PAPERS ON PHYSICS 



291 



DETERMINATION OF THE VAPOR PRESSURE OF 



MERCURY BY MEANS OF THE KNUDSEN 



PRESSURE GAUGE 



Prof. C. F, Hill, University of Illinois 



While engaged in experimental work in the spring of 

 1920, the question arose as to the pressure of saturated 

 mercury vapor. Upon looking in the tables for the values 

 at temperatures ranging from 0- to 20 -C., it was noticed 

 that there is little or no agreement between different ob- 

 servers. Data for but three direct methods was found listed 

 at the above temperatures. An accurate knowledge of the 

 vapor pressure of mercury is important in vacuum work 

 since mercury is almost always involved in the vacuum in 

 some way, either in gauges or pumps. Due to this fact, 

 and to the lack of agreement of former observers, ana also 

 to the small amount of data really taken before, it was 

 decided to try to devise a more dependable method. To 

 this end a Knudsen pressure gauge was used. This in- 

 strument is independent of the gas used and gives ac- 

 curate readings on pressures of the order of those of mer- 

 cury vapor at ordinary working temperatures. The cali- 

 bration of the instrument was accomplished by connecting 

 the apparatus as shown in the Fig. 1. All vapors were re- 

 moved from the Knudsen gauge side and kept out by 





FIG. 1 



