(luring the Absorption of Electrons hy Platinum. 203 



identical in the cases compared. It is also necessary that the 

 heat communicated should be similarly distributed about 

 the cross section of the grid and that the latter should be 

 uniform. 



One possible source o£ error that we considered arises from 

 the cooling effect produced by the escape of the thermions 

 from the osmium filament. This will make the temperature 

 of the filament lower when the thermionic current is on 

 than when it is off, and hence the thermal energy radiated 

 to the grid will be less when the thermionic current is on. 

 We should therefore expect a change of temperature of the 

 grid independently of any of the causes discussed hitherto. 

 This effect was tested for in the following manner. 



The grid and its connexions, which were otherwise insu- 

 lated, were connected by a wire with the negative end of the 

 filament. A suitably high potential difference, which could 

 be reversed, was applied between the filament and the sur- 

 rounding brass box, so that the thermionic saturation current 

 could be made to flow at will from the filament to the box 

 when desired. AVith this arrangement there is no thermionic 

 current from the filament to the grid, so that any change 

 produced in the temperature of the latter will arise from 

 changes in the thermal radiation it received from the 

 filaments. Under these circumstances it was found that no 

 change was produced in the resistance of the grid when the 

 thermionic current from the filaments to the case was turned 

 on or off. This shows that under the conditions of the 

 experiments the cooling effect arising from the emission of 

 the electrons by the filaments is too small to exert any 

 appreciable influence on the temperature of the grid through 

 the change in thermal radiation which it causes. This 

 possibility can therefore be entirely left out of consideration 

 as a disturbing factor. 



§ 15. Comparison with the Work done during the emission of 

 Electrons from Hot Metals. 

 Without being able to assign any very satisfactory reason, 

 we are inclined to think that the most probable values of the 

 effect are those which are obtained after the high values in 

 the tables are omitted. Thus for platinum saturated with 

 oxvgen by electrolysis in nitric acid the value of (j> probably 

 corresponds very closely with the work done in falling 

 through a potential difference of 5'5 volts, whereas for 

 platinum saturated with hydrogen by electrolysis of dilute 

 sulphuric acid the mean value obtained, when the last three 

 series of observations are neglected, is 4*5 volts. 



