INSTRUMENTS, OBSERVATIONAL PROCEDURE, AND CONSTANTS 



17 



preliminary quantity for use in subsequent determination 

 of the radium-emanation content. This determination, 

 which necessitates a careful analysis of the curve ob- 

 tained by plotting tj against time, was carried out at 

 Washington rather than on ship. 



The value of t) is computed from the formula 



7} = C5V(t"^ - t-l)/300We 



where C is the capacitance of the apparatus; 6V is the 

 voltage change corresponding to the change in deflection 

 of the electrometer fiber over a selected range, 6, of 

 scale divisions; r is the time in seconds for the fibers 

 to move over the selected range, 6; t is the leakage cor- 

 rection based on a leak-test made before the copper foil 

 is introduced into the ionization chamber and derived as 

 explained on page 8 under computation of conductivity; e 

 is the electronic charge; and W is the number of cubic 

 centimeters per second of air drawn past the collecting 

 cylinder during the period of collection of active deposit. 

 For cruise VII the capacitance, C, was taken as 8.9 

 cm for radioactive content apparatus 4, as compared 

 with 8.7 cm used for cruise VI. Twenty measurements 

 of radioactive content were obtained on as many days, 

 during the period between September 22 and November 

 18, 1929. The observational procedure was such that the 

 copper foil was exposed in the collecting apparatus for 

 3600 seconds, during which time air was flowing past the 

 foil at the rate of approximately 130 liters per second. 

 The high potential generator maintained the foil within 

 the range of 1500 to 3000 volts, negative with respect to 

 the air -flow tube, for each collecting period, with the 

 most frequently obtained potentials lying in the range 

 between 2000 and 2500 volts. 



After transfer of the foil from the collecting appa- 

 ratus to the ionization chamber, observations with the 

 latter were made over a period of two to three hours, 

 during which time five or six, and occasionally more, 

 sets of measurements were made of the ionization pro- 

 duced by the active deposit. From these several sets of 

 data the form of the decay curve could be obtained. 



Owing to a late beginning of the observations of 

 radioactive content on cruise VII and the sudden ending 

 of the cruise, only twenty measurements were obtained, 

 as has been mentioned earlier. One measurement was 

 made September 22, 1929, and the remaining between 

 October 8 and November 18, 1929, all in the Pacific 

 Ocean. To assemble these into groups of about ten for 

 the purpose of drawing an average decay curve for each 

 group, as was done for the data from cruises IV, V, and 

 VI, would yield only two or three determinations of em- 

 anation content. This number is small compared with 

 the thirty-two derived values of emanation content for 

 the Pacific Ocean obtained from 271 measurements 

 made with apparatus 4 on cruises IV, V, and VI. Fur- 

 thermore, the measurements over all oceans on these 

 cruises totaled 497, and the derived values of emanation 

 content totaled 63. These have been tabulated and dis- 

 cussed by Swann (13) and Mauchly (14). The two or 

 three values to be derived from cruise VII measure- 

 ments have been considered, therefore, to be too few to 

 warrant the lengthy and involved analysis which would 

 be required to obtain them, and too few to add signifi- 

 cantly to the information obtained from the work of pre- 

 vious cruises. The data from this part of the program 

 of cruise Vn accordingly are being omitted from the 

 present publication. 



