116 
3. The size of a charge-cloud which has developed to 
the stage where lightning of average intensity may oc- 
cur is comparable to that of a sphere of at least 1-km 
diameter and probably is several times that dimension. 
For a cloud of smaller size, having a charge of 20 
coulombs, the dielectric strength would be exceeded in 
air containing water drops, ice pellets, or crystals. 
4. The region of primary electrical activity (loca- 
tion uncertain) where the initial charge separation and 
the large-scale separation occur jointly, doubtless has a 
cross section less than that of the charge-clouds. 
5. The average cloud charge developed between 
lightning flashes is 20 to 30 coulombs. 
6. The average rate of regeneration for charge-clouds, 
after lightning discharges, is about 4 amp but values as 
ereat as 20 amp are sometimes indicated. In regenera- 
tion the charge approaches an equilibrium value in 
approximately an exponential manner. One may ac- 
cordingly speak of a relaxation time. An average value 
of the latter is about 5 sec. The relatively small depar- 
ture of individual values from this average seems to 
have some significance. 
7. No convincing evidence has been found of ab- 
normally large electrical conductivity of the air above 
thunderclouds or under thunderclouds at the earth’s 
surface. 
These items will be referred to by number in later 
paragraphs. 
The initial separation of charge has been attributed to 
various processes: The disruption of drops of pure water 
will effect a separation of electric charge with positive 
charge on the drops and negative charge in the air (a 
few thousandths of one per cent of some salts as an 
impurity annuls this effect). G. C. Simpson developed 
a theory which depended on this process. This theory 
was in vogue for more than a decade, but it became 
untenable when it was found that the positive charge- 
cloud is usually above the negative cloud. 
Ton-capture is the fundamental process in a theory 
developed by C. T. R. Wilson. This process may be 
illustrated as follows. A water drop located in the nor- 
mal electric field will have a positive charge induced on 
its lower surface and a negative charge on its upper 
surface. If the atmosphere contains ions (large ions as- 
sumed by Wilson), the positive ions will drift downward 
and the negative ions upward both with a velocity 
much less than that of the falling drop. The negative 
ions will be attracted by the positive charge on the 
bottom of the drop and those located in or near the 
path of the drop will be captured. The positive ions, 
however, will be repelled by the charge on the bottom 
of the drop and escape capture because when such an 
ion is in position to be attracted by the negative charge 
on the top of the drop, that attraction is not great 
enough to effect a capture. In this way larger drops may 
acquire a net negative charge. The larger drops with 
their negative charge will accumulate in the lower part 
of the thundercloud while positive charge, without in- 
volving the ion-capture process, will accumulate at a 
higher level. This is a favorable aspect of Wilson’s 
theory, since it is in accord with the observed orienta- 
ATMOSPHERIC ELECTRICITY 
tion of the principal charge-clouds. Although there are 
reasons for thinking that pellets or crystals of ice may 
also capture ions in this way, this requires more in- 
vestigation if the ion-capture process is to be considered 
active in the region of sub-zero (centigrade) tempera- 
ture where the principal charge-clouds are found. This 
ion-capture process requires that a relatively large con- 
centration of ions, preferably of low mobility, obtains 
in the electrically active part of a storm cloud. Such a 
condition has not yet been observed, except as a local 
phenomenon of relatively rare occurrence, namely at 
times when glow or brush discharge (St. Elmo’s fire) 
occurs. Unless there is some potent source of such ions 
other than the glow or spark discharge, Wilson’s 1on- 
capture process can act only in a secondary role after 
fields capable of initiatmg glow discharge have been 
developed by another mechanism. 
The collision of ice particles has been suggested as a 
process for the initial charge separation. Charge de- 
veloped by drifting snow (positive in the air) is more 
conspicuous than that for splashing rain but it 1s doubt- 
ful whether this process is effective in the atmosphere 
remote from the earth’s surface. 
Evaporation, condensation, and sublimation acting 
singly or in combination have been assumed as primary 
processes [11, 16]. Findeisen’s experiments [11] mdi- 
cated that these processes are much less effective than — 
is the process which is involved in the formation of sleet 
(Vergrawpelung). Dinger and Gunn [9] found an effect 
associated with the freezing of water and the melting of 
ice. Gunn [16] also assumed that a raindrop is essen- 
tially a concentration cell, and he indicated how it may 
acquire a net negative charge during condensation and 
a net positive charge during evaporation. Frenkel [13] 
developed a theory in which the electrokinetic potential 
of a raindrop, or cloud droplet, was proposed as the 
basis for the initial separation of charge. The funda- 
mental element of this theory is similar to that of 
Gunn’s theory. 
A very active process of charge separation was dis- 
covered by Workman and Reynolds [24]. This occurs 
during orderly freezing of water in which very small 
quantities of certain salts are dissolved. The effective- 
ness and the direction of the process depend upon the 
concentration and nature of the solute. In most solu- 
tions for which a pronounced effect was reported, nega- 
tive ions are captured by the ice. A prominent exception 
to this was found in solutions of the ammonium salts 
for which the solid phase acquires a positive charge. 
The charge developed by this process in the freezing 
of one cubic centimeter of water is extraordmarily large 
for a number of solutions which were examined. For 
solutions of NaCl the greatest effect was for an 0.83 
> 10-? normal concentration. This gave a charge of 
9.2 X 10! stat coulombs from the freezing of one cubic 
centimeter of solution. The solid phase acquired a nega- 
tive and the liquid phase a positive charge. The largest 
value reported was for a CsF solution of 11.1 X 10 
normal concentration. This is 44 X 104 stat coulombs 
em-*. These values are of a much greater order of mag- 
nitude than the largest value reported by Lenard and 
