108 
be compared even though made by different observers 
and at different places. 
During this latter period, measurements have been 
made on most of the representative areas of the earth, 
including a number of series for the polar regions, many 
widely distributed measurements on the oceans, and 
measurements made during balloon flights (all im 
Europe). Continuous registration has been used at most 
land stations since about 1910 and was also successfully 
employed at sea during Cruise VII of the Carnegie. The 
features of the electric field which are clearly shown by 
these data are as follows for fair-weather conditions. 
The electric field strength E is negative, or the potential 
gradient is positive, wherever and whenever fair weather 
prevails. The exceptions to this are rare and transitory. 
For example, a positive field (negative gradient) is 
registered while a dust whirl passes near the station. 
An example of such an effect appears on Fig. 5 between 
1440™ and 14550™. Since at the surface of a conductor 
E = —0V/dZ = 4c, where dV/0Z is the potential 
gradient along the outwardly directed normal and o 
is the surface charge density, one concludes that the 
charge of the earth in fair-weather areas is negative and 
that the potential of the air increases with altitude. 
The potential gradient (or —H) at the surface varies 
considerably from place to place. The average potential 
gradient at sea during Cruise VII of the Carnegie was 
130 v m~. At some places on land an average consider- 
ably less than 100 v m™ has been reported, but values 
larger than that for the oceans are prevalent in densely 
populated areas: at the Kew Observatory near London 
the average value exceeds 300 v m7. In the polar re- 
gions and in open country, far from sources of atmos- 
pheric pollution, the average is about the same as that 
for the oceans. Apparently a value of about 130 v m™ 
is a fairly representative average for the preponderant 
part of the earth’s surface. 
The potential gradient decreases with altitude. The 
measurements of gradient made on 57 balloon flights, 
mostly over central Europe, as summarized by Schweid- 
ler [17], satisfy the following empirical equation: 
aV/dZ = 90 exp (—3.5z) + 40 exp (—0.23z), (4) 
where 0V /dZ is expressed in volts per meter for z, the 
altitude in kilometers. There is a sharp decrease from 
the surface up to about 1-km altitude followed by a 
slower decrease. The value at z = 0.5 km is less than 
half the value at the surface, and at z = 9 km it is less 
than 4 per cent of the surface value. The data for alti- 
tudes less than 1 km are scattered much more about 
the general trend than are those for higher altitudes. 
This may be due to variable pollution in the air near the 
earth, a circumstance that is indicated by other types of 
atmospheric electric observations such as the contrast 
between measurements of H made on the Wiffel Tower 
and of those made at a neighboring ground station. 
A decrease of H with altitude, if widespread and of 
the magnitude and character indicated by these data, 
doubtless is attributable chiefly to a corresponding in- 
crease of air conductivity. For these conditions \H = 2 
is independent of altitude if the electric space-charge 
ATMOSPHERIC ELECTRICITY 
density p does not vary with time and provided convec- 
tion plays a negligible part in the transport of electricity 
in the atmosphere. The latter conditions doubtless are 
satisfied during most fair weather. The drifting of snow, 
or of dust, however, is accompanied by the generation 
and convection of electric charge and although this is a 
source of conspicuous modifications of the electric field 
near the earth on some occasions, for average fair- 
weather conditions the magnitude of electric convec- 
tion-current density calculated on the basis of available 
data is negligibly small. 
Measurements of both \ and # were made on only a 
few balloon flights. For these the product \# was not 
invariable with altitude, but this result is probably at- 
tributable to a variation of the air-earth conduction 
current with either, or both, time and position [10]. 
The potential gradient of fair weather also decreases 
with altitude in the lowermost few meters owing to a 
depletion of the negative ions in this region, the so- 
called electrode effect. Under the action of the electric 
field, negative ions drift away from the earth and since 
such ions are not emitted from the earth at an appreci- 
able rate, the flux of negative ions, and accordingly the 
concentration, must vanish at the surface. But for 
steady conditions, the flux of positive ions from air to 
earth is nearly continuous in the lowermost few meters, 
and at the air-earth boundary is equal to the total elec- 
tric flux at higher levels. Two attending conditions are 
(1) positive ions predominate or a positive space charge 
prevails in air that is contiguous with the earth, and 
(2) the potential gradient decreases with altitude. These 
general aspects of the electrode effect may be modified 
or obscured when either or both the electric convection- 
current component and the displacement-current com- 
ponent are appreciable, or when the vertical distribu‘ion 
of air conductivity near the surface is abnormal. The 
conclusion that the electrode effect is a universal char- 
acteristic of the electric field of the atmosphere depends 
chiefly on the observed fact that \1/)2 is usually greater 
than unity in fair weather and that it mereases as the 
gradient increases. 
The temporal variations of potential gradient are usually 
of complex origin, however there are types which may 
be classified on the basis of origin, as follows. From the 
several equivalent expressions for air-earth current den- 
sity, namely +H and —!}‘/r, one obtains the relation 
13) = NV), (9) 
which is valid except for the rather unusual condition 
that 2 is dependent upon altitude, or that the displace- 
ment current is appreciable. On this basis the follow- 
ing four types of change in # may be adopted: type 
(a) in which E is independent of V, \, and 7, but d de- 
pends upon Z#; type (6) in which # « 1/), with V and 
r constant; type (c) for which # « 1/r, while both X 
and V are constant; type (d) m which # « V, while \ 
and r are constant. In this discussion # and X are gener- 
ally used to denote values observed in air practically 
contiguous with the earth, V denotes the potential of 
the ionosphere (or upper stratosphere) relative to the 
earth, and r denotes the columnar resistance—the elec- ~ 
