190 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1953 



1.5 



6 8 10 15 



ACCELERATION 



20 



30 40 



60 80 100 

 X 10" 



NUMBER DENSITY 



Fig. 4 — Drift velocity in an electric field of Ne+ ions in neon gas. Comparison 

 of observed results with an "asymptotic" straight line of slope 3^^ 



in the parent gas, observed for He+, Ne+, A+, Kr+ and Xe+. The plot 

 is a log-log plot of these quantities against a/'N ^ a variety of fields 

 having been used to determine each point. The data are taken from 

 measurements of J. A. Hornbeck^^' ^^ and R. N. Varney.^^ These data 

 verify in the first place that the drift velocity depends on a and N only 

 in the combination aIN . Beyond this we see that the curves consist of 

 two straight line portions: in the lower field portion (c^) is proportional 

 to aIN , in the higher is proportional to -s/a/N. We recognize in this 

 latter region the high field dependence predicted in equation (26b). 

 We learn from this that the collision cross section between noble gas 

 atoms and their ions is approximately constant in the experimentally 

 significant velocity range. To determine these collision cross sections 

 the computation of only a single number, namely the one entering into 



»• Hornbeck, J. A. and G. H. Wannier., Phys. Rev., 82, p. 458, 1951. 

 " Hornbeck, J. A., Phys. Rev., 84, p. 615, 1951. 

 » Varney, R. N., Phys. Rev., 88, p. 362, 1952. 



