1897 - 98 .] Thermodynamics of Volta-contact Electricity. 119 
at one temperature by giving heat to it, or taking heat from it, 
where required for the fulfilment of this condition. 
§ 3. Suppose the relative thermo-electric quality of the two 
metals is such that, when a complete metallic circuit is made of 
them, and the two junctions are kept at different temperatures, the 
thermo-electric current is (according to the old French rule for 
bismuth and antimony) “ against the alphabet through hot ” — that 
is, from Y to X through the hot junction and from X to Y through 
the cold junction. It is in this direction for some pairs X and Y 
(as, for example, X zinc, Y gold), of which X is Yolta-positive to 
Y, and is in the opposite direction for others ; but, by allowing 
negative values to some of the quantities, this latter case is in- 
cluded in our supposition which we make as a preliminary to 
avoid circumlocutions. 
§ 4. Consider now the Peltier thermal effect of the current pro- 
duced as in § 2 by drawing the movable slab of Y-metal outwards. 
The current crosses the junction J in the same direction as the 
natural thermo-electric current in a closed circuit with J the cold 
junction : and the thermal effect is therefore production of heat at 
J ; according to a thermodynamic hypothesis which I adopted as 
fulfilled so far as the sign of the Peltier effect was concerned, in 
Peltier’s splendid original discovery for bismuth and antimony, 
and verified * by myself experimentally for copper and iron below 
* Verified by Le Roux and Jahn for several other pairs of metals for which 
they also measured the value of the Peltier effect. Their results verify the 
thermodynamic hypothesis absolutely in respect to the sign, and tend to con- 
