MEASUREMENT OF ATMOSPHERIC ELECTRICITY 
resistance, serves as an index of the efficiency of a col- 
lector. The value of « fluctuates from approximately 
GAGA y VILLAS SS 1 VU LAL Z 
Fic. 8.—Hlectric field conditions produced by a collector. 
unity for glow collectors to 50-100 X 10-” for highly 
radioactive collectors. 
The “external resistance,” in other words, the resis- 
tance of the insulation of the collector from the ground, 
must be great compared to the transition resistance of 
the collector, as otherwise its readmgs become inaccu- 
rate. Attempts to operate the collector ‘“‘short-circuited”’ 
were found to be subject to disturbances, for example, 
by the wind, and should therefore be avoided [77]. 
Exact field measurements can only be performed by 
means of the Wilson test-plate on completely level 
terrain. Any other type of arrangement disturbs the 
field and yields only more or less acceptable approxi- 
mations. The so-called technique of reduction to the 
free plane can only be used as an approximation [16]. 
For this reason a comparison of “absolute values” of 
the field will always remain somewhat problematical. 
Therefore, consideration of the relative periodic and 
aperiodic variabilities of the electric field is funda- 
mentally more important. 
With respect to the selection of “undisturbed days” 
in the treatment of recorded data, see, among other 
sources, Israél] and Lahmeyer [72]. 
The Vertical Current 
The difficulties involved in measuring and recording 
the vertical electric current stem from the generally 
minute current density of about 10—” amp m~ and the 
disturbing effects of the electrostatic inductance of the 
field or of its changes on the receiver system. Galvano- 
metric methods are applicable only to measurements of 
vertical currents intensified by thunderstorms. There- 
fore, accumulation methods (accumulation of inflow of 
charges over a given time) are used in most cases 
(Ebert [34], Simpson [128], and Wilson [146-149]). The 
disadvantage of these methods is that in the accumu- 
lation period the potential of the collecting body changes 
somewhat. Simpson circumvents this disadvantage by 
continuous draining of the charges by means of a water- 
dropper collector, whereas Wilson obviates the diffi- 
culty by a compensation method (see Fig. 9). Methods 
149 
for continuous recording are described by Scrase [125] 
(accumulation method, quadrant electrometer) as 
shown in Fig. 10, and by Kasemir [77] (direct recording 
by use of a d-e amplifier). Scrase compensates for the 
effects of field fluctuation by using a supplementary 
field collector connected to alternate quadrants in the 
quadrant electrometer, whereas Kasemir largely sup- 
D 
ELM. 
7 VA 
Fic. 9.—Diagram of the Wilson test-plate method (K is a 
variable condenser and HLM is the electrometer). 
y J Af f 
Wied LAAT 
Fic. 10. Schematic diagram of the vertical-current recorder 
after Scrase (/ is the radioactive collector, P is the test plate, 
and HLM is the quadrant electrometer). 
presses these effects by increasing the capacitance. The 
vertical current can also be computed from the poten- 
tial gradient and the conductivity, according to equa- 
tion (6). 
The Space Charge 
Measurement or recording of the space charge is 
made by means of the following methods: The cage 
method consisting of the measurement of the potential 
difference between the wall surface and the center of 
a wire cage whose volume ranges from approximately 
one to several cubic meters [75, 76]; the method of the 
change of the potential gradient with altitude according 
to Poisson’s equation [13, 97, 145]; Obolenski’s filter 
method [102]; or the method of synchronous counts of 
positive and negative ions [53, 82]. The cage method is 
disturbed in most cases by Volta effects [15]. 
