E. M. Wellisch — Mobilities of Ions in Air. 595 



either an electron or an ion. At first sight the presence of the 

 bend in that part of each curve where the ions begin to appear 

 might be construed as evidence of the presence of carriers 

 intermediate in nature between the electron and the ion. 

 Against this view, however, is the fact that a similar bend 

 occurs in the curves for both the positive and the negative 

 ions at the higher pressures. This is shown in the curves of 

 iig. 4. The three curves which correspond in that figure to the 

 three higher pressures were obtained with relatively small fre- 

 quencies of alternations. They all exhibit this characteristic 

 bend, which is in all probability due to a distortion of the field 

 in the neighborhood of the gauze ; as a result of this some ions 

 arrive slightly ahead of the main body and thus appear to have 

 greater mobility. The curve corresponding to p = 62 mm is 

 interesting as affording an illustration of the advantages which 

 arise from the higher resolving power obtained by using large 

 alternation frequencies ; the curve K of tig. 7 shows that at 

 least part of the bend in the corresponding curve of fig. 4 must 

 be due to electrons. 



For pressures below l mm the values of the critical potential 

 (Y ) were of necessity small and consequently the observation 

 error was proportionately large ; it is however safe to con- 

 clude from the experimental results that the value of pk for 

 the negative ion is the same at the lowest pressures employed 

 as at one atmosphere, showing that the nature of the nega- 

 tive ion remains unaltered over a range of pressures from one 

 atmosphere clown to *15 mm . The mean value of K for the 

 negative ion came out as 1*93, which is higher than is usually 

 assigned. 



The curves of fig. 7 show not only that the negative carriers 

 consist of electrons and ions, but that the proportion of elec- 

 trons increases with diminishing pressure. It must be remem- 

 bered, however, that these curves give information only with 

 regard to the relative number of electrons and ions which con- 

 stitute the current between the gauze and the electrode e. This 

 is not the same as the relative number of electrons and ions pro- 

 duced in the antechamber owing to the unequal rate of diffusion 

 through the gauze. A rough estimate which was made showed 

 that at a pressure of 12-2 mm the ions were still at least 80 per 

 cent of the total number of carriers produced, and even at 

 the lowest pressure (/15 mm ) were still over 50 per cent.* 



The mobility of thejpositive ion was determined at various 



pressure intervals from 1 atmosphere down to — mm. At the 



lower pressures where the error of observation in the value of 



Y was relatively large, the mobilities were also determined by 



a method which was suggested to the writer by Professor 



* See footnote p. 598. 



