604 



BELL SYSTEM TECHNICAL JOURNAL 



Electromagnetic waves of high frequency often emanate from gases 

 intensely ionized by a flow of direct current, and their wave-lengths 

 have several times been measured, but for one reason or another the 

 comparison with equation (24) was not and cannot be made. In a 

 mercury-vapor arc Tonks and Langmuir detected oscillations of 

 frequencies nearly as high as 10^ accompanied by others ranging 

 downwards as low as lO*'; the former, they believe, were plasma- 

 electron vibrations; but unluckily they were unable to estimate N 

 with any degree of exactness. 



Fig. 10 — Correlation of quantities serving as measures of high-frequency con- 

 ductance and dielectric constant of rarefied ionized hydrogen. (H. Gutton; Comptes 

 Rendus.) 



If a region populated with free electrons is pervaded by a constant 

 magnetic field, yet another natural frequency exists. Say, to begin 

 with, that there is no electric field; then any moving electron, instead 

 of continuing in a straight line, is constrained to describe a spiral path 

 (the axis of the spiral being parallel to the magnetic field). The 

 velocity of the electron affects the curvature of the helix, but not the 

 time of traversing a single winding thereof. The reciprocal of this 

 time, the number of windings described by any moving electron per 

 second, is the "natural frequency" aforesaid, and is given by the 

 formula: 



VH — nil 12 IT = ell/lTTUic, (26) 



