JAN. 4, 1921 proceedings: philosophical society 21 



PROCEEDINGS OF THE ACADEMY AND AFFILIATED 



SOCIETIEvS 



PHILOSOPHICAL SOCIETY OF WASHINGTON 



833rd meeting 



The 833rd meeting was held at the Cosmos Club, March 27, 1920. 

 President Sosman presided and about 75 persons were present. The 

 speaker of the evening was Dr. Saul Dushman, of the General Electric 

 Co., who presented a paper on Methods for the production and measure- 

 ment of extremely low pressures. 



The pressures dealt with under this heading range from one mil- 

 lionth of an atmosphere (1 bar) to pressures lower than 10"^ bar. For the 

 proper operation of a number of hot cathode devices, it is absolutely 

 necessary to obtain pressures which are well below 10"- bar. The 

 methods used for the production of high vacua may be classified for 

 convenience under three headings: (1) Mechanical pumps, of which 

 the rotary mercury or oil pump and Gaede molecular pump are typical 

 illustrations. With the former, pressures as low as 10~^ bar can be 

 attained, while the Gaede molecular pump is capable of producing 

 vacua as high as 10"^ bar. (2) Mercury vapor pumps. The Gaede 

 diffusion pump and Langmuir condensation pumps are both capable of 

 producing extremely high vacua. The latter is, however, much more 

 rapid in its operation. (3) Physico-chemical methods. By means of 

 charcoal immersed in liquid air it is possible to obtain extremely low 

 pressures in devices in which gas is being evolved continually either from 

 metal parts or glass walls. The volatilization of calcium has been 

 suggested by Soddy as another method for producing low pressures in 

 sealed-off tubes. 



In the actual exhaust operations, it is of importance to consider the 

 resistance to flow of glass tubes, as well as the speed of the pump. 

 The laws of flow of gases at low pressures have been studied by Smolu- 

 chowski and Knudsen and it is possible by means of the formulae de- 

 rived by the latter to calculate the effect of glass tubing of given dimen- 

 sions on the speed of exhaustion. 



Methods of measuring low pressures. — The McLeod gauge is the sim- 

 plest but is not always applicable and is not very sensitive for pressures 

 below 10~^ bar. Pirani and Hale have developed an electrical re- 

 sistance method which is capable of measuring pressures as low as 10~" 

 bar. The molecular gauge devised by Langmuir is sensitive to 10~^ 

 bar, but is difficult to construct. Knudsen developed a radiometric 

 type of gauge and various modifications of it have been designed by 

 which pressures as low as 10"^ bar can be measured. The simplest 

 method of measuring low pressures is the ionization gauge. This is 

 very convenient for measuring the pressure in sealed-off hot cathode 

 devices and incandescent lamps. 



