186 



Bronson — Radio-active Measurements. 



whole was protected from air currents. The electrometer had 

 a sensitiveness of about 150 scale divisions per volt, and with 

 the plates 2 cms apart, the ionization currents were practically 

 proportional to the deflections throughout the entire length of 

 the scale (500 divisions). When the distance between the 

 plates was increased to 2'7 cms the same deflection was produced by 

 an ionization current only one third as large, that is, the sensi- 

 tiveness of the apparatus was tripled ; on the other hand, the 

 current was proportional to the deflection for only about 150 



/6o no /e,o 



Time in minutes. 



divisions. The currents measured varied from about 10" 12 to 

 10~ 10 amperes. 



The advantages of this method are obvious ; deflections are 

 independent of the capacity, measurements can be made over 

 a large range without readjustment, and observations can 

 be taken in as rapid succession as desired. In some cases obser- 

 vations were taken as often as once in Ave seconds. Figs. 1 and 

 2 show the accuracy which can be obtained by single sets of 

 observations. 



A and B, fig. 1, are two similar logarithmic decay curves of 

 the excited activity of actinium, and should therefore be paral- 

 lel. The time was at least 20 minutes after the emanation was 

 removed, so that the initial rise of the excited activity is not 

 shown. The equation of the decay of any radio-active sub- 



