88 Prof. J. S. Townsend on the Potentials required 



It will be observed that the rise of potential V — V 

 for a given current is diminished by drying the gas. 

 The velocities are therefore reduced by the presence of 

 water vapour when the ions move away from the strong 

 field in which they are generated. The mean value of 

 ^1^/760 = 415, deduced from the experiments in dry air at 

 176 mm. pressure, is in good agreement with the velocities 

 408 and 427 obtained by Zeleny* and Langevinf by direct 

 methods. 



The mean values 491 and 620 obtained at the pressures of 

 84 and 30 mm. respectively, show that the velocity of positive 

 ions increases more rapidly than the inverse of the pressure. 

 Langevin, whose determinations were made with smaller 

 forces, found that k x p was practically constant for pressures 

 between 75 millimetres and 1420 millimetres. 



The numbers given in Table IV. show that even at the 

 pressure 176 mm. the negative ions move much faster than 

 the positive ions, the velocity being eight times greater at 

 the pressure 30 millimetres. This is a well known effect, 

 and is due to the tendency of the negative ions to assume 

 the electronic state when the value of X/p increases. The 

 phenomenon only occurs at the higher pressures when the 

 intensity X is large, and has not been observed at the higher 

 pressure by the direct methods of finding the velocities. 



The theory also gives satisfactory results when applied 

 to the experiments made by Watson J and Schaffers§ on the 

 discharges at higher pressures. Watson determined the 

 potential required to maintain currents from a wire *35 

 millimetres radius and a co-axial cylinder 10'2 centimetres 

 radius. The values of Jc-i and k 2 for positive and negative 

 ions deduced from these experiments are given in Tables V. 

 and VI., the current i per unit length of the wire and the 

 potential V being in electrostatic units. 



There is a good agreement between the values of the 

 velocities deduced from experiments at the same pressure 

 with different currents. The mean values are rather low for 

 dry air. This may be due to the oxides of nitrogen which 

 are formed by the discharge as they would condense on the 

 ions and diminish the velocities, an effect which may be 

 appreciable with the larger currents when the air is im- 

 perfectly dried. 



* J. Zeleny, Phil. Trans. A. cxcv. p. 193 (1900). 



t P. Langevin, Annates de Chemie et de Physique, [7] xxviii. p. 289 

 (1903). 



J E. Watson, * Electrician,' February 11, 1910. ■ 

 § V. Schaffers, Pht/s. Zeits. xv. (1914). 



