180 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1953 



X = rr- = mean free path of ion between collisions with molecules 



(may be a function of 7). 



r = — — = mean free time for the ion between collisicfhs with mole- 

 Nay 



cules. 



r, = same parameter for ''spiralling" collisions. 



a = — 1 — — + 1. It is assumed constant in Section ID. 

 dlny 



X, Xc , Xtt = angle of scattering of ion and molecule in the center of 

 mass system. 



K = angle of scattering of the ion by a molecule in the laboratory system. 



€ = scattering azimuth of ion and molecule in the center of mass system. 



CO = scattering azimuth in the laboratory system (azimuth of the initial 

 ion velocity about the final ion velocity). 



I?, ^' = angle between velocity vector and field direction. 



^p, (p, d, <}), d = other angles (these angles are defined on spherical tri- 

 angles which are exhibited in Figs. 8 and 15. 



d{Cj r, t) = density function of ions in phase space. 



^(c) = ( — ) exp {—^mc) = Maxwellian velocity distribution function 

 for ionic mass. 



M(C) = I- — j exp (—^MC^) = Maxwellian velocity distribution func- 

 tion for molecular mass. 



h(c) — "high field" distribution function of the ions for the case that 

 the spatial distribution is uniform (the exact meaning of this 

 term is to be explained in the text). 



f(c) = true velocity distribution of the ions for the case that the spatial 

 distribution is uniform. 



gf(c) = correction to /(c) or h{c) for the case of a constant relative con- 

 centration gradient k. 



5(c) = vectorial 6-f unction in velocity space. 



Ei(x) = / -J- d^ (suppression of two minus signs). 

 Jx k 



Io{x) = modified Bessel function of order 0. 



Ko{x)j Ki{x) = Modified Hankel functions of order 0, 1. (Alteration of 



2 

 Macdonald function by a factor — ). 



T 



P,{x) = Legendre Polynomials. 



