62 McCiettanp—lJonization in Atmospheric Arr. 
number comes out surprisingly small; values were found varying from 13 to 40 
ions per cubic centimetre; that is, on some days as small as the number of ions 
produced per second im a closed vessel. 
As Rutherford himself shows, we should expect that the total ionization at any 
instant would be very much greater than the number of ions produced per 
second. 
If recombination were the only source of loss of conductivity, the number (7) 
of ions per c.c. would increase until the rate of production (q) is equal to the rate 
of loss,-or 
gq = an 
where a is a constant. 
The constant a has been determined by various experimenters in the somewhat 
similar case of ionization by Réntgen rays, and we may take it as 3500 #, where 
E is the charge on the ion, or 8-4 x 10. Substituting this value for « and for ¢ 
its observed value in closed vessels, viz. 15, we obtain 
nm = 3800. 
Of course the value assigned to a may be considerably in error, when applied to the 
case under consideration, and there are other sources of loss of ionization in addi- 
tion to recombination. Besides, if a considerable percentage of the ionization 
inside closed vessels is due to a radiation from the walls of the vessels, as may 
follow from the work of Strutt and M‘Lennan, the rate of production of ions in 
such vessels may be greater than in the same volume of free air. Still, we should 
expect the number of ions per ¢.c. near the surface of the Earth to be much 
greater than the number produced per ¢.c. per second in a closed vessel. ‘T’o test 
this point some observations have been made with the apparatus described in 
fig. 1; the plates between which the current passes were freely exposed to 
the air of the room, the cover PQ having been removed. 
If the room is fairly air-tight, the current, as a rule, rapidly diminishes from 
the value it has when fresh air has just been admitted, the rate of production of 
ions being less than the loss. Cases occurred in which this diminution was not 
so marked; this occurred where the ionization in the free air was below the 
average. Observations were made with the room not closed, doors and windows 
being opened in such a manner as to maintain a steady renewal of the air without 
any appreciable draught. 
With such an arrangement an electrometer deflection of as much as 20 scale 
divisions in 4 minutes was often obtained. The electrometer gave a deflection of 
60 scale divisions for ene volt potential difference between the pairs of quadrants, 
and the capacity of the electrometer and connected system was 62 electrostatic 
