CONTEMPORARY ADVANCES IN PHYSICS 599 



higher pressure of 21 mm. Hg differ very seriously from the theoretical 

 form. 



Reverting to the question of testing equation (8) for the dielectric 

 constant, that is, for the current in quadrature with the voltage: 

 one might try the test with a gas-free space, populated by electrons 

 derived from a hot filament or from some other source. This in fact 

 has been tried, the electrons being shot across the interspace between 

 the plates of the "ionization-condenser," in directions nearly parallel 

 to the plane of these plates; from the (controllable) speed of these 

 electrons, and from the charge which they bear per unit time to a 

 collector located beyond the condenser, one may estimate the number- 

 per-unit-volume heretofore denoted by N. 



Since the density of the gas may easily be made so low that few 

 electrons collide with even one atom between the condenser-plates, 

 one expects the coefficient g to vanish. Strictly speaking, it does; 

 and yet there is in effect a component of current in phase with voltage. 

 The electrons, being pulled aside by the transverse high-frequency 

 field as they traverse the condenser, acquire kinetic energy; and this 

 absorption of energy from the high-frequency circuit produces the 

 same reaction, and is measured in the same way, as an ordinary 

 conductance. Benner, who developed these ideas, derived (.with 

 certain simplifying assumptions) these expressions for the coefficients 

 e and a of a cloud of streaming electrons which individually take the 

 time T to cross the condenser : 



e=l-47r- — ^ 1 ^^ . 0" = — ^(1-coswr), (18) 



and they have been tested with satisfactory results by Jonescu and 

 Mihul. 



We turn now to the possibility for which preparation was made in 

 equations (10); that an electron in an ionized gas may experience, 

 in addition to the force due to the applied field (eE) and the quasi 

 frictional force due to the gas (gx) yet a third force proportional to 

 its distance from some fixed point. One is accustomed to postulate 

 this last for electrons bound to molecules or atoms. (Such electrons, 

 by the way, contribute to the current-component in quadrature with 

 field strength, so that the dielectric constant of a gas in absence of 

 ionization is not quite unity as I have been writing). But to imagine 

 such a force acting on free electrons must seem strange — as if one 

 were denying them the quality of freedom. Still, for a cloud of 

 electrons mingled with positive ions there is such a force, and therefore 

 a "natural frequency"; and another natural frequency in addition, 

 if there happens to be a magnetic field. 



