880 MISCELLANEOUS GEOPHYSICAL METHODS [Chap. 12 



where e is the elementary quantum of electricity or 4.77- 10"^° e.s.u.'s, 

 V the volume of the chamber in cm^, and dE/dt is in volts ■sec~\ 



In the calibration of the equipment, it is first necessary to be certain 

 that saturation current is measured at all times. For this purpose the 

 voltage loss dE/dt is determined for a given ionization and for various 

 voltages. This gives the voltage range for which the current is constant. 

 According to Kohlhoerster/* a 4-liter chamber has its saturation current 

 for potentials exceedmg 100 volts and a 2-Uter chamber for potentials 

 greater than 80 volts. If these measurements are to be made accurately, 

 the air in the chamber must be completely dry. The same is required of 

 the field observations. The volume V is determined by filling the chamber 

 with water and weighing it. The capacity C may be obtained by cali- 

 brating the chamber with a standard UaOs preparation of known disper- 

 sion, usually rated for 1 cm capacity and 1 cm radiating surface. To 

 avoid infesting the chamber with radioactive decay products, a duplicate 

 chamber may be tested instead. Another method of measuring the ca- 

 pacity uses a UsOs preparation and an additional condenser of known 

 capacity.^^ 



It has also been suggested that bore holes be used as ionization chambers 

 and that a dispersion rod with electroscope be introduced after stationary 

 conditions have been established. 



(&) The activity of the soil-air emanation (mostly radon) may be meas- 

 ured with the instrument shown in Fig. 12-4.^^ A hole several feet deep 

 and about 1-1 1 inches in diameter is pounded down in the surface soil 

 with a steel bar. A pipe having a bulge midway of its length to prevent 

 the influx of atmospheric air is inserted in the hole. This pipe is con- 

 nected to the intake of a suction pump whose outlet in turn connects by a 

 rubber hose to the dryer and filter tube of the instrument. The soil air 

 is thus forced under pressure through the chamber. After several strokes 

 of the pump, the chamber is shut off by two cocks, and the measurement is 

 made in the usual manner by observing the dispersion rate. An alcohol 

 lamp is provided to obviate insulation difficulties resulting from moisture. 

 It is necessary to sample the soil air always at the same depth ; if possible, 

 soils of nearly identical consistency should be used and observations should 

 be taken within the same time interval after the air has been pumped into 

 the chamber. 



" W. Kohlhoerster, Phys. Zeit., 27, 62 (1926); 31, 280-288 (1930). 



15 Kohlhoerster, loc. cit.; V. F. Hess and A. Reitz, Phys. Zeit., 31, 284 (1930); 

 R. A. Millikan and G. H. Cameron, Phys. Rev., 31, 921 (1928); and A. Lomakin, 

 Inst. Pract. Geophys. Bull. 4, 151-156 (1928). 



i« J. Koenigsberger, Zeit. Prakt. Geol., 34, 187-190 (1926). 



17 R. Ambronn, Phys. Zeit., 28(12), 444-446 (1927). A similar apparatus is de- 

 scribed by A. Lomakin, op. cit., 3, 124-136 (1927). 



