Design of Soft Thermionic Valves. 481 



for soft valves because the pressure did not remain constant 

 under working conditions. Special experiments were made 

 to investigate this effect. Life tests carried out at Ports- 

 mouth indicated that a fairly rapid absorption of nitrogen 

 occurred when the filament was running and a voltage of 

 the order of 20, or more, was applied to the anode. Very- 

 little nitrogen was left after 50 hours of such treatment. The 

 remaining gas began to ionize at a potential of about 

 13 volts, and its spectrum showed lines of ar^on and the 

 usual carbon monoxide bands due to electrode gas. Atmo- 

 spheric nitrogen had been employed and this had apparently 

 been absorbed, leaving a residue of argon, which would 

 ionize at about 12*5 volts, and at the same time the elec- 

 trodes had given out carbon compounds. 



In order to investigate this effect more fully special 

 experiments were carried out in the Cavendish Laboratory. 

 A valve was attached to a " dark-space tube" (see para- 

 oraph VI. above") and was exhausted and filled with nitiogen 

 until the dark space was 9 mm. in length. No change of 

 pressure occurred as the result of sealing this apparatus off 

 from the pump, nor was there any change alter the valve 

 had been allowed to stand idle for a week. The filament 

 current was then turned on, and it was found that after 50 

 hours running the length of the cathode dark spare was 

 reduced to 8'5 mm. At the same time the discliar, e had 

 become somewhat less pink in colour. Thus a small quantity 

 of 2 - as must have been liberated from the electrodes, or from 

 the walls of the tube. A potential of 20 volts was then 

 applied to the anode, and after 7 hours the length of the 

 dark space was found to have increased to 11 mm. At the 

 end of 30 hours' continuous running under these conditions 

 the dark space was 14*5 mm. in length. At the same time 

 the colour of the discharge had altered from pink to blue. 

 It appeared, therefore, that nitrogen was being absorbed and 

 that electrode gas was being given out. 



To test this point further a small spectrum tube, provided 

 with external tin-foil electrodes, was attached to another 

 valve, and the arrangement was then exhausted and tilled 

 with nitrogen at a pressure of about 0'5 mm. Initially the 

 spectrum consisted chiefly of nitrogen bands, but a few faint 

 argon lines and carbon monoxide bands were also visible. 

 The general colour of the discharge was pink. The 

 apparatus was sealed off from the pump and allowed 

 to stand idle for 10 days. This produced no change in 

 the spectrum, and there was no great alteration after the 

 filament had been run for 33 hours. As soon as a potential 



