-02 Profs. Richardson and Cooke on the Heat developed 



Another difficulty that we have to contend with arises 

 from the fact that the hot filaments continually sputter 

 particles on to surrounding objects, so that the grid very 

 rapidly becomes covered with a layer of material deposited 

 on it from the osmium filaments. We are not sure, however, 

 that this alters the magnitude of the observed effect, as in 

 some cases, for example oxygen No. 4, the value 5*58 was 

 obtained after a continuous heating of some twenty hours ; 

 whereas the preceding experiment gave a value of ;V()2. In 

 other experiments a change seemed to be observed after 

 continued heating, but there is no conclusive evidence that 

 the change was due to the sputtered material. The possi- 

 bility of an alteration both from the pressure of the gas and 

 from sputtering hinds us in a dilemma, because to get the 

 apparatus down to a really low pressure it is necessary to 

 heat the filaments continually for a long time, whereas to 

 avoid the accumulation of the sputtered material it would be 

 advisable to take the observations as quickly as possible after 

 the apparatus had been set up. 



We have already pointed out that the rate at which the 

 grid heats up depends not only on the total rate of heat 

 production within it, but also on its mode of distribution. 

 For instance, for a given rate of heat production the increase 

 of temperature in a given interval is not the same when the 

 heat is produced throughout the volume of the grid by 

 increasing the current in the Wheatstone's bridge circuit 

 as it is when it is produced by the impact of the electrons. 

 A source of error of somewhat similar character may 

 possibly arise when the heating effects at different voltages 

 are compared with one another by comparing the rise in 

 temperature in equal times ; since it is probable that the 

 distribution of the current into the grid will be different at 

 different voltages. We should expect that the electrons 

 would be more likely to be collected into a small region in 

 the centre of the grid when the difference of potential 

 between the filaments and the grid is relatively large. This 

 might introduce a difference in the rate of rise of tem- 

 perature of the grid as between high and low voltages, even 

 if the actual total rate of heat production were the same. It 

 is difficult either to test for this effect or to eliminate it if it 

 occurs, but it seems fairly certain that it cannot lead to very 

 big errors. It is to be borne in mind that since the resistance 

 of the grid is proportional to the absolute temperature at 

 every point, the increase in the total resistance will be inde- 

 pendent of the distribution of the heat communicated to 

 it, provided that the total amount of heat communicated is 



