﻿produced by an Alpha Particle from Polonium. 673 



Measurement of Ionization Current. 

 The brass plug containing the polonium, with the cap over 

 it for limiting the emergent beam of rays, was placed in an 

 apparatus similar to that used in previous. experiments *, and 

 the Bragg ionization curve determined. Such a curve, thus 

 obtained, is shown in figure 2. The plug with the cap over 

 it was then placed in the apparatus (fig. 1) in position " B," 

 and the screw, upon which it was fixed, turned so that the 

 polonium was as close to the ionization-chamber as possible. 

 This distance was 9 millimetres. The pressure of the gas in 

 the air-tight vessel surrounding the ionization-chamber was 

 then adjusted so as to be exactly 76 centimetres of mercury; 



Fig-. 2. 

 7 



/ 2 "6 4 



The ordinates are the deflexions in scale-divisions of the electrometer- 

 needle per second. The abscissae are the distances in centimetres of 

 the polonium from the ionization-chamber. 



This adjustment gave the alpha particles the same range in 

 the different determinations. With a potential of 750 volts 

 on the outer coating of the ionization-chamber, observations 

 were made of the deflexion of the electrometer for intervals 

 of 5 minutes. This deflexion of the electrometer-needle gave 

 a measure of the ionization current that was produced by 

 (3*86 — 0*90) 2*96 centimetres of the range of the alpha 

 particle. That is, it was proportional to the ionization that 

 is represented by the area under the ionization curve in 

 fig. 2 to the right of the ordinate corresponding to the 

 abscissa 09 centimetre. 



* Taylor, Phil. Mag. vol. xviii. p. 604, October 1909. 

 Phil Mag. S. 6. Vol. 23. No. 136. April 1912. 2 Y 



