408 Profs. Richardson and Cooke on the Heat liberated 



The weighted means were judged by inspection of the 

 experimental points. It is questionable whether they are 

 much more reliable than the others, as it is our opinion that 

 the experiments are affected by causes, which we are not 

 able to control, which may remain constant during a single 

 set of experiments. In the case of silver and aluminium 

 the results of the experiments seemed less trustworthy than 

 in the other cases. Often it was impossible to get the same 

 value twice, for the same thermionic current and the same 

 potential-difference, owing to some change with time which 

 was going on. Moreover, in several experiments with these 

 metals the heating effect did not turn out to be a linear 

 function of the applied potential-difference. This was pro- 

 bably due to some parts of the grid being different from 

 others, and the heating current moving from one part to the 

 other as the potential-difference was changed. Probably 

 most of the difference from one part of the grid to another 

 was due to the effect on it of the heating and sputtering. 

 This suggestion is supported by the appearance of the grids. 

 The aluminium ones, after they had been used a few. >times, 

 were quite changed in texture and were so much altered that 

 they crumbled to pieces as they were unwound. They were 

 also very badly discoloured. 



The experiments made with iron grids led to a discovery 

 which we think likely to account for a considerable part of 

 the irregularities which have been noted with all the metals. 

 It seems that under the conditions of the experiments on 

 iron, and probably on the other metals, there is a kind of 

 instability in the thermionic emission of the osmium filaments. 

 The nature of this instability is best described by considering 

 the way the thermionic emission changes as the temperature 

 of the filament is altered. Starting at a comparatively low 

 temperature, the emission increases rapidly with increase of 

 temperature following the usual inverse exponential law. 

 This goes on until temperatures of a certain value are reached, 

 when the current shows a tendency to sag off. If the tempe- 

 rature is now raised and maintained at a certain value, there 

 is a sudden drop in the thermionic emission. In favourable 

 cases the new current may be as little as one-thirtieth of what 

 it previously was at the same temperature. When the tem- 

 perature is raised further the current is found to be stable 

 again, and increases with temperature according to an inverse 

 exponential law. 



The behaviour of the emission as the temperature is lowered 

 is the reverse of what has been described. We have first a 

 quick but regular decrease in current following the inverse 



