during the Absorption of Electrons by Platinum. 201 



R4/R5 was comparable with 200, so that x is practically 

 equal to the whole length / o£ the grid. Let us, therefore, 

 put as an approximation ,v = / and x x jx Q = 0. So that 

 SQ = m(%fl 2 -ijl). In this case we also have R7 = S, the 

 resistance of the grid, and jl = J. So that 



8Q/J = S(^J-i). 



In the experiments the maximum values of these quantities 

 were respectively S = 15 ohms, J = 3xl0~ 4 amp., and 

 i = 17 x 10— 3 amp. So that the maximum value of this 

 correction SQ/J = '25 volt. 



Although the above calculation of the correction has only 

 been carried out for the case where j is constant, it can 

 readily be seen that a similar result will follow for any 

 distribution of j which is symmetrical about the central point 

 of the grid. As this condition was very nearly fulfilled in 

 practice, the above method of calculation has been used in 

 estimating the magnitude of the correction for the direct 

 heating effect of the thermionic current. The relation 

 between the direction of the thermionic current and the 

 direction of the bridge current was such that this correction 

 involved a deduction from the observed value in all cases. 

 The actual values of the amounts which have to be sub- 

 tracted on this account are given in the last column but one 

 of the various tables above. 



§ 14. Possible Sources of Error, 



It will readily be conceded that the foregoing results leave 

 no doubt as to the existence of the effect under investigation 

 or of its order of magnitude ; but, at the same time, it is 

 very difficult to obtain results of a high order of accuracy. 

 We are unable at present to locate the source of such 

 inconsistencies as have been experienced. A glance at the 

 tables would seem to indicate that there is a correlation 

 between the magnitude of the observed effect and the 

 pressure of the gas in the apparatus. If it is desired to 

 carry out experiments of this nature with reasonable rapidity 

 it is difficult to reduce the pressure of a gas below '005 mm. 

 owing to the fact that the development of heat in the osmium 

 filament raises the temperature of the whole apparatus very 

 considerably and causes an inconvenient amount of gas to 

 be evolved. We have made direct experiments in order to 

 see if an increase of gas pressure gave rise to an increase in 

 the apparent value of the effect. These experiments show 

 that if there is any effect due to pressure it is probably too 

 small to account for the observed differences. 



