0. C. Lester — Emanation Electroscopes. 



231 



mean of seven such percentage scales was obtained for cham- 

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 closely than 1 per cent. Actual trial showed that after 40 min. 

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Fig. 3. Constant-pressure curve. 



the same exactness that it could be observed. Such a scale is 

 therefore among the most useful constants of the electroscope. 

 When activities are expressed in terms of the ionization cur- 

 rent or in terms of units based upon the ionization current, 

 such as theMache unit, more constants of tlie electroscope must 

 be determined. These constants include the electrical capacity, 

 the drop in potential per division of the eyepiece scale, the 

 percentage of the maximum activity due to the active deposit, 

 and the amount of ionization absorbed by the walls of the 

 chamber. 



