A proper mobility formula for large, heavy particles in gases in any regime

The age-old problem of the derivation of a proper formula for the mechanica l (or electrical) mobility of large, heavy (l.h.) particles in a simple neu tral gas in any regime is carefully examined and solved under the usual hyp otheses that the l.h. particles are subject to the action of a constant ext ernal force and undergo only elastic collisions with the gas particles. By a convenient procedure, a general mobility formula is obtained which not on ly properly corrects the well-known result derived by Cunningham for an l.h . hard sphere in a hard-sphere gas, but also applies to the general case in which both the l.h.-particle-gas-particle and gas-particle-gas-particle in teractions are arbitrary. In addition, on the basis of the new formula, the fair success of the Cunningham formula (regarded as a semiempirical formul a) in fitting the experimental results is explained. Other interesting aspe cts of the new formula are also examined, and its limits of validity are br iefly discussed. (C) 2000 Elsevier Science B.V. All rights reserved.