Cathode Fall front /lot (\irhon. VJ"1 



coil of thick wire (resistance =1*245 ohms") immersed in an 

 oil-bath of large capacity. This served a> the standard of 

 comparison for the resistance of the filament. To get the 

 resistance when cold, the current from one Leclanche cell was 

 used by momentarily depressing a key. It was found that 

 when working at high temperatures the resistance of the cold 

 electrode steadily increased (from 7*91 to 8*75 ohms during 

 live weeks' continuous work — ten to twelve hours a day). 

 Instead of the actual resistance as measured, I Lave therefore 

 throughout the present paper calculated its ratio (Ja on the 

 curves) to the resistance of the filament measured imme- 

 diately before and after. This has the further convenience 

 of being directly referable to the temperature determinations 

 of Le Chatelier*, obtained by a photometric comparison and 

 generally accepted as the most satisfactory determination of 

 the glowing carbon filaments that has been made f. 



Before starting electrical measurements the discharge-tube 

 was pumped out to 1T ^ of a millimetre, and both filaments 

 cautiously heated up to redness, pumping being continued 

 all the time. The apparatus was then slowly filled with 

 nitrogen (prepared by the action of potassium nitrite on 

 ammonium chloride), and again pumped down to the lowest 

 possible degree accompanied by the heating of the filaments 

 to a still higher temperature. This process was repeated four 

 times. A pocket-spectroscope showed the nitrogen lines only. 

 It was originally intended to use also hydrogen and other 

 gases, but the apparatus cracked at the high temperature 

 reached in the last experiments with nitrogen. 



All workers with carbon at low pressures have had trouble 

 on account of its occlusion of gases. Prof. J. J. Thomson J 

 found that so long as a carbon filament continued to give 

 off gas, it gave a positive leak with almost greater facility 

 than a negative leak. This irregularity was not got rid of 

 until the occluded gas was expelled, after which it gave a 

 negative leak only, even at comparatively low temperatures. 

 By virtue of the high potentials used in my experiments. 

 thi< difficulty did not present itself in the same form, and the 

 following observations may even throw, some light on the 

 manner of those irregularities. 



With a carbon cathode, it was found that an enormous 

 quantity of nitrogen was absorbed during the passage of fhe 



* Journal (le P/n/.sit/>ic, :'.][;<• serie, vol. i p. 203. 



j- Cf. E. Blaker, Phys. Kev. vol. xiii. p. 346 (1901), whose summary 1 

 havM had to make use of, a- the Journal de Physique i- not available in 

 Calcutta. 



: Phil. Mag rol. xlviiip. 547 i 1899). 



