In gaseous electric conduction, the average velocity with which a given ion drifts through a specified gas under the influence of an electric field of unit strength. Mobilities are commonly expressed in units of meters per second per volt per meter (m s−1(V m−1)−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 m s−1(V m−1)−1 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 m s−1 (V m−1)−1 at sea level, their sluggishness being due to their great mass.