418 Ultra- Violet Ligld emitted by Electrical Discharge. 



the lowest pressure investigated '29 mm. of mercury, fig. G, 

 the left-hand maximum has almost disappeared, the centre 



Fij 



;eo 



.'30 







p 



V-. 



- -29 mm> 



- 820 



























■"+1 



light having become equally important, and the highest 

 maximum is then on the right. In these last cases the dis- 

 charge lit up the whole tube and discharges occurred round 

 the back of the electrodes. 



The effects were all due to UV light, no trace of Rontgen 

 rays was found nor of Entladungstrahlen, for no deflexion 

 could ever be obtained if the zinc plate was charged positively, 

 or if when negatively charged a very thin sheet of aluminium 

 was interposed to cut off the light. 



The tube was unfortunately too short to show much positive 

 column, and I doubt if more than 2 or 3 cm. of column were 

 present in any experiment. It would have been interesting 

 to have continued the measurements to still lower pressures, 

 but the potential difference required to drive the discharge 

 soon gets so enormous that there were not sufficient cells 

 available to proceed further. 



The results, however, without assuming any theory as to 

 the actual mechanism by which the short waves of light are 

 emitted, are certainly sufficient to show that, after the pressure 

 corresponding to the minimum sparking potential for two 

 fixed electrodes has been passed, the character of the dis- 

 charge is radically changed. 



This research was suggested by and conducted in the 

 laboratory of Professor Townsend, to whom my thanks are 

 due for very much encouragement and assistance. 



