596 E. M. Wellisoh — Mobilities of Ions in Air. 



Erikson of the University of Minnesota. The mobility is 

 given by 



fc = -=zr ' - — - where V is 



V ji — i 



a potential near Y ; i' is the current corresponding to the 

 potential Y when the alternating Held is employed : i is the 

 current corresponding to the potential Y directly applied. There 

 was no evidence of any anomalous behavior and over the whole 

 range the product ph remained unaltered, indicating that the 

 nature of the positive ion in thoroughly dried air remains the 

 same over this range of pressures. The mean of a series of 

 determinations was found to be 1*23 cm/sec per volt/cm, which 

 is lower than is usually given. 



To sum up, the experimental results indicate that the positive 

 ion remains the same in nature over the whole pressure range 

 employed ; the negative carriers, however, consist, in general, 

 of two kinds, electrons and ions, the former increasing in 

 number relatively to the latter with diminishing pressure. In 

 the present experiments the electrons came into evidence at 

 pressures below 8 cm , but it is probable that they exist in small 

 numbers at much higher pressures and that they would be 

 detected by more sensitive measuring instruments. 



There is no evidence of any transition stage in the nature of 

 the ion ; the electrons appear to travel freely through the 

 molecules of the gas without attaching themselves and the ions 

 manifest a similar behavior. The negative ion remains the 

 same in nature over the whole pressure range employed. 



4. Brief Discussion of Results. 



As mentioned in the introduction, many observers have 

 undertaken a determination of the mobility of gas ions at rela- 

 tively low pressures. They all arrived at the same general 

 conclusion, viz., that the product ph increases in the case of the 

 negative ion when the pressure is reduced below about 10 cm ; 

 this result was interpreted as denoting a progressive diminution 

 in the size and mass of the ion as the pressure is reduced. Pro- 

 fessor Townsend and his students showed that at low pressures 

 when the gas was thoroughly dried the negative ion tended to 

 assume the electronic state ; the velocity of the negative ion was 



expressed in the general form v=f( — \ which implies that 



the nature of the ion is determined by the field as well as the 

 pressure. In his recently published treatise* (pp. 120, 121) 

 Professor Townsend sums up the results of a large number of 

 experiments by stating that the negative ion consists in general 

 * Electricity in gases, Oxford, 1915. 



