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Reports on Special Researches 



Observations on the amount of radioactive material in the atmospliere were made by 

 Elster and Geitel's method.^ Except on one occasion, the length of the collecting- wire 

 was always 7 meters, and it was usually exposed for about 1 hour. Wliile the collecting- 

 wire was being exposed, the testing-electroscope was charged and the rate at which its 

 potential fell noted, in order to measure the natural leak. The latter was almost invariably 

 found to decrease with tune, sometimes very regularly, and was generally nearly constant 

 and small by the time the radioactivity test was begun. After the wire was inserted, the 

 electroscope deflections were read at frequent intervals and the fall of the potential with 

 time thus obtained. The results were plotted on cross-section paper, the ordinates repre- 

 senting potentials and the abscissae times. A smooth cui've was drawn through the 

 points thus plotted, and from this smoothed curve an activity curve was di-awn, the ordinate 

 at any point of which was proportional to the gradient of the first curve at the time repre- 

 sented by the abscissa. The times were measured from the time of discharge of the wire. 



On days when a comparatively large quantity of deposit was collected and the condi- 

 tions of observation were good, the curves obtained for the decay of the activity were fairly 

 regular and similar m character. The deposit appears to be derived from radium emana- 

 tion. Table 73 shows roughly the relative amounts of activity collected on different days. 

 The activity is here measured by the fall of potential in volts, produced in the electroscope, 

 by the deposit in 1 hour, starting 15 minutes after the discharge of the wire. The capac- 

 ity of the electroscope was about 15 cm. 



Table 73. — Relative Amowils of Radioaclivity on the Carnegie's First Cruise. 



On December 16, 1909, latitude 20?0 N, longitude 314?2 E, the wire was charged to 

 a high positive potential for 1 hour, but no active matter appeared to have been collected. 



On January 1, 4, and 12, 1910, the charged wire was exposed for a long period, in order 

 to detect if possible the presence of thorium products in the atmosphere. On January 1 , 

 after 5 hours, there appeared to be still left on the wire about 3 per cent of the activity 

 exhibited 10 minutes after discharging. This effect, however, may have been due to an 

 increase of the natural leakage which was liable to take place in the increased dampness 

 after nightfall. Unfortunately no determination of the leakage could readily be made at 

 the close of this experiment. On the other two days no sign of activity could be detected 

 after a few hours. On January 12, 1910, the observations were taken in Hamilton Harbor, 

 Bermuda, under good conditions, so that a very slight activity should have been detected. 



'See digest of P. H. Dike's report on the third cruise of the Galilee, page .'i64. 



