Excitation of soft characteristic X-rays. 1017 



molecules appear at lower voltages than those from the 

 target and, if any considerable amount of gas is present, 

 completely swamp the effect sought. The device was 

 therefore exhausted with the most extreme precautions and, 

 protected by two liquid-air traps, was held constantly on a 

 very fast diffusion pump throughout the measurements. 

 The electrodes in the discharge chamber were so related 

 that they could be used as an ionization manometer, and the 

 pressure fluctuations in the immediate neighbourhood of the 

 target followed during the observations. The pressure 

 during the runs recorded below was of the order 10 " 8 to 

 10~ 9 mm. 



*> 20 



260 230 300 



£j<-c/t/'/?y Vo/ts<?e. 



320 



360 



The emission of ra liation has been detected with carbon, 

 molybdenum, copper, and tungsten targets. In the case of 

 carbon and molybdenum, the critical potential at which the 

 radiution sets in has been determined with considerable 

 exactness, as will be seen from the sample data plotted in 

 the accompanying diagram. The details of the experiments 

 with these two elements are as follows : — 



Carbon. — The target was prepared by precipitating carbon 

 on a copper disk from a solution of india-ink. A smooth 

 coherent deposit was obtained. In making the observations, 

 the exciting thermoionic current was kept constant by 

 reducing the temperature of the filament as the operating- 

 voltage was increased. Although the pressure was o( the 

 order 10" 8 to 10" 9 mm.,, a radiation effect was present at 

 Phil. Mag. S. 6. Vol. 42. No. 252. Dec. 1921. [) X 



