The Discharge of Electricity through Gases, 531 



Behaviour of Platinum Electrodes. — We owe to Messrs. Elster and 

 Geitel* a series of most instructive experiments, in which they have 

 discussed the unilateral conductivity of gases in several cases. Their 

 research was chiefly directed to an investigation of some observed 

 electromotive forces between glowing bodies and the surrounding gas. 

 These electromotive forces, as well as those discovered by Righi, 

 which appear under the action of ultra-violet light, will, I believe, 

 prove to be of the greatest importance in clearing up many electrical 

 questions, for they seem to me to belong to a class of phenomena 

 which can be brought under the domain of the second law of thermo- 

 dynamics. I am, however, for several reasons, anxious to avoid, for 

 the present, a discussion regarding them, except in so far as they 

 seem to have a bearing on the question of a possible chemical action 

 at the electrodes. The facts discovered by Elster and Geitel are 

 briefly as follows : — 



If a white-hot platinum wire and a cold metal are brought near 

 each other in an enclosure filled with gas at various pressures, a 

 difference of potential is observed between the two metals ; when the 

 gas is air or carbonic acid, the hot platinum is electro-negative to the 

 cold wire, while in hydrogen or hydrocarbons 'the opposite is the case- 

 It is also found that platinum in air discharges positive electricity 

 more readily than negative, while again the opposite is the case in 

 hydrogen. 



Messrs. Elster and Geitel bring these facts into connexion with 

 each other. The curious relationship between unilateral conductivity 

 and electromotive force, which they have traced, is an important one,, 

 but may only be one of those reciprocal relations (Peltier effect and 

 thermo-electric force) so often met with in all parts of physics. 

 As regards the explanation of these phenomena, several considerations 

 occur to me which should make us careful not to pronounce a definite 

 opinion at present. The authors take it for granted that the electro- 

 motive force has its seat at the hot junction of metal and air. That is 

 probable, but it will be safer for the present not to commit ourselves 

 to more than is actually observed, namely, that if a cold platinum 

 wire in air is placed near a hot platinum wire, the hot wire will be 

 electro-negative towards the cold metal, while in hydrogen it will be 

 electro-positive. At first sight the behaviour of platinum wire seems* 

 exactly the reverse to that of copper ; but platinum shows some 

 curious effects on long continued heating, which lead me to the 

 opinion that complicated effects here determine the nature of the 

 phenomenon. Elster and Geitel have examined the changes in the 

 behaviour of a platinum wire kept glowing in hydrogen. Two 

 platinum wires were kept in the same enclosure ; one of them was 



* ' Wiedemann, Annalen,' vol. 19, p. 588 (1883) ; vol. 31, p. 109 (1887) ; Vienna 

 Academy ' Sitzungsberichte,' vol. 97 (Abth. 2. a.), p. 1175 (1888). 



