McCiettanp—Jonization in Atmospheric Air. 61 
4.—Amount of Ionization in Atmospheric Air. 
It is possible that the amount of ionization in atmospheric air may be of great 
importance as a meteorological factor, and it is, therefore, important to know how 
the amount of ionization changes from time to time. We know, for example, 
from the work of C. T. R. Wilson, that charged particles, such as the ions result- 
ing from the ionization of a gas, can act as centres of condensation, and take the 
part formerly assigned entirely to dust particles, of acting as condensation nuclei. 
It is, therefore, obvious how important may be a supply of such ions in causing 
condensation of water-vapour in the form of rain. 
Again, the amount of ionization in the atmosphere, with the accompanying 
currents to or from the Earth’s surface, may be of importance, as affording an 
explanation of the variations of the Earth’s magnetism. 
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There are various methods we may follow in observing the amount of the ioniza- 
tion in the air. We may enclose the apparatus in an air-tight case, and observe the 
current. In this case we get the rate of production of ions, and not the actual amount 
at any time. The actual amount of ionization at any time depends not only on the 
rate of production of ions, but also on the rate of loss resulting from recombination 
of positive and negative, and from other causes. Wilson’s experiments give the 
rate of production of ions in a closed vessel as about 15 per second per cubic centi- 
metre. Rutherford has made some experiments, in which he draws a stream of 
air at a known velocity through a tube and through his measuring apparatus, and 
the electric current observed gives the ions in a cubic centimetre of air. This 
