(369i1>) 
the second meta!. The law of the thermo-electric series may imme- 
diately be inferred from this formula. However, in order to obtain 
this result, it has been necessary to adopt the hypothesis expressed 
by (58). 
I shall terminate this discussion by indicating the way in which 
our formulae have to be modified, if, in the direction of the circuit, 
the electrons are acted on not only by the electric force caused by 
the differences of potential, but also by some other force proportio- 
nal to their charge and whose line-integral along the circuit is not 0. 
Let us denote this force, per unit charge, by /, and let us write for 
its line-integral 
| Eede = F,. 
This latter quantity might be called the “external electromotive 
force” acting on the cireuit. Now, in the formulae (54), we must 
replace EL by E+ L.. Consequently, (55) becomes 
ND SJN D= IDS 
ij | 1 1 af 
2 
and treating this equation in the same way as we have done (55), 
we find instead of (61) 
ide elle 
eens Ee 
$ 22. I shall not enter on a discussion of the conduction of heat, 
the Perrier-effect and the THomson-effect. 
In the theory which admits two kinds of free electrons, all ques- 
tions relating to these phenomena become so complicated that I 
believe we had better in the first place examine more closely the 
Harr-effect and allied phenomena. Perhaps it will be found advisable, 
after all, to confine ourselves to one kind of free electrons, a course 
in favour of which we may also adduce the results that have been 
found concerning the masses of the electrons. These tend to show 
that the positive charges are always fixed to the ponderable atoms, 
the negative ones only being free in the spaces between the molecules. 
If however a study of the Harr-effect should prove the necessity 
of operating with both positive and negative free electrons, we shall 
be obliged to face all the difficulties attending this assumption. 
