when an Electron escapes from Surface of a Hot Body. 215 



from this by nearly 7 per cent. It seemed to the writer 

 that this discrepancy was great enough to leave a reasonable 

 doubt as to the exact equivalence o£ b and <f>. 



On account of the importance of the relationship it seemed 

 advisable to obtain more specific information on this point. 

 The present measurements on tungsten were taken with this 

 end in view. Tungsten was selected for the purpose because 

 it is the only metal for which extensive measurements have 

 been made under ideal conditions to determine the value 

 of b. Cooke and Richardson investigated only one speci- 

 men in their measurement of the cooling effect. It was 

 thought possible that an extended investigation might yield 

 results more in harmony with the published values of b. 

 The mean of the eighteen values found for tungsten gives 



<£ = 4-478, 

 from which 



b = 52130. 



This value is within *7 per cent, of Langniuir's, and differs 

 from the weighted mean of Langmuir's and Smith's value 

 by 1*6 per cent. Each of these differences is well within 

 the limits of accuracy of the present experiments. 



The above result leaves little doubt as to the exact equi- 

 valence of c/> and b within the limits of accuracy of the above 

 determinations, and consequently justifies the assumption 

 in regard to b involved in the Richardson equation, and lends 

 additional support to the findings of Richardson and Brown 

 concerning the kinetic energy of electrons. 



2. Effect of Gases on the Value of <£. 



Langmuir has shown that residual gases strongly influence 

 the value of b. It seemed worth while to test for corre- 

 sponding effects in the value of <f). Osmium when heated 

 gives out gas copiously. An osmium filament mounted and 

 treated in the usual manner but not completely glowed out, 

 would give up gases that are probably free from water- 

 vapour. These gases react with the hot cathode to form 

 compounds which condense on the walls of the tube, so that 

 there is a gradual improvement in the vacuum, a gas-free 

 state being finally arrived at. A tube so treated gave 

 initially for the cooling effect 5*93, after a few hours heating 

 the value was 5*16. The filament burned out before the 

 gases were all consumed, so that the final value was not 

 obtained. A tungsten filament similarly treated but not at 

 first sealed off from the charcoal tube gave values as 



