126 
When the large ions are numerous, then in accordance 
with the assumptions made in deriving (5), 
1 
a Se (9) 
Over the oceans the value of 7, will generally be around 
5 or 6 min. Over land, in areas where the large ions are 
few, the mean life 7, may be expected to be only 10 or 
20 per cent of the above. In localities where large ions 
are numerous, the value of 7, will be still smaller, prob- 
ably only 5 per cent or so of the ocean value. 
The mean life ty of the large ion is given approxi- 
mately by the equation: 
1 
M121 : 
iN 
Over the oceans and over land where the large ions are 
not very numerous, the value of ty may be between 15 
and 20 min. In areas where the large ions are numerous, 
the value of ty may easily exceed an hour. 
Because of the relatively short life of the small ion, 
the small-ion concentration may be expected to follow 
with little lag those factors which tend to produce a 
change in the concentration. Much greater lag may be 
expected in the changes of large-ion concentrations. 
Outstanding Problems in the Field of Atmospheric Ions 
A number of investigations have been carried out for 
the purpose of evaluating the factors which control or 
regulate the small-ion content of the lower atmosphere. 
There is growing evidence that some of the factors, for 
example the value of the combination coefficients be- 
tween the small ions and the charged and uncharged 
condensation nuclei, may vary from place to place. This 
is probably due to a difference in character of the nuclei 
which results in a difference in nuclei size. A close 
correlation would therefore probably be found between 
the values of the combination coefficients and the mobil- 
ity of the large ion. In future work, a recognition of this 
possibility may assist in harmonizing results which 
otherwise might appear to be inconsistent. 
Another small-ion regulating factor which is difficult 
to evaluate is q, the rate of ionization of the atmosphere. 
Probably the most promising method of evaluating this 
factor is through the use of a thin-walled ionization 
chamber. This cannot be accomplished, however, with- 
out certain difficulties. Cognizance must be taken of 
the low radioactive content of the air within the cham- 
ber compared to that of the outside air and of the fact 
that the ionization within the chamber will depend 
upon the wall thickness and upon the particular voltage 
applied to the central electrode. The latter arises from 
the fact that complete saturation is never achieved, due 
probably to columnar ionization inside the chamber. 
A full discussion of this problem is not possible within 
the limits of this article. 
It is quite probable that meteorological conditions 
play an important role in altering the efficiency of some 
or all of the small-ion regulating factors. Temperature, 
humidity, and pressure of the atmosphere, for example, 
are likely to exert an influence on such factors as com- 
ATMOSPHERIC ELECTRICITY 
bination coefficients, recombination coefficients, and the 
mobility of ions. There is urgent need for careful experi- 
ments designed to secure much-needed information 
along such lines. 
Multiply charged large ions should be examined as to 
regularity of, and conditions of, occurrence and as to 
their effect on large-ion mobility and on establishment 
of small-ion equilibrium conditions. 
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