l'4o Barss -Ionization by Collision in Gases and Vapors. 



The ionization in vapors was much more intense than in air 

 under similar conditions of pressure, distance between plates 

 and field strength. The thing which next suggested itself was 

 to take air at a pressure that would give about the same ioni- 

 zation as that obtained with vapors (under saturating field 

 strength) and to investigate for evidence of the crossing effect 

 for different distances between the plates. Unfortunately 

 when the experiment was tried, sufficient potential was not 

 available to produce ionization by collision at the pressure 

 required. 



Keversing the electric field gave the same general results, 

 the curves crossing for both positive and negative applied 

 potentials. 



Experiments with a Cylindrical Chamber. 



It was thought that the apparent contradiction in the case of 

 vapors might be due in some way to the collision effect of the 

 positive ion ; for, as stated in an earlier part of the paper, the 

 ionic mobilities in vapors have given evidence of a characteris- 

 tic behavior, that of the positive ion in some cases exceeding 

 that of the negative ion. 



To test for the relative ionizing powers of the positive and 

 negative ions, a cylindrical ionization vessel was employed. A 

 brass vessel 12"7 cm high and 6 cm in diameter was coated on the 

 inside with powdered uranium oxide. A central brass elec- 

 trode - 3 em in diameter, provided with an earthed guard ring, 

 was connected to one pair of quadrants of the electrometer and 

 an electromotive force was applied to the case of the chamber. 



It has been shown that when the potential difference is small, 

 the current through a gas in such a vessel is approximately the 

 same whether the case has a positive or negative potential ; but 

 when the potential difference is large, the currents are no longer 

 equal. When the case is negative, a large increase in current 

 is obtained when the electromotive force is increased. In this 

 case all the negative ions traverse the field of strong electric 

 force near the central electrode, and consequently acquire a 

 velocity sufficient to ionize by collision the molecules of the gas. 

 When the case of the chamber is positive, the increase in cur- 

 rent is relatively very small when similar increases in the electro- 

 motive force are made. This time the positive ions traverse the 

 strong field near the central electrode and experiment has shown 

 that they do not acquire sufficient velocity (no doubt due to their 

 greater mass) to ionize by collision the molecules of the gas. 



This effect was tried in air at a pressure of 3 - 8 mm , the result- 

 ing curves agreeing very well with those obtained by Kirkby.* 

 * Phil. Mag., vol. iii, p. 212, 1902. 



