102 
surface, or (2) by an indirect method? which involves 
the measurement of three factors: (a) potential gradient 
or electric field strength H, (6) the electrical conductiv- 
ity of the air attributable to the positive ions \, and 
(c) the conductivity attributable to negative ions )o. 
The electric current density then is obtained from the 
relation 
a = (\i + Yo). (1) 
The sum (A; + As) = 2X will here be called the total 
conductwity, or simply conductivity when the latter en- 
tails no ambiguity. The technique for making automatic 
registrations of these three factors is now more satis- 
factory than that for registration by the direct method. 
This, and the advantage for analytical purposes of 
knowing how the several factors vary, is the reason that 
the indirect method has been used in most long series of 
registrations. 
ELECTRICAL CONDUCTIVITY OF AIR 
The conduction of electricity in air and other gases 
became a concrete conception in the last years of the 
nineteenth century. Coulomb found in 1785 that an 
electrically charged body when exposed in air loses 
charge at a rate given by the law which bears his name. 
However, his discovery received little attention until 
1887 when W. Linss made measurements, two times 
each day for two years, of the proportional rate of 
dissipation, or the coefficient of dissipation a, of elec- 
tricity from a charged body exposed in the open air. 
The coefficient a is defined by Coulomb’s law, namely, 
dQ/di = —aQ, where Q represents the quantity of 
electricity on the body and dQ/di represents the rate 
at which charge is lost. These measurements showed 
that, im present-day terminology, (1) the electrical con- 
ductivity of air, which is roughly proportional to a, 
varies considerably from time to time, (2) it is greater 
in summer than in winter, and (8) during the year the 
conductivity on the average varies inversely as the 
potential gradient. This inverse relationship is fre- 
quently found. It implies that the electric conduction- 
current in fair weather tends to vary less than at least 
one of the two component factors, namely, potential 
gradient and conductivity. But at some places where 
the air conductivity is small the air-earth current den- 
sity is considerably less than normal. 
The conductivity of air was earlier thought to arise 
from’ the presence of impurities, such as particles of 
dust, smoke, fog, or water in its various forms, until J. 
Elster and H. Geitel, about 1895, from their numerous 
measurements of electrical dissipation, showed the re- 
verse, namely, that air generally conducts electricity 
best when pure and relatively dry. These observations 
were clarified when the conception of gaseous ions was 
introduced near the end of the nineteenth century. 
But how are ions formed in the open air? Elster 
2. Methods of measuring the elements of atmospheric elec- 
tricity are described in the article in this Compendium by H. 
Israé] entitled ‘Methods and Instruments for the Measurement 
of Atmospheric Electricity.” 
ATMOSPHERIC ELECTRICITY 
and Geitel, i search for an answer to that question, 
discovered that the air over land generally contains 
radioactive matter, that most of the important con- 
stituents of the earth’s crust contain measurable 
amounts of radioactive matter and that the former is 
doubtless derived from the latter. Since it had recently 
been found that radiations from radioactive substances 
form ions in the surrounding air, the conductivity of 
the air over land, but not of that over the oceans, 
seemed to be largely accounted for. 
The first clue of the ionizing agent which is active 
at sea appeared when Hlster and Geitel and C. T. R. 
Wilson in the first years of this century found that air 
from which radioactive matter had been carefully re- 
moved continued to be somewhat conductive. These 
observations apparently stimulated a series of investi- 
gations by other physicists, which eventually led to the 
discovery of what are now commonly called cosmic rays. 
That these ionizing rays are of extraterrestrial origin 
was first clearly indicated by observations made by 
V. Hess (1911) during ten balloon flights. These obser- 
vations were verified and extended by W. Kolhorster 
in 1913. 
Begining in 1915 many measurements of the cosmic 
radiation were made on all oceans during cruises of the 
Carnegie. These showed that the intensity is about the 
same at sea as at sea level on land. Furthermore, meas- 
urements over the open seas showed that there the 
radioactive content of air is not more than two per cent 
of that found on the average over land. Other measure- 
ments made on these cruises showed that the amount of 
radium in sea water, far from land, is less than one per 
cent of the amount found in the soil. The results of 
these observations strongly corroborated those of Hess 
and Kolhérster and showed that this radiation is doubt- 
less of universal distribution and constitutes the pre- 
ponderant ionizing agent over the oceans. 
Furthermore, balloon observations of the several fac- 
tors involved indicate that everywhere in the tropo- 
sphere and stratosphere, at altitudes greater than one 
or two kilometers, the air is ionized almost exclusively 
by the cosmic radiation. This radiation is, accordingly, 
an all-important factor in determining the character of 
the universal aspects of atmospheric electricity. 
But this statement does not apply in the region above 
the stratosphere, namely, the ionosphere extending up- 
ward from an altitude of 60 km. There the electrical 
conductivity is much greater than would prevail if 
cosmic radiation were the only ionizing agent. The in- 
tense ionization of the ionosphere is attributed to ultra- 
violet light and corpuscular radiation from the sun. 
Perhaps the comparatively great electrical conduc- 
tivity in the lower part of the ionosphere plays a part 
in promptly distributing the supply current from the 
thunderstorms to remote areas over the earth, but this 
has not yet been proven. This distribution may occur 
at a lower level. 
Since, aside from the possibility just mentioned, the 
ionosphere is presumably not mvolved in the phe- 
nomena which are usually regarded as belonging in the 
category of atmospheric electricity, the interesting elec- 
