Measurements on the Gases Evolved from Glasses 

 of Known Chemical Composition 



By J. E. HARRIS and E. E. SCHUMACHER 



Synopsis: This paper has a very direct bearing upon the pumping or 

 exhausting of the vacuum tubes used in telephone repeaters and similar 

 thermionic tubes. The telephone repeater bulb, as is well known, holds 

 a vacuum of the order of 10 =6 mm. of mercury. In order to produce this 

 vacuum it is necessary during the manufacture of the tube to not only 

 remove the air from the space within the bulb but also to allow very con- 

 siderable amounts of various gases to diffuse out from within the glass of 

 the bulb and the metal parts of the tube structure. The volume of gas 

 which is frequently removed from the metal plates, for instance, may be 

 roughly estimated as 100 times the volume of the plates themselves, the 

 volume of the gas being measured at atmospheric pressure. To remove 

 these gases from the bulb and metal parts, it is necessary to maintain 

 during the pumping process a temperature which is far above the normal 

 temperature and a fair degree of vacuum within the bulb for a period of 

 time which varies from a few minutes to an hour or more depending upon 

 the type of tube. 



With a view to simplifying the pumping process, the authors have found 

 that a glass relatively free from absorbed gases can be produced by using 

 special precautions in manufacture. The authors have also measured and 

 analyzed the gases evolved from glasses of various composition. Seven 

 different samples representing four distinct types of glass have been exper- 

 imented with. Six of these samples of glass have been carefully analyzed 

 and definite relations found between the amounts and kinds of gases evolved 

 and the chemical composition of the glasses. — Editor. 



THIS investigation of the gases evolved by certain glasses when 

 heated was undertaken with a view to securing a glass which, 

 after an initial period of heating, should cease to give off appreciable 

 quantities of gas. The use of such a gas-free glass would obviously 

 be desirable in the experimental investigation of vacuum tubes, in 

 which the filament and other parts within the tube may be affected 

 adversely by gases evolved from the heated glass during manufac- 

 ture of the tube. 



The work of Guichard (11), Langmuir (19), Sherwood (30), Wash- 

 burn (37), and others has established the following points with respect 

 to the gases evolved by the glass when heated. The gases may be 

 held as an adsorbed film or dissolved throughout the glass; the ad- 

 sorbed gases are evolved readily at temperatures less than 300°C, 

 whereas the dissolved gas, although it begins to come out of the sur- 

 face layers at 200°C, comes out slowly, by reasons of the slowness 

 of diffusion through the glass, even at much higher temperatures. 

 Consequently the total gas evolved at each of a series of temperatures 

 is a maximum somewhere between 200°C. and 400°C; this decreases 

 to a minimum, and rises again at temperatures approaching the 

 softening range of the glass. 



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