236 0. C. Lester — Emanation Electroscopes. 



to the insulated system wliicli is clearly not intended by its 

 definition. 



For the reasons just mentioned the .work of European 

 observers in general presents an almost hopeless confusion 

 when accurate comparisons are attempted. It is trne that 

 much work had been done before suitable units and methods 

 were devised, and we find therefore many results expressed in 

 terms of the fall of the leaf in volts per unit time or in units 

 even more arbitrary. Such results can not be compared with 

 other work. Still other units nsed are the Milligram-Second, 

 Milligram-Minute, Gram-Second, etc., meaning the amount of 

 emanation produced by a given amount of radio-active sub- 

 stance in the specified time. The substance is usually the ele- 

 ment radium or a radium salt, and when this is specified, as 

 well as its degree of purity, measurements based upon such 

 units can be reduced to curies. 



Most European observers, outside of France and England, 

 express their results in terms of the mache unit. Generally 

 the corrections which have been made are clearly stated, but 

 not always. Furthermore tlie correction for absorption by the 

 walls of the chamber (Duane's factor) has usually been omitted 

 in work where most of the other corrections have been applied. 

 This has been pointed out by Berndt* in an elaborate series 

 of calculations undertaken with the aim of making possible the 

 comparison of the results of different observers. He shows 

 that, depending upon the size of the ionization chamber, the 

 correction for absorption alone may amount to from 10 per 

 cent to 155 per cent. 



A given instrument can be calibrated simply and accurately 

 in terms of a known quantity of radium emanation. If the 

 mache unit is to be retained it would seem easier and more 

 accurate to reduce results measured in curies to this unit by 

 means of the theoretical relation between them, than to calcu- 

 late mache units from the ionization current which involves 

 the determination of several more constants and the applica- 

 tion of troublesome corrections. As has been shown above, the 

 two methods, when all corrections are applied, give identical 

 results within the limits of experimental error. 



For the drawings which accompany this article and for effi- 

 cient aid in securing the data upon which it is based the author 

 wishes to express his indebtedness to Mr. J. H. V. Finney, 

 instructor in Physics in the University. 



Hale Physical Laboratory. 

 University of Colorado, 

 Boulder, Colo. 



* Log. cit. 



