232 0. C. Lester — Emanation Electroscopes. 



The capacities of the electroscope were measured by the 

 divided charge method. A condenser made of two coaxial 

 cylinders, having cylindrical guard rings and a calculated 

 capacity of 25"^"" was used as a standard. The mean results are 

 g.^cm f(^j, chamber K"o. 2, T-S^-^ for No. 3, and S'O^'" for ]Sos. 6 

 and 7. During a visit in the summer of 1915 Professor J. C. 

 Hubbard very kindly checked some of these measurements by 

 a different method. 



The calibration of the eyepiece scale in volts gave quite 

 accurately straight lines for each chamber. The number of 

 volts per division varied from 1'22 to 1*35. The average 

 potential used on the leaf system, over the working part of the 

 scale, was about 310 volts. 



The per cent of activity due to the active deposit at maxi- 

 mum activity was determined for each ionization chamber in 

 several different ways ; by projecting backward the activity- 

 time curves to the axis of zero time ; by projecting backward 

 the curves representing the decay of the active deposit when 

 maximum activity had been reached ; by determining graphi- 

 cally and by calculation the maximum activity due to Ka. C. 

 alone and from this estimating the total activity due to the 

 deposit ; by calculation from the formula of Curie and Duane^ 

 and by tables. f The various methods gave fairly concordant 

 results for each chamber. Four different methods gave for 

 chamber No. 2 the percentages 42*5, 41-9, 42-7 and 43-2. 

 While each of these numbers is the mean of several determina- 

 tions so close an agreement was scarcely to be expected and is 

 no doubt accidental. The values adopted were 42 per cent for 

 chamber No. 2, 46 per cent for chamber No. 3 and 44"3 per 

 cent for chambers No. 6 and 7. 



Among persons interested in the physiological effects of 

 radioactivity and especially among those interested in the 

 therapeutic properties of mineral waters, the mache unit is 

 widel}" used as a unit of radio-activit}^ In spite of the fact 

 that this unit is difficult of definition and comparison, and, in 

 the author's opinion, is needless, its extensive use gives it an 

 importance that can not be ignored. 



The mache unit is defined as 1000 times the saturation ioni- 

 zation current due to one curie of emanation without disinte- 

 gration products when all the radiation is absorbed in the air 

 of the ionization chamber. ;{: In the case of the chambers under 

 discussion it is evident from their dimensions, the density of 



*C. E., vol. cxxxvi, p. 304, 1903. 



f Duane, Jour, de Phys. (4), vol. iv, p. 605, 1905. Dnane & Laborde, Le 

 Eadmrn, vol. vii, p. 1()2, 1910, Mine, Curie. Traite de Radioactivite. vol. 

 ii, p. 338. 



X^. Mache and St. Meyer, Rad. in Biol, und Heilk., vol. i, p. 350, 1912. 

 Gockel, Radioactivitat von Boden und Quellen, p. 82. 



