ATMOSPHERIC ELECTRICITY IN HIGH LATITUDES. 83 



It would appear from these results as though the double daily period were confined 

 to places having a large daily temperature variation. The daily variation of the 

 temperature was much the greatest in Karasjok during the autumn three months : 

 the sun not rising during the winter three months, not setting during the summer 

 three months, and the snows still being on the ground during the spring, all tended 

 to keep the daily temperature variations low during these seasons. In all places 

 having a double daily period of the potential gradient the midday minimum is 

 always greater during the summer than during the winter, which supports the same 

 conclusion. 



ATMOSPHERIC RADIO-ACTIVITY. 



In 1901 ELSTER and GEITEL* made the very important discovery that the 

 atmosphere always contains more or less radio-active emanation. Since the discovery 

 several workers have repeated the observations and confirmed the results. During 

 the whole of 1902 ELSTER and GEITEL t made daily observations of the radio-activity, 

 and found that the amount of emanation in the atmosphere depends largely on some 

 meteorological conditions, such as the rising or falling of the barometer and tempe- 

 rature ; and, as a result of their work, made the suggestion that the emanation in the 

 air is supplied entirely by the radium or radio-active emanation contained in the soil. 



The method used by ELSTER and GEITEL to detect and measure the emanation in 

 the air, which has been adopted by other observers, consisted of stretching a wire 

 about 10 metres long between insulators in the open air. This wire was then charged 

 to a negative potential of between 2000 and 2500 volts. After the wire had been 

 exposed to the air at this potential for two hours, it was removed and wrapped round 

 a net cylinder fitting inside the "protection cylinder" attached to their dissipation 

 apparatus (specially closed at the bottom as well as the top for this measurement), 

 and the rate at which the electroscope discharged was determined. When one metre 

 of the wire discharged the electroscope one volt in one hour the atmospheric activity 

 was said to be unity and written A = 1. 



Using ELSTER and GEITEL'S method, I made observations of the atmospheric radio- 

 activity in Karasjok. I first started by making odd observations every now and 

 again, but found that the values obtained were so much higher than anything which 

 had up to that time been recorded that I determined to make a thorough investi- 

 gation of the matter. In December, 1903, 1 started a series of observations, observing 

 three times each day. As each observation occupied over two hours, it was impossible 

 to take them so frequently without interfering with my other work, therefore I 

 decided to take three observations each day for a month, then wait a month, then 



* ' Phys. Zeit.,' 2, p. 590, 1901. 



t ' Phys. Zeit.,' 4, p. 526, 1903. 



M 2 



