Gases on Contact Potential Difference between Metals. 171 



The displacement for this limit of pressure would thus be 

 BV= -078 vo.lt. 



For T, = 1460° K : when p = 0'112 mm. V=0'629 volt, and 

 when p=0'0013mm. V = 0'501 volt. 



For this value of T 2 , SV=0*125 volt. 



The experimental value of the displacement over this range 

 of pressure for T ? =1460°K. was 0"1G volt; so that the 

 observed effect is in fair accordance with the theory. It 

 should also be added that it is in the prescribed direction. 



There is one point in this discussion which perhaps has not 

 received sufficient emphasis, namely, that it appears vit'il in 

 order to reconcile the results with the type of theory dealt 

 with to assume that the hydrogen is in equilibrium not 

 merely with the surface of the hot platinum wire but with 

 that of the cold platinum foil as well. If the hydrogen did 

 not modify the cold platinum surface this would act as a 

 reference surface of constant potential, and a calculation 

 along similar lines for this case shows that the effects should 

 be considerably greater than those observed and in the 

 opposite direction. This is in accordance with the require- 

 ment that the hydrogenated platinum surface should be 

 electropositive to a clean platinum surface at all temperatures. 

 The physical reason for the opposite direction of the effect as 

 given by the experiments and supported by the calculations, 

 lies in the fact that the temperature coefficient of the electron 

 work function is negative at all pressures and increases with 

 the pressure of the hydrogen. 



Experiments with Tungsten. 



We have made a large number of experiments with 

 tungsten filaments using the type of tube shown in fig. 6. 

 The anodes were of copper foil and were 5'0 cm. long 

 and 1*9 cm. in diameter. The tungsten filaments were 3*0 

 cm. long and 0'127 mm. in diameter except in the data shown 

 in fig. 7 where the diameter was 0*076' mm. The saturation 

 currents were of the same order as before, 10 -7 amp. The 

 wires were kept taut along the axis of the cylinder by means 

 of the molybdenum wire springs shown at the ends. These 

 were under slight tension and took up the slack due to 

 expansion on heating. 



For some reason we have found it more difficult to get 

 consistent results with tungsten than with platinum. This 

 may be due partly to inexperience in controlling the condi- 

 tions as the tungsten experiments were, in point of fact, 



