416 Messrs. Stead and Gossling on Relative Ionization 



characteristic is reproduced in the lower parts of the curve; 

 but the subsequent upward turn indicates, of course, that 

 the higher applied potentials are competent to produce a 

 larger current than in the high-vacuum case. In view of 

 Langmuir's theory of the effect of the space-charge it seems 

 difficult to account for this larger current except by the 

 assumption that positive ions have appeared in the neigh- 

 bourhood of the cathode. Positive ions would be expected 

 to produce just such an effect as is observed, for they would 

 tend to neutralize the opposing field due to the elections in 

 transit, and so permit the passage of a larger electron current 

 for any given value of the applied voltage. As Professor 

 Sir J. J. Thomson * has pointed out in a slightly different 

 connexion, a very small proportion of positive ions produces 

 a marked effect on the field by reason of their low velocity 

 and consequent large contribution to the space-charge in their 

 neighbourhood. 



(2) Discussion of Method. — As mentioned above, some of 

 the methods which have been used in attempts to measure 

 ionization potentials are open to the serious objection that 

 they fail to distinguish between the picking up of positive 

 ions by the collecting electrodes and the photo-electric 

 emission of negative electricit} r from these electrodes. The 

 method described in the present paper is free from this 

 objection, as the collecting electrodes are always at a positive 

 potential with respect to the filament, and their function is 

 to collect electrons, so that photo-electric emission o£ electrons 

 would diminish rather than increase the current. If photo- 

 electric emission from the filament itself is supposed to occur, 

 this will merely slightly increase the saturation current from 

 the filament, and will have no influence whatever on the 

 current through the tube. 



There remains the possibility that the sudden increase in 

 the slope of the current-voltage curve is due to the production 

 of positive ions, not by the thermions from the filament, but 

 by more rapidly moving photo-electrons emitted by either 

 the filament or the collecting electrodes under the influence 

 of ultra-violet radiations from the gas molecules. Such a 

 view, however, appears to be inconsistent with the quantum 

 relation Ye = Jiv, for this relation not only determines the fre- 

 quency of the ultra-violet light necessary to cause an emission 



* Roy. Inst. Lecture, " Engineering," 103. p. 563 (1917). Proc. R. 

 Inst. xxii. p. 175 (1917). 



