Heat produced by Emission of Ions from Hot Bodies. 637 



A glance at the Table will show that the experiments 

 Involve the following range of variation of the variables 

 entering into the formula : — the thermionic current T from 

 about 20 to 800 microamperes, the heating current i 1 from 

 0*430 to 0'687 ampere, the resistance R of the filament from 

 4-216 to 5*533 ohms, the potential V from 12*10 to 24*4 

 volts, Dt from 2*4 to 198 divisions and D E from 20*4 to 

 107-5 divisions. The results thus afford a very thorough 

 test of the constancy of <f> under widely varying conditions. 



Naturally the accuracy which may be expected in the 

 final result differs very considerably in different cases. It 

 is obvious that a determination which involves a deflexion 

 of only 2*4 divisions as a factor cannot be comparable in 

 accuracy, as regards errors of measurement, with one in 

 which the corresponding deflexion is 200 divisions. There 

 is of necessity a slight steady drift of the galvanometer spot 

 due to uncontrollable causes in experiments of this character. 

 The absolute effect of this is just the same for both large 

 and small values of Dt, so that its importance in the final 

 result is much greater for small than for large deflexions. 

 Under the circumstances it is almost surprising that the 

 measured values of </> exhibit so much constancy and con- 

 sistency over so wide a range. 



We might have picked out some of the experiments as 

 being more reliable than others, but we have not done so, 

 as our aim has been to examine the question of the constancy 

 and definiteness of cf> over a wide range of variation of the 

 thermionic emission and other conditions. In this connexion 

 it is worth repeating that the values deduced from experi- 

 ments with large values of T (and consequently of Dt) are 

 subject to an error due to effects which do not reverse with 

 %i and which are only roughly calculable. These effects are 

 unlikely to amount to more than 2 per cent, of (f> with 

 T = 8xl0 -4 amp., the largest value of T used, but they 

 would be serious if T were appreciably larger. Conversely 

 it is probable that they may be neglected with the small 

 values of T used in most of the experiments. 



Taking R*A= ^^r x 10- 6 erg/°C. x E.S.U., = 2000° K, 



00 = 290° K, the term 2 — (0-0 o )=O'3 volt witn sufficient 



accuracy. This estimate cannot be wrong by more than 

 0*05 volt for the average temperature, but it may be 0*05 volt 

 higher at the highest than at the lowest temperatures. 



