Probability Variations in Distribution of a Particles. 699 



observers, including Kohlrausch *, Meyer and Regener f, 

 and H. Geiger J, by noticing the fluctuations when the 

 ionization currents due to two sources of « rays were balanced 

 against each other. The results obtained have been shown 

 to be in good agreement with the theoretical predictions of 

 von Schweidler. 



The development of the scintillation method of counting 

 a particles by Regener, and of the electric method by 

 Rutherford nnd Geiger, has afforded a more direct method 

 of testing the probability variations. Examples of the dis- 

 tribution of a particles in time have been given by Regener § 

 and also by Rutherford and Geiger || . It was the intention 

 of the authors initially to determine the distribution of 

 a, particles in time by the electric method, using a string 

 electrometer of quick period as the detecting instrument. 

 Experiments were made in this direction, and photographs 

 of the throws of the instrument were readily obtained on a 

 revolving film ; but it was found to be a long and tedious 

 matter to obtain records of the large number of a particles 

 required. It was considered simpler, if not quite so accurate, 

 to count the a particles by the scintillation method. 



Experimental Arrangement. 



The source of radiation was a small disk coated with 

 polonium, which was placed inside an exhausted tube, closed 

 at one end by a zinc sulphide screen. The scintillations 

 were counted in the usual way by means of a microscope on 

 an area of about one sq. mm. of screen. During the time 

 of counting (5 days), in order to correct for the decay, the 

 polonium was moved daily closer to the screen in order that 

 the average number of a. particles impinging on the screen 

 should be nearly constant. The scintillations were recorded 

 on a chronograph tape by closing an electric circuit by hand 

 at the instant of each scintillation. Time-marks at intervals 

 of one half-minute were also automatically recorded on the 

 same tape. 



After the eye was rested, scintillations were counted from 

 3 to 5 minutes. The motor running the tape was then 

 stopped and the eye rested for several minutes ; then another 

 interval of counting, and so on. It was found possible to 

 count 2000 scintillations a day, and in all 10,000 were 

 recorded. The records on the tape were then systematically 



* Kohlrausch, Wiener Ahad. cxv. p. G73 (1906). 

 t Meyer and Regener, Ann. d. Phys. xxv. p. 757 (1907). 

 % Geiger, Phil. Mag. xv. p. 539 (1908). 



§ Regener, Verh. d. D. Phys. Ges. x. p. 78 (1908) ; Sttz. Per. d. K. 

 Preuss. Akad. Wiss. xxxviii. p. 918 (1909). 



|| Rutherford and Geiger, Proc. Roy. Soc. A. lxxxi. p. 141 (1908). 



