PROCEEDINGS 
OF THE 
NATIONAL ACADEMY OF SCIENCES 
Volume 2 JULY 15. 1916 Number 7 
ON THE MOBILITIES OF GAS IONS IN HIGH ELECTRIC 
FIELDS 
By Leonard B. Loeb 
RYERSON PHYSICAL LABORATORY, UNIVERSITY OF CHICAGO 
Received by the Academy. May 20. 1 9 1 6 
Up to the present two theories have been advanced to explain the 
low order of magnitude of the mobility of the ordinary gaseous ion. 
The first theory known as the 'cluster' theory is due to Langevin.^ It 
assumes that the ion consists of a cluster of neutral molecules sur- 
rounding an electron or a positive atomion. The second theory was 
proposed by Wellisch^ and is known as the 'small ion' theory. This 
assumes that both positive and negative ions consist of single charged 
molecules. In its path through the gas such an ion might be retarded 
abnormally due to the fact that its charge, acting on neutral gas mole- 
cules, drags in a greater number of collisions than would an uncharged 
molecule. 
On the 'cluster' theory it would be expected that if the ion acquired 
sufficient kinetic energy, it would begin to break up. Such energy 
might be acquired in traversing a mean free path under a sufficiently 
high field. The break up would then be indicated by an abnormally 
high mobihty. One would certainly expect such a breaking up of the 
cluster before field strengths great enough to cause ionisation by 
collision were reached. The other theory would expect no such 
break up. It is, however, conceivable on the 'small ion' theory that 
when the kinetic energy reaches a very high value, near that at which 
ionisation by collision begins, the negative ion might lose its electron. 
Experiments which were performed by a number of observers on the gas 
ions at low pressures (i.e., where with a small value of the field the ion 
can pick up considerable energy on a long mean free path), indicated 
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