from Long Vacuum Tubes. 739 



dark-space must be reduced to 5 mm. again and the tube 

 operated tor another hour or two. Blue-white strata in 

 rapid motion (with a vacuum of 1 mm. dark-space) are 

 a sure sign that we are very near the white stage. A 

 strong vermilion colour shows that the tube requires further 

 treatment. It may take several days to bring the tube into 

 proper condition, especially if it is a new one. 



After the tube l.as reached the white stage, with H^ 

 practically invisible, it may be exhausted to a 5 mm. dark- 

 space and operated at this pressure with a moderate current, 

 say 5 amperes, in the transformer. After running for some 

 time, H^ will appear again and gradually increase in 

 intensity. It can be banished again by introducing fresh 

 hydrogen at I mm. dark-space and running the tube for 

 half a minute or so as before. 



A brief summary of the treatment for brinoino- the white 

 stage follows : — 



(a) Operate tube with heavy current and Pd hydrogen 

 for six hours, exhausting and refilling to 5 mm. 

 dark-space every twenty minutes. Current stopped 

 during exhaustion. A ten-second application of the 

 hydrogen flame (Kipp generator and fine-pointed 

 capillary burner) to the Fd tube is usually sufficient 

 to introduce sufficient gas. 

 (/>) Operate tube continuously with same hydrogen for 

 2 hours, admitting fresh gas from time to time if 

 dark-space widens much. 



(c) Stop current, exhaust, and refill to 1 mm. dark-space. 



If tube is red at the start and does not change to white 

 in fifteen or twenty seconds, repeat process c. If the 

 tube still remains red after twenty seconds, and H^ 

 shows no tendency to fade, repeat operation b. 



(d) If H^ fades appreciably, two or three repetitions of 

 operation c will generally wipe it out, and may 

 even reduce H a to a brick-red colour (the white 

 stage par excellence). 



I have usually found that this condition seldom occurs 

 until the tube has been operated for several hours at the 

 white stage, repeating operation c from time to time as 

 the pink colour returns. It is important in this treatment to 

 stop the current each time at the moment when Hp seems 

 to have reached its minimum intensity, exhaust immediately, 

 and refill to 1 mm. dark-space (hydrogen flame tor a minute 

 or two). 



I have given rather explicit directions for producing the 

 white stage, for the reason that a tube cleaned out by this 



