THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Art. XXI. — Ionization by Collision in Gases and Vapors ; 

 by W. R. Barss.* 



Introduction. 



The experiments described in this paper were undertaken 

 with the object of extending the results which have already 

 been obtained in connection with the ionization current which 

 passes through a gas when the pressure is not too high. The 

 theory of Ionization by Collision has been developed by Town- 

 sendf and verified for many gases at low pressures by various 

 experimenters, mainly Townsend and the students associated 

 with him. Bishop:}: has submitted this theory to a more rigid 

 test by extending the observations to higher pressures and also 

 to mixtures of gases, the properties of the constituent pure 

 gases being known. He has shown that Townsend's theory 

 holds over a range of pressures up to forty centimeters in air, 

 carbon dioxide, hydrogen and various mixtures of these gases. 



An ionization current through a gas may be considered as 

 passing through three distinct stages as the electromotive force 

 is increased. In the first stage Ohm's law is approximately 

 obeyed, the current rising proportionately to the electromotive 

 force applied. In the second stage the current remains practi- 

 cally constant with small, if any, variations for large additions 

 to the electromotive force. In the third stage, the current 

 rises rapidly when the electromotive force is increased by a 

 small amount. Townsend has shown that when the applied 

 electromotive force is sufficiently strong, an ion will acquire 

 sufficient velocity to produce new ions by colliding with the 

 neutral molecules of gas which may be in the path of the ion. 



* Thesis presented to the Graduate School of Yale University, Jane, 1912, 

 for the Degree of Doctor of Philosophy. 



•f The Theory of Ionization of Gases by Collision. 

 X Phys. Rev., vol. xxxiii, p. 325, 1911. 



Am. Jour. Sci.— Fourth Series, Vol. XXXIV, No. 201.— September, 1912. 

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