January 20, 1905.] 



SCIENCE. 



83 



is analogous to the sloping line which in 

 the steam-boiler-engine diagram indicates 

 the absorption of heat by the feed water 

 from the condenser in mixing with the hot 

 water in the boiler. The slope of each line 

 implies that the working agent, electricity 

 in one case and water in the other, takes 

 in the particular quantity of heat repre- 

 sented by the area under the line at a tem- 

 perature below the highest of the cycle, and 

 therefore, does not make the best possible 

 thermodynamic use of the heat supplied 

 and of the range of temperature available. 

 Similarly the inclined line I'l, which indi- 

 cates that heat is absorbed by the electric 

 current in passing through the metal M^, 

 from temperature T' to the lower tempera- 

 ture T, is analogous to the line of the steam 

 cycle which indicates the recovery of heat 

 from the cylinder wall during expansion 

 after cut-off. 



Furthermore, the horizontal lines CT 

 and IC, indicating the absorption or emis- 

 sion of heat by the electric current in pass- 

 ing, withoiit change of temperature, from 

 one metal to the other, are analogous to 

 those horizontal lines of the steam cycle 

 which indicate absorption or emission of 

 heat in the act of evaporation or of con- 

 densation. To this analogy we shall pres- 

 ently return. 



Let us for the moment occupy ourselves 

 with a reexamination of the prevailing 

 opinion as to the relation between the heat 

 absorption or emission at the junction of 

 two metals and the difference of potential, 

 or the electromotive force, at that junction, 

 that is, between the thermal aspect and the 

 electrical aspect of the Peltier effect. We 

 shall find the situation not quite so clear 

 as it is often supposed to be. 



Maxwell* states that the amount of heat 

 taken up or given out by unit quantity of 

 electricity in going from one metal to an- 

 other at any temperature is a measure of 



* ' Electricity and Magnetism,' § 249. 



'the electromotive contact force at the junc- 

 tion ' ; and he says that ' this application 



* * * of the dynamical theory of heat to 

 the determination of a local electromotive 

 force' is due to Sir Wm. Thomson.* He 

 then goes on to declare that — "The elec- 

 tromotive force at the junction of two 

 metals, as determined by this method, does 

 not account for Volta 's electromotive force. 



* * * The latter is in general far greater 

 than that of this article, and is sometimes' 

 of opposite sign," etc. 



But it is a remarkable fact that Thom- 

 son, years after he had pointed out the 

 method which Maxwell approves for de- 

 termining contact electromotive force, came 

 out (in 1862) with a letter giving a 'New 

 Proof of Contact Electricity,' his famous 

 'divided ring' experiment, in which letter 

 he says "For nearly two years I have felt 

 ([uite sure that the proper explanation of 

 voltaic action in the common voltaic ar- 

 rangement is very nearly Volta 's," etc. 



I do not feel called upon to take up the 

 cudgels for Thomson or for Volta. The 

 point of immediate interest is that Thom- 

 son, after proposing the thermodynamic 

 method of determining contact electro- 

 motive force, found it possible to hold a 

 view contradictory to the soundness of this 

 method. This fact may give the rest of us 

 courage to question the finality even of 

 IMaxwell's opinion as to the relation be- 

 tween electromotive force and heat in the 

 Peltier effect. I believe, too, that Poin- 

 care, in article 292 of his ' Thermodynam- 

 ique,' holds that the opinion supported by 

 Maxwell may be wrong. Let us see what 

 we can do with the question thus raised. 



By difference of potential, D^.i, between 

 two points I shall mean the net amount 

 of work which must be done beeaiise of 

 the attractions and repulsions of electric 

 charges (to use the convenient terms of 



* Proc. Roy. Soc. Edin., Dec. 15, 1851; Tra7is. 

 Hoy. 8oc. Edin., 1854. 



