The electron, its isolation and measurement and the determination of some of its properties

by Robert Andrews Millikan

FROM MOBILITIES AND DIFFUSION COEFFICIENTS

Fig. 38

If we assume that gaseous ions, which are merely charged molecules or clusters of molecules, act exactly like the uncharged molecules about them, they will tend to diffuse just as other molecules do and will exert a partial gas pressure of exactly the same amount as would an equal number of molecules of any gas. Imagine then the lower part of the vessel of [Fig. 38] to be filled with gas through which ions are distributed and imagine that these ions are slowly diffusing upward. Let

be the ionic concentration, i.e., the number of ions per cubic centimeter at any distance

from the bottom of the vessel. Then the number