Mr. J. Parker on Thermoelectric Phenomena. 353 



the conditions are the same for all, viz. breadth of slit, length 

 of exposure, and source of light. 



3. Hypotheses in regard to coincidences of gaseous and 

 metallic spectra cannot be safely based upon existing measure 

 ments of spectra in the ultra-violet. 



4. The limit of the copper lines is extended by our investi- 

 gation. 



Jefferson Physical Laboratory, 

 Cambridge, U.S. 



XXXYIII. On Thermoelectric Phenomena. By J. Parker, 

 B.A., late Scholar of St. Johns College, Cambridge*. 



WHEN two different metals at the same temperature are 

 put in contact with one another, they assume a dif- 

 ference of potential, and if a current be made to cross the 

 junction, a cooling (or heating) effect is observed. These two 

 facts are evidently dependent on one another, and a relation 

 must exist between them ; but though the subject of Thermo- 

 electricity has been frequently considered, a satisfactory ex- 

 planation appears yet to be found. 



Duhem, writing in 1886, says : — " L'analogie des lois qui 

 regissent le phenomene de Peltier avec les lois des differences 

 de niveau potentiel au contact de deux substances differentes 

 a porte certains physiciens a chercher une relation entre ces 

 deux phenomenes, et a regarder le degagement de chaleur 

 qui correspond au phenomene de Peltier comme proportionnel 

 a la chute que subit la valeur de la fonction potentielle. 

 I/experience a depuis longtemps demontre que cette propor- 

 tionnalite n'existait pas." 



In this paper a very simple relation is obtained between the 

 Peltier effect and the difference of potential. I have also 

 found the difference of potential and the thermal effect at the 

 junction of two portions of the same metal at different tem- 

 peratures. Combining these results, we easily find an expression 

 for the electromotive force of a thermo-electric couple, and 

 obtain an analytical proof of the fact, observed by Magnus, 

 that no current is produced by unequal heating in a homoge- 

 neous circuit. 



It is well known that when a current crosses a metallic 

 junction kept at a given constant temperature, the heat 

 absorbed (or given out) at the junction depends only on the 

 quantity of electricity that passes, and is independent of the 

 strength of the current. 



* Communicated by the Author. 

 Phil. Mag. S. 5. Vol. 26. No. 161. Oct. 1888. 2 A 



