( iT4 ) 



modified by an electromotive contact force). After tiie condition 

 lias become stationary, there will still exist a transverse current of 

 ions, and so on one side free ions must be captivated, a process 

 by which heat is generated, and on the other side new ions must 

 be freed, which is accompanied by loss of heat; in this way a transverse 

 dift'erence of temperature ((/alva no-magnetic difference of temperature) 

 arises ; together with this a displacement of unelectrified particles 

 must be brought about. 



So far as the free ions are concerned, Ave must imagine the cur- 

 rent of heat (now that no displacement of substance may be assumed) 

 to be such, that although across a surface perpendicular to the direc- 

 tion of the current of heat an equal number of ions goes in both 

 directions per unit of time, yet the ions have on an average a 

 greater velocity during a displacement in the direction of the current 

 of heat than they have during a displacement in the reverse direction. 

 The electromagnetic force may then cause transveise displacements, 

 which have opposite directions for positive and for negative ions. In 

 the same way as with the electric current there will arise in general 

 a difference of potential (transverse ihermo-maijnctic effect) and a 

 difference of temperature (rotation of isotJiermals). The answer to 

 the question in how far these suppositions might lead to a quanti- 

 tative explanation must be left to further investigations. 



7. It is important to inquire whether molecular theories of 

 thermo-electricity enable us to determine the sign of the variation in 

 the magnetic field of the thermo-electric difference, for instance between 

 bismuth and cop[ier, only by means of the hypothesis mentioned in 

 the previous communication ^) : viz. that the longitudinal phenomena 

 would be explained by a variation of the number of free ious, due 

 to the magnetic field. This however appears not to be the case, as 

 new suppositions must still be introduced. 



As an instance 1 choose the formula for the electromotive contact 

 force, lately published by RiECKK -), which can be put down in 

 the form : 



Vb-Va = c(log ^- log-^^ 

 where the quantities with A relate say to copper, and those with 



1) Proceediugs Iloyal Acad. Amsterdam. March 25tb. 1899, p. 116 Comiu. Phys. 

 Lab. Leiden No. 48 p. 20. 



-) Wied. Aun. 66, p. 534, 189S. . . 



