the atmosphere, the amount of excited activity I_{t}, produced on a wire exposed under given conditions, will, after exposure for a time t, be given by I_{t} = I_{0}(1 - e^{-λt}), where I_{0} is the maximum activity on the wire and λ is the constant of decay of the excited activity. Since the activity of a wire after removal falls to half value in about 45 minutes, the value of λ is 0·92 (hour)^{-1}. Some experiments made by Allan[1] are in rough agreement with the above equation. Accurate comparative results are difficult to obtain on account of the inconstancy of the radio-activity of the open air. After an exposure of a wire for several hours, the activity reached a practical maximum, and was not much increased by continued exposure.
We have seen (section 191) that the carriers of the active deposit of radium and thorium move in an electric field with about the same velocity as the ions. We should expect therefore that a long wire charged to a high negative potential would abstract the active carriers from the atmosphere for a considerable distance. This does not appear to be the case, for Eve (see section 281) has found that the carriers are only abstracted from the air for a radius of less than one metre, for a potential of the wire of -10,000 volts. It seems probable that the carriers of the active matter are deposited on the numerous fine dust particles present in the air and thus move very slowly even in a strong electric field.
The amount of excited activity produced on a wire, supported some distance from the surface of the earth, should increase steadily with the voltage, for the greater the potential, the greater the volume of air from which the radio-active carriers are abstracted.
The presence of radio-active matter in the atmosphere will account for a considerable portion of the ionization of the air observed near the earth. This important question is discussed in more detail in section 281.
275. Radio-activity of freshly fallen rain and snow.
C. T. R. Wilson[2] tried experiments to see if any of the radio-active
material from the air was carried down by rain. For this
purpose a quantity of freshly fallen rain was collected, rapidly
