438 Prof. Richardson and Dr. Bazzoni : Experiments 



are not due to the presence of traces of contaminants such as 

 hydrogen, to which the spectroscopic test is not very sensitive 

 at these low pressures. No hydrogen lines could be detected in 

 the mercury spectrum under the conditions of experiment (b). 



At pressures greater than about 0*01 mm. the jump to the 

 higher range was always accompanied by the establishment 

 of a visible glow discharge. The spectrum of the glow is 

 that of the mercury arc. The behaviour of the luminosity 

 when the potential across the gap is changed presents very 

 interesting features. We have not been able to observe any 

 luminosity except that due to the light coming from the hot 

 filament when the discharge is on the low-current range. 

 On the high-current range at the lowest potential which 

 would maintain the discharge a faint glow appeared close to 

 the surface of the anode. On raising the potential slightly 

 the glow grew out from the anode into the surrounding space. 

 A further slight increase of the potential difference, in some 

 of the experiments at least, caused the glow to spring away 

 from the auode. In this condition there was a bright patch 

 in the space between the electrodes with a dark space round 

 both the anode and the cathode. A further small increase of 

 potential difference made the glow settle round the cathode. 

 After this stage was reached the luminosity was greatest 

 near the cathode. Further increase of potential difference 

 caused the luminosity to spread out further from the cathode 

 and to increase in intensity. In this condition there was a 

 gradual falling off in intensity towards the anode, and the 

 luminosity did not appear to have a sharp boundary. 



At high pressures the changes in the potential difference 

 which are necessary to cause the glow to move over from the 

 anode to the cathode are quite small, being comparable with 

 1 volt. Observations similar to the following were frequently 

 made with pressures of mercury vapour of the order of 1 cm. 

 With - 98 amp. heating current and the quartz tube at 

 225° C. the following observations of the luminosity and 

 discharge current at different potentials were taken : — 



Volts 11 12 14 25 50 



Luminosity None. On anode. Crossed Filled whole Filled 



to cathode. space, whole space. 



Current (microamps.) 16 84 184 315 



With a larger heating current (1*05 amps.), but under 

 conditions otherwise similar to the foregoing, the following 

 observations were taken : — 



Volts 11 



Luminosity None. 



Current (microamps.) 20 



12 



13-2 



25 50 



On anode. 



Crossed 



Filled whole Filled 





to cathode. 



space, whole space. 



124 



544 



884 



