Mobilities are commonly expressed in units of meters per second per volt per meter [m s-1
]. In a vacuum, a single gaseous ion subjected to any nonzero potential gradient
would accelerate indefinitely; but in the midst of a gas the ion continually experiences collisions with gas molecules. These encounters tend to break up its trajectory
into a series of short intervals of acceleration
punctuated by deflections. The net result is that the ion's gross motion resembles drift
at a uniform velocity
. The mobility
depends not only upon the nature of the ion and gas but also upon the density
of the gas, for the latter controls the mean free path
of the ion. In atmospheric electricity
, the mobilities of small and large ions weight their relative importance in atmospheric conduction
. Small ions have mobilities of about 1.3 × 10-4
in air at sea level
with negative small ions exhibiting slightly greater values than do the positive small ions. High humidities suppress small ion
mobilities slightly. Large ions
have mobilities of only about 4 × 10-7
at sea level, their sluggishness being due to their great mass.
Wait, C. R., and W. D. Parkinson 1951. Compendium of Meteorology. 124–125.
Israël, H. 1951. Compendium of Meteorology. 146–147.
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