178 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1953 



section (6) is bigger. This situation is accentuated in an actual scattering 

 calculation which shows an attractive force to be generally more efficient 

 than a repulsive force of equal range. 



A detailed numerical discussion of these questions is found in Massey 

 and Mohr" for the case of He"*" ions moving through He gas. Their 

 interest is in the low field mobility. They show that for this problem the 

 repulsive force makes so little difference that it could be neglected en- 

 tirely without much affecting the results. It does finally come out that 

 the polarization force gives a mobility which is too big by a factor of two. 

 But the additional scattering is due to an effect which we listed above 

 under (c) : namely a resonance attraction between the He atom and the 

 He"^ ion for which the cross section is abnormally large. It should be 

 possible to eliminate this effect by increasing the cross section (6) until 

 it masks even this special effect. Lowering the field is not sufficient to 

 achieve this because of the temperature motion; it would be necessary 

 in addition to reduce the absolute temperature by a sizeable factor and 

 so to decrease the value of 7 in (6). Thus we are led to the prediction 

 that if the temperature of He is reduced the mobility of He^ ions in He 

 should gradually rise from its "anomalous" value of 12 cmVvolt sec to 

 the ''normal" value of 22 cmVvolt sec, which one gets by taking account 

 of polarization forces only. 



IB. GLOSSARY 



The complicated appearance of equations in gaseous kinetics suggests 

 special care in the use of symbols and a convenient arrangement for the 

 reader to find their meaning. It is hoped that the glossary to follow will 

 accomplish this purpose. It explains all symbols except those used at 

 one location only. 



Generally, Latin capital letters will refer to the gas molecules and 

 Latin lower case letters to the ions; Greek letters will have no special 

 relationship; exceptions will be made for generally recognized symbols. 

 Thus we define 

 E, E = electric field. 



Xy y = cartesian coordinates at right angles to the field direction. 

 z — cartesian coordinate along the field direction, 

 r == position vector with components x,y,z. 

 t = time. 

 m — ionic mass. 

 e = ionic charge. 



» Massey, H. S. W., C. B. O. Mohr, Proc. Roy. Soc, 144A, p. 554, 1931. 



