I] IONIZATION THEORY OF GASES AT 
The negative ion thus more readily acts as a centre of conden- 
sation than the positive ion. The greater effect of the negative 
lon in causing condensation has been suggested as an explanation 
of the positive charge always observed in the upper atmosphere. 
The negative ions under certain conditions become centres for the 
formation of small drops of water and are removed to the earth by 
the action of gravity, while the positive ions remain suspended. 
With the apparatus described above, it has been shown that 
the positive and negative ions are equal in number. If the ex- 
pansion is large enough to ensure condensation on both ions, the 
numbers of drops formed on the right and left of the vessel in 
Fig. 7 are equal in number and fall at the same rate, 2.e. are equal 
in size. 
Since the ions are produced in equal numbers from a gas 
electrically neutral, this experiment shows that the charge on 
positive and negative ions is equal in value but opposite in sign. 
36. Charge carried by an ion. For a known sudden ex- 
pansion of a gas saturated with water vapour, the amount of water 
precipitated on the ions can be readily calculated. The size of the 
drops can be determined by observing the rate at which the cloud 
settles under the action of gravity. From Stokes’ equation, the 
terminal velocity w of a small sphere of radius r and density d falling 
through a gas of which the coefficient of viscosity is w is given by 
nae 
Beas 
where g is the acceleration due to gravity. The radius of the drop 
and consequently the weight of water in each drop can thus be 
determined. Since the total weight of water precipitated is known, 
the number of drops present is at once obtained. 
This method has been used by J. J. Thomson! to determine the 
charge carried by an ion. If the expansion exceeds the value 1:31, 
both positive and negative ions become centres of condensation. 
From the rate of fall it can be shown that the drops are approxi- 
mately all of the same size. 
* Plul. Mag. p. 528, Dec. 1898, and March, 1903. Conduction of Electricity 
through Gases, p. 121. 
