of Electricity through Rarefied Air. 5 



§2. 



The apparatus employed in the experiments were: — a Holtz 

 electrophorus with a double rotatory disk, the crank of which 

 was turned, with a constant velocity, as a rule once in two 

 seconds; a reflecting galvanometer of a construction previously 

 described*, specially applicable to electric discharges, the coil 

 of which consisted of forty turns of copper wire 1'3 millim. in 

 thickness, surrounded with gutta percha; a Topler's mercury 

 air-pump of improved construction by Bessel-Hagenf, as well 

 as a RuhmkorfF induction-apparatus, which, however, could 

 only give very short sparks in a space filled with air. The 

 intensity of the induced current was measured by a magneto- 

 meter, the deflections of which were read with the aid of a 

 telescope and scale, 



I proved, several years since, by experiment, that there 

 exists in the voltaic arc, as well as in the electric spark, an 

 electromotive force sending a current in the opposite direction 

 to that which calls forth the electromotive force J. In order 

 to distinguish it from other currents, it received the name of 

 disjunction-current. The above-mentioned result has been 

 verified by Sundell, in regard to the electric spark, by means 

 of a series of detailed experiments §. In those experiments 

 the electric spark was formed in a space filled with air. An 

 investigation which I subsequently made showed that the force 

 discovered by me in a space filled with air existed also when 

 the air was rarefied; but the resources at my disposal on that 

 occasion did not permit me to carry the rarefaction far enough; 

 moreover it was not in the programme of the investigation to 

 devote at that time more attention to this matter || . I will 

 therefore now show, pursuing the same method as that which 

 I employed in 1868, the manner in which the electromotive 

 force is modified with the density of the air. Of the experi- 



* Ofversigt af Kongl. Vetenskaps-Akademiens Forhandlingar, 1868? 

 Pogg. Ann. cxxxvi, (1862) j Ann. de Chim. et de Phys. [4] xvii. ; Phil. 

 Mag. [4] xxxviii. p. 169. 



t Wied. Ann. xii. p. 425 (1881). 



% Of vers, af Kongl. Vet.-Akad. Forhandl. 1867 & 1868; Pogg. Ann. 

 cxxxi., cxxxiv. pp. 250, 337 ; Phil. Mag. [4] xxxvi. & xxxvii. 



§ Pogg. Ann. cxlv. 



|| In a communication to the Paris Academy of Sciences (Comptes 

 Rendus for 1881, t. xcii. p. 710), M. Le Roux states that my investigation 

 on the voltaic arc has merely led to this result — that the arc does not 

 behave exclusively like a simple resistance ; and he then gives a proof that 

 there really exists an electromotive force. Now this proof is identical 

 with that which I employed in 1867 to demonstrate the existence of the 

 same force. 



