be d48e dl 
L. Note on a slight Error in the customary Specification of 
Thermo-electric Current-direction, and a Query with regard to 
a point in Thermodynamics. By Outver J. Lopes, D.Se., 
Professor of Physics in University College, Liverpool™. 
[Plate III.] 
1 lea a paper written for the British Association at Montreal, 
“On the Seat of the Electromotive Forces in the Voltaic 
Cell,” printed in the Philosophical Magazine for March, April, 
and May this year, there occurs the following footnote to 
section 23 (Phil. Mag. May 1885, vol. xix. p. 355):— 
“Tt is always easy to tell from thermoelectric data which 
way the force acts at a junction ; but it is not always the 
same way as the current flows, by any means. A current 
often flows against the H.M.F’. even at a hot junction, and it 
may flow against the force at both junctions. This is the 
case, for instance, in a copper-iron circuit with one junction 
above 275°, and the other below that temperature by a greater 
amount. It is customary to say that the current flows across 
a hot. junction from the metal of higher to the metal of lower 
thermoelectric value: this is not necessarily true. The safe 
statement is to say that the H.M.F. acts from high to low 
thermoelectric value at either junction.” 
A short statement like this is apt to be buried in a long 
paper on other matters, and though not of any high import- 
ance, it is always worth while to keep as clear as possible on 
all elementary matters, if only for the sake of one’s students. 
The point is perhaps most easily illustrated by considering 
special cases; but I may first briefly explain my general 
point of view in case there may be some objection to it. 
It is well known that the whole H.M.F. of a thermoelectric 
circuit is obtainable by integrating, all round, a certain func- 
tion of the temperature and of the metals in circuit, which may 
be called the Peltier function, I, 
a 
where the @ stands for absolute temperature. On Tait’s 
assumption that what Thomson called ‘the specific heat of 
electricity in a metal” is proportional to @—equal, say, to k,0 
for metal A, and to 4,6 for metal B—the value of II is 
T= (ka—hy)6(8)—9), 
where @ is a constant to be determined by experiment, just 
as k, and k, are to be determined. 
* Communicated by the Author. 
