Dadourian — Radio-activity of Underground Air. 17 



A circular hole, A A, 50 cm in diameter and 200 cm deep, was 

 dug in the ground. At the top of the cavity was plastered a 

 rectangular board, CC, with a circular opening 4rO cm in diam- 

 eter. A sheet metal cover, D, screwed on to the board, over a 

 rubber gasket, served as a partition between the outside air and 

 the air inside the cavity. £B is a wooden frame, which con- 

 sists of a 175 cm long rod, provided with a circular board, of 

 30 cm diameter, at each end. A piece of copper wire, J mm thick 

 and about 50 meters long, 

 was wound about the frame, 

 so as to form a cylinder. 

 The wire was put into this 

 form in order to have it as 

 near the walls of the cav- 

 ity as possible, also to secure 

 a larger field. This cylinder 

 of wire was then hung from 

 a hook, E, which was insu- 

 lated from the sheet metal 

 cover and was connected to 

 the negative terminal of a 

 Wimshurst machine,the other 

 terminal being to earth. 



The wire was charged for 

 three hours, keeping a par- 

 allel spark- gap of about 2 mm . 

 In the meanwhile the air in 

 the cavity was sucked out by 

 means of a filter pump, con- 

 nected to the stopcock, F, in 

 order to bring fresh under- 

 ground air into the field of 

 the negatively charged wire. 

 At the end of three hours the 

 wire was removed from the 

 wooden frame and was put 

 into a testing vessel. This 

 was a cylindrical condenser 



which consisted of a galvanized sheet-iron cylinder and a cen- 

 tral brass rod, insulated from the cylinder and connected to 

 one pair of the quadrants of an electrometer.* The 

 needle of the electrometer and the testing cylinder were con- 

 nected to the negative electrode of a set of dry cells giving 

 a potential difference of 105 volts, the other electrode of the 



* For a description of the electrometer, the testing cylinder and connec- 

 tions, see February (1904) number of this Journal. 



Am. Jour. Scl— Fourth Series, Yol. XIX, No. 109.— January, 1905. 



Fig. 1, 



f 



