UNIVERSAL ASPECTS OF ATMOSPHERIC ELECTRICITY 
trical resistance of a vertical column of air 1 cm? cross 
section extending from the observation point up to the 
ionosphere. 
Variations of the first three types generally are 
marked by local characteristics; many occur at irregular 
intervals but some follow a diurnal or other periodic 
routine, which varies much from place to place. The 
only thing in common among these variations is that 
usually factors of local origin tend to increase the gra- 
dient during daylight hours more than at night, but 
the contrasts in the amplitude and character of these 
variations at different places are remarkable. An ex- 
ample of the contrast between periodic local variations 
of this sort may be seen by comparing an electrogram 
for potential gradient (Fig. 3) obtained at the Watheroo 
Magnetic Observatory, Western Australia, with a cor- 
responding electrogram (Fig. 5) made at the Huancayo 
Magnetic Observatory near Huancayo, Peru (observa- 
tories established and operated by the Department of 
Terrestrial Magnetism of the Carnegie Institution of 
Washington). In the latter electrogram, the gradient at 
©. AUGUST 29, 1936 
a 5 2 16 
POSITIVE CONDUCTIVITY 
aA AliAs, AL SAL Latatn Wy ate a! Ke 44, a Mn aN re Bien, 
4 - . i ‘ - 
VERTICAL SCALE IN VOLTS FER METER 8 
Fic. 3—Electrograms, Watheroo Magnetic Observatory, 
8530 (local time) is about five times the value regis- 
tered two hours earlier. In the former, there is a rela- 
tively small and gradual increase of gradient, beginning 
at about 8" local time and contiuing throughout the 
daylight hours. The universal diurnal variation, which 
will be discussed later, is, of course, present in both 
these cases, but allowance for that would not change the 
order of contrast seen here. Analyses of such records 
from these two stations indicate to the author that the 
local aspect of the diurnal variation of potential gradi- 
ent at Watheroo is largely of type (c) while that for 
Huancayo is chiefly of type (b). 
At the Kew Observatory, where the influence of local 
air pollution is prominent, a semidiurnal variation of 
gradient of local origin appears with maxima at about 
8» and 21" local time. These apparently are a combina- 
tion of types (b) and (c) with type (6) dominant in the 
morning and type (c) coming into evidence later in the 
day [22]. The latter is an interesting example of a local 
influence which may be explained as follows. 
The introduction into the air in the general vicinity 
of the observatory of substances which serve as nuclei 
for the formation of large ions proceeds at an enhanced 
rate during the period 4" to 20". Near the surface the 
120° EAST MERIDIAN HOURS 
20 () : : 2 : : 8 
a Ee Ee pee, mi es 
{ ae 
‘ 
“VERTICAL SCALE IN UNITS OF a ESU 
Naar aaa Tt 
VERTICAL SCALE IN UNITS OF 10% EsUS 2 8 28 eK 
POTENTIAL GRADIENT 
a oe ue Feat o 
109 
concentration of these nuclei increases rapidly between 
65 and 8+ in winter (3 to 9" in summer) and as a conse- 
quence ) decreases; then as convection and turbulence 
increase during the day, the local contamination de- 
creases somewhat, and \ increases; but when the air 
becomes more stable in the evening, local contamina- 
tion increases and A decreases again until checked when, 
after 20", both the rate of introduction of the nuclei and 
their concentration decrease. Throughout the daylight 
hours the columnar resistance r increases to a maximum 
at about 19". This doubtless indicates that the content 
of nuclei in the air column has been increasing during 
this period. A decrease of r which occurs during the 
night is doubtless the combined effect of (1) decreased 
rate of supply, and (2) scattermg and other modes of 
dissipation of the nuclei. 
Potential gradient variations of type (a) are most 
prominent during stormy weather. Examples of such 
effects are exhibited in Figs. 4 and 6. These figures are 
reproductions of unretouched electrograms obtained at 
the Watheroo Magnetic Observatory and at the Huan 
Qe4 6 ee 
200 400 600 800 
“quiet” day. 
cayo Magnetic Observatory, respectively. Figures 3-6 
inclusive are samples selected from the thousands of 
such registrations made during the years 1922-46 at 
Watheroo and 1925-46 at Huancayo. Points for zero 
gradient are recorded each hour. Ordinates above this 
line of ‘‘zeros”’ correspond to positive values of gradient. 
During the 24-hr period of record shown in Fig. 4, three 
periods of rainfall were indicated by the station rain 
gauge. These are indicated at the top of the figure. The 
corresponding storm effects in potential gradient are 
exhibited in the lowest electrogram. Most of these were 
of moderate intensity on that day, and varied from 
positive to negative values of gradient. 
During the ten-minute period beginning at 18'10™ the 
gradient was entirely negative and at its greatest in- 
tensity exceeded 400 v m7, the limit of registration. 
During the period 08 to 050", several changes from 
negative to positive gradient occurred. The trace of 
these may not appear in the reproduction—the changes 
were rapid and the extreme values exceeded the limits 
of registration, namely, —400 and 700 v m~. But for- 
tunately, in cases where the trace for potential gradient 
is not clearly shown, the sign of the gradient can usually 
be ascertained by an examination of the traces for con- 
