

 spoken of as having a certain sectional density or intensity in 

 each of the wires, it is clear that this quantity need not vary 

 from the first wire to the second. What has varied is the 

 number and the internal or molecular distribution of the com- 

 ponent paths. The postulated expansion is a configu rational 

 change within the fixed bulk of the wires. An example 

 might be given of charges traveling bivalently in one metal 

 and univalently in the other, though I do not wish to limit 

 myself to such a case. 



8. In the further pursuit of the subject it is advisable to 

 proceed from a different point of view, by taking advantage 

 of vant'Hoff's* conception of a solid solution. Seeing more- 

 over that in a large and important class of thermo-couples 

 (iridio-platinum and other platinum alloys, iron and steel, etc.) 

 the metals consist of what may be regarded as two different 

 states of concentration of the same alloy, Helmholtz'sf results 

 for " Concentrationsstrome" are available, particularly in the 

 form given in a remarkable paper by Nernst4 Eliminating 

 the osmotic pressures in the usual way, an equation results in 

 which the electromotive force of the couple is expressed in 

 terms of the concentrations and of the velocities of the ions 

 at the temperatures of the junctions together with those tem- 

 peratures. The problem then consists in finding the condi- 

 tions or approximations under which the equation reduces to 

 Tait's form. I have carried this out obtaining results of a cer- 

 tain degree of interest but not enough so to make it worth 

 while to reproduce the long computation here. 



9. In closing the present paper I had hoped to make a test 

 of equation (3) above, by the aid of data obtained in a direct 

 comparison§ of the platinum-iridioplatinum couple with the 

 air thermometer. Moreover as the corresponding electromo- 

 tive forces of this couple and of platinum-rhodioplatinum, in 



*vant'Hoff: Zeits. Phys. Chem., v, p. 322, 1890. Ideas of this kind seem to 

 have originated with Matthiessen, cf. Rep. Br. Assoc. 1866, p. 15, and Bulletin 

 TJ. S. G. S., No. 14, 1885. 



■j- Helmholtz : Zur Thermodynanrik chem. Vorgange, Verh. d. Berliner Ak. , 

 July, 1882, p. 489. 



JNernst: Zeits. Phys. Chem.. iv, p. 129, 1889. 



§ Barus: Bull. U. S. Geolog. Survey, No. 54, p. 216etseq„ 1889; cf. Phil. 

 Mag., July, 1892, p. 1. 



