of Electricity through Rarefied Air. 21 



two cases must therefore depend on the modification of the 

 electromotive force e. The experiments made show that this 

 force is dependent on the chemical nature of the poles, and 

 also that in this respect the two poles play essentially different 

 parts. These facts are in accordance with the observations of 

 Hittorf, according to which an induced current traversing a 

 space of rarefied air meets with a notable resistance at the 

 negative pole, but less if that pole consists of aluminium than 

 if it is of platinum*. Now, as we have seen from what pre- 

 cedes, the obstacle encountered by the current at the negative 

 pole does not consist of an electric resistance in the usual 

 acceptation of the term, but of an electromotive force tending 

 to send a current contrary in direction to the principal 

 current. 



It has been observed that the negative pole is strongly 

 heated in a space of rarefied air, and that, cceteris paribus, this 

 heating is augmented with the rarefaction of the air. I have, 

 in a previous memoir, proved that, when a current traverses 

 an electromotor in an opposite direction to the current which 

 the electromotor tends to produce, there results from it an 

 evolution of heat proportional to the electromotive force mul- 

 tiplied by the intensity of the currentf. Now, since (accord- 

 ing to what precedes) the electromotive force e in the spark is 

 notably augmented with the rarefaction, it follows that the 

 heat evolved at the negative pole must be increased when the 

 rarefaction is increased, which accords with the results of ex- 

 periment. Here it should be remarked that in the voltaic arc 

 it is the positive pole that is most strongly heated, from which 

 we may be authorized to conclude that the electromotive force 

 shown by irrefutable proofs to exist in the voltaic arc has its 

 seat at the positive pole. This force is not thermoelectric, 

 and it is not produced, as M. Le RouxJ assumes, by one 

 of the poles being more highly heated than the other ; on the 

 contrary, it is the electromotive force that is the cause of the 

 heating. It has been proved that the force continues to exist, 

 even if the negative pole of the voltaic arc be heated, by a 

 Bunsen burner or in any other way, so as to become hotter 

 than the positive pole§. 



The researches of which I have just given a brief summary 

 confirm therefore, in all respects, the results to which I had 



* Pogg. Ann. cxxxvi. p. 25 (1869). 



t Kongl. Svenska Vetenskaps-Akademiens Handlingar, xiv. (1876); 

 Pogg. Ann. clix. ; Phil. Mag. [5] iii. 



% Comptes Rendus, xcii. p. 710 (1881). 



§ Ofversigt af Kongl. Vetvemk.-Akad. Forhandl. 1868, p. 12 ; Pogg. 

 Ann. cxxxiv. ; Phil. Mag. [4] xxxvi. 



