66 LYNN C. HOWELL 
stant with time. The sensitivity is such that the readings can usually 
be taken over time intervals sufficiently short so that the temperature 
changes are too slight to change appreciably the deflections of the 
fibers by expansion or contraction of the frame holding the fibers. 
In the field procedure, readings taken with a stop-watch, of the 
discharge of the fibers, usually over a range of four divisions on a roo- 
division scale in the eye-piece, are begun immediately after the soil 
gas is sucked into the chamber from a hole bored in the ground with 
an augur. The pumping is usually maintained uniform by making a 
given number of strokes of the pump. These readings are repeated 
until a fairly constant value is found. The thoron in the soil gases at 
some points causes a more rapid discharge in the beginning, but since 
it decays very rapidly with a half-life of 54.5 seconds, its effect is soon 
negligible. There is, of course, an increase in activity while the active 
deposit is coming into equilibrium with the radon, but this increase is 
fairly slow after a short time. 
RESULTS 
As an example of consistency which could be obtained under favor- 
able conditions, the following data are presented which show the rates 
of discharge at one station on different days. 
Rate of Discharge 
Date Divisions per Second 
Dec. 8 -22 
Dec. 21 a2: 
Dec. 23 Siig 
Dec. 29 arr 
Dec. 31 -II 
SNE Bry, 
The first five readings were taken with gas taken from different holes 
spaced along a line with an interval between holes of the order of 4 
feet. Even the discrepancy between the two extreme readings is not 
serious, since in a field survey, a “radioactive low” reading is of an 
entirely different order of magnitude from a “radioactive high” read- 
ing. 
The following data were obtained in another area at several differ- 
ent stations, the interval between stations in general varying from 
about 200 to 1,500 feet. The readings at these stations were repeated 
after the capacity, and hence the sensitivity, of the electroscope had 
been changed. A new hole was drilled at each station within a few feet 
of the original hole for these check readings which were made several 
days after the first readings. 
588 
