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Contemporary Advances in Physics, XXIV 

 High-Frequency Phenomena in Gases, First Part 



By KARL K. DARROW 



This is an account of the behavior of conducting gases subjected to 

 high-frequency electrostatic fields— behavior which can be interpreted, in 

 many cases with striking success, by supposing that the free electrons 

 wandering in the gas are set into motion by the field, and oscillate and 

 drift according to laws which can be derived from our knowledge of the 

 response of free electrons to steady fields. When a constant magnetic 

 field coexists with the high-frequency forces, the phenomena become more 

 complicated, but remain predictable. There are also peculiar phenomena 

 indicating that the electrons in a conductive gas have certain natural 

 frequencies of oscillation. Applications are made to the absorption of 

 radio-frequency waves in ionized gases. 



N this article I will describe some of the phenomena which are 

 observed when the voltage across a region filled with gas is varying 

 quite rapidly. Considered as a function of time, the voltage may be 

 periodic, a sine-wave with uniform amplitude and of a frequency 

 somewhere between a few thousands and a few hundreds of millions 

 of cycles per second. It may be a succession of highly-damped 

 wavetrains, each commencing with a rapid rise of voltage and con- 

 tinuing in oscillations of high frequency but swiftly declining ampli- 

 tude, which die away into nothing and after an interval (it may be of 

 a few hundredths or a few thousandths of a second) are followed by 

 another train. It may be a brief irregular spasm of electromotive 

 force, of which the highest value of the voltage is measured or merely 

 guessed. In the gas itself there may be the phenomenon of sudden 

 and violent breakdown; or the establishment of a self-sustaining 

 luminous discharge, like in aspect to a glow or an arc; or merely a 

 vibratory motion of electrons, freed by other agencies and set in 

 motion by the oscillating field. 



One might regard this as a subject which physicists had better leave 

 alone, until they have full understanding of the seemingly much 

 easier problems of discharges across a gas exposed to a constant 

 voltage. So great are the apparent advantages of steady or "direct- 

 current" discharges for the student, so great the apparent inconveni- 

 encies of high frequencies, that one might reasonably think it futile to 

 assail the latter with weapons which have not yet overcome the former. 

 Thus, in a self-sustaining glow maintained by a constant voltage, there is 

 a peculiar distribution of space-charge, which distorts the field between 



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