172 Prof. Richardson and Mr. Robertson : Effect of 



made first. It is probable, however, that the main factor 



in this is the superior chemical activity of tungsten and 



copper as compared with platinum. Another difficulty 



arises from the very marked " clean up " 



effects with tungsten, which for example Fig. 6. 



when working with hydrogen cause the 



pressure of the gas to vary considerably 



in a single experiment. It would take too 



much space to discuss the whole mass oE 



data which have been obtained in detail, 



and we shall merety point out the more 



important conclusions which follow from 



the experiments. 



The first point tested was as to whether 



this admission of small amounts of mercury 



vapour at a pressure of the order *001 mm. 



would displace the curves along the voltage 



axis. No displacement was observable, in- 

 dicating that mercury vapour does not 



affect the contact potential between hot 



tungsten and cold copper. This is in 



agreement with the observation * that the 



electron emission from tungsten is not 



affected by mercury vapour. In these 



experiments the mercury vapour was only 



admitted for short intervals. No doubt 



if enough were admitted to- amalgamate 



the surface of the cold electrode some 



displacements would be anticipated. 

 The problem of the effect of hydrogen 



was attacked in a similar way to that in 



the case of platinum. The experiments show that any genuine 



effect of small quantities of hydrogen is small, and we have 



not been able to convince ourselves that there is such an 



effect. The most complete set of data gave the following 



values for the potentials V on the voltage axis at which the 



currents had attained half the saturation value at the pressures 



stated : — 

 p(mm.)-> -1795 1745 -1452 -132 -1275 "123 '0855 -078 -075 -0722 

 V (volts.)-* -1-38 -1-37 -1-32 -111 -1'42 -115 —111 —110 —113 -113 



Means. 



p (mm.)-> -0155 

 V (volts)-* -1-57 



0085 



•00625 



•0035 



177 



132 



•0777 



•008 



-1-51 



-1-51 



-116 



-1-375 



-110 



—112 



-T51 



* Kichardson, Phil. Mag. vol. xxvi. p. 347 (1913). 



