THEORY OF ELECTROMAGNETISM. 
763 
(1.) If P be assumed to be dependent upon F, the theory of r ever sib ility suffices to 
explain all the known experimental facts of thermoelectricity. 
(2.) If as there is some reason to believe , P be independent of F, the main thermo¬ 
electric effects must be explained on the theory of reversibility , but the Thomson effect 
and the thermoelectric effects observed in a circuit of a single metal differently stressed, 
in different parts must be explained on the theory of irreversibility. In this case 
there is no such connection between the Thomson effect and other thermoelectric effects 
as is usually supposed. 
(3.) On the present theory it is impossible to expjlain thermomagnetic phenomena, 
by the theory of reversibility. 
94. There is little to be said with regard to the thermomagnetic phenomena them¬ 
selves, as our knowledge of them is almost confined to what is expressed by equations 
(2) and (9a). It is necessary to remark, however, that the C of equation (2) may be— 
probably is—not the main cause of what has been described in §79 above as the 
longitudinal effect, since an apparent longitudinal effect would be caused (§88 above) 
by any interference with the lines of flow of electricity, and by the variation in the 
resistance due to any cause. More than one effect of these I 
In eh 
v. i 
notice 
bel 
eiow. 
But, of course, the existence of B and C may involve results of a kind other than 
thermomagnetic, which are practically measurable. Besides equations (2) and (9a), 
B and C occur in equations (13a) to (18a). Equations (13a) (14a) do not require 
notice, since in the present state of accuracy of experimental knowledge of thermo¬ 
electric quantities the influence of B and C in these equations is negligible. With 
regard to equations (16a) to (18a), we can trace approximately the effects of B and C 
in one important class of cases. 
95. Suppose we have a plate of uniform thickness (small) in which a current is 
flowing placed in a strong uniform magnetic field. On account of the current, of 
course, the uniformity will be disturbed, but only slightly if the strength is great 
enough. Outside the plate (except in certain conducting wires) we assume that there 
is no current. Hence by equation (18a) we see that at every point of the boundary 
of the plate 
SU*> (VBK© - 2CHSK0) = 0.(26). 
Equation (17a) gives by equation (4) § 5 for any portion of the plate 
(VBK0 — 2CHSK0) = 0 ....... (27). 
This is the form in which can be most easily discussed the effect of equation (17a). 
Let the region to-which equation (27) refers be taken as a cylinder, one face of the 
cylinder being in .one face of the plate, and the parallel face somewhere inside the 
5 E 2 
