459 



librium with the emanation and can be used again. By noting the ionization 

 current or the "leak" of the electroscope other samples of radium can be 

 compared with the first by putting sample No. 2 through the same process. 

 The Bureau of Standards at Washington is prepared to standardize radium 

 solutions by comparing them with a standard in its possession. 



If no standard is at hand the electroscope can be standardized by using 

 Duane's empirical formula. (Le Radium Vol. XI, P. 5, 1914; Ann. der Phys. 

 Vol. 38, P. 959, 1912; Compt. Rendus Vol. 150, P. 1421, 1910; Jour, de Phys. 

 Vol. 4, P. 605, 1905), which is, 



2.49 X10 6 (1— 0.517 S/V) 



6.31 X10 6 (1—0.572 S/V) 



Where, e = amount of emanation in the electroscope. 



io = initial current, expressed in E. S. units. 



i max = maximum current (current at end of three hours) expressed 

 in E. S. units. 



S = inside surface of ionization chamber of electroscope. 



V = volume of ionization chamber. 

 This equation applies to a cylindrical ionization chamber with a central rod. 

 The volume of the chamber must be about one liter and the height is from 

 one to three times the diameter. 



The ionization chamber can be a cylindrical metallic chamber with an 

 insulated rod extending through the center. This rod can be connected to 

 an electrometer or to an electroscope in order to determine the potential of 

 the rod. For very delicate measurements of small amounts of emanation a 

 sensitive electroscope is better than an electrometer. In an electroscope the 

 ionization current, i, is measured by knowing the capacity, C, of the elec- 

 troscope; the change of potential, dV, of the insulated rod, in the time, t; 

 according to the equation, 



C dV 



i = ■■ 



t 



In order to measure a small current in a short time, C, the capacity 

 of the electroscope must be small and dV, the change of potential, must be 



