208 Mr. H. H. Lester : Determination of Work Function 



pump was broken and exhaustion was completed with charcoal 

 and liquid air. Liquid air was also placed on a trap near the 

 furnace so as to prevent mercury vapour and other eon- 

 densible gases from diffusing back into the tube. As soon 

 as it had cooled sufficiently the furnace was opened and the 

 filament was glowed out at a temperature near its melting- 

 point. This process was accompanied by a considerable 

 evolution of gas. This evolution did not continue long, so 

 that after about fifteen or twenty minutes the pressure 

 decreased to about 2 x 10 -5 mm., in most cases with the wire 

 hot. The tubes were now sealed off and were ready for use. 

 In one case readings were taken on a tungsten filament before 

 sealing off. 



The above procedure was not followed with carbon except 

 in one case where the tube was sealed off when the pressure 

 was down to 1 X 10 ~ 4 mm. with the wire hot. It was found 

 that the carbon gave up a very great quantity of occluded 

 gas. It seemed impossible to glow it out to a point where 

 no more gas was given up. For reasons that will be dis- 

 cussed later it is thought that no serious error arose from 

 the lack of ideal conditions in the case of carbon. 



The temperature of the filaments, except in the case of 

 tungsten, were estimated by comparing the colour of the 

 filament with that of a tungsten filament for which a tempe- 

 rature-current curve was available. The tungsten tempe- 

 ratures were given by the curve referred to above, for which 

 thanks are due to Dr. Irving Langmuir, of the General 

 Electric Company Research Laboratory. Dr. Langmuir 

 was kind enough to furnish most of the filaments used in 

 this research. 



The tungsten and tantalum filaments were fine drawn 

 wires similar to those used in the construction of metallic, 

 filament lamps. The molybdenum was in the form of a fine 

 wire about *1 mm. in diameter. The carbon filaments were 

 taken from burned-out carbon lamps of the " metalized " 

 filament type. The filaments of these lamps are baked out 

 in the process of manufacture at a temperature of 3000° C, 

 thus changing most of the carbon to graphite and greatly 

 purifying the material. The other materials were regarded 

 as pure, though no investigation was made except that a test 

 on tungsten showed no trace of thorium. 



The values of </> are given below in the form adopted by 

 Oooke and Richardson. In their work they neglected the 

 factor 1/i in the expression for BR e . This omission did not 

 materially affect their values for tungsten, but it is very 

 important for many of the values found here. 



