0. C. Lester — Emanation Electroscopes. 227 



Most of the work wa? done with vessels No. 2 and No. 3, 

 the latter being the most sensitive of the four. Nos. 6 and 7 

 were constructed after the experience of the first summer, and 

 are, on the whole, the most satisfactory. They were designed 

 to have approximately a volume of S'liters and a distance of 

 6^="" between the electrode and the outer wall. 



Each electroscope consisting of the common head and an 

 ionization chamber with its appropriate electrode was carefully 

 standardized a number of times by means of known quantities 

 of radium emanation obtained from pitchblende as suggested 

 by Boltwood."^ For this purpose some finely-ground pitch- 

 blende giving 2'10xl0~" curie of radium emanation per mg. 

 on direct solution was kindly furnished by Richard B. Moore 

 of the United States Bureau of Mines, Denver. The emana- 

 tion from several milligrams of pitchblende was introduced 

 into the electroscope and the activity observed every few min- 

 utes until it reached its maximum. In each case a complete 

 and typical activity-time curve was platted. Then from the 

 known amount of emanation present and the observed maxi- 

 mum activity, that fraction of a curie which will produce a 

 movement oF the leaf of L div. per min. at maximum activity 

 can be calculated. This fraction of a curie is called the con- 

 stant of the electroscope. 



Numerous trials show that readings must always be taken 

 between the same points on the scale or symmetrically about 

 the middle point of the portion used in calibration. Thus if 

 the electroscope is standardized for the portion of the scale 

 lying between 70 and 30, the same maximum activity will be 

 found and hence the same constant will hold for readings taken 

 between 60 and 40 but not, for example, if they are taken 

 between 70 and 40 or between 60 and 30. The shorter distance 

 is sometimes convenient when dealing with weak activity. 



The constants of each ionization chamber as determined at 

 Boulder at a pressure of 62*5'^™ and at a temperature of about 

 22^C are 2-34x10-^° curie for No. 2, 1-89x10-^" curie for No. 

 3, and 2-07x10"'° curie for Nos. 6 and 7. These values are 

 the means of six or more concordant determinations for each 

 chamber. Strictly speaking these constants hold only for a 

 given pressure and temperature in the case of chambers whose 

 volume or air density is not large. As the springs examined 

 are at elevations varying approximately from 5,000 ft. to 10,000 

 ft. which causes changes in pressure from about 64'^''' to SS'^"' 

 the constants given above were of little value in the field work. 

 This made necessary an investigation of the way in which the 

 '• constants " varied with the pressure. Previous investigations 

 on the variation of ionization with pressure such as those of 

 *Tliis Journal (4), vol. xviii, p. 378, 1904. 



