﻿produced by an Alplia Particle from Polonium. 6*71 



brass plate " A," about 8 centimetres long by 8 millimetres 

 wide, was insulated by an amber plug as shown, and con- 

 nected to one pair of the quadrants of a sensitive Dolezalek 

 electrometer. The plate " A " was surrounded by a brass 

 box about 8'3 centimetres long, 12 millimetres wide, and 12 

 millimetres deep. This box had a small opening, as shown 

 at the left side, for allowing the alpha particles to enter it. 

 This box together with the plate "A" comprised the ioni- 

 zation-chamber, since " A " was connected to the electrometer 

 and the box surrounding it was joined to n high-potential 

 battery of 750 volts. This voltage was sufficiently high to 

 produce saturation current under normal atmospheric 

 pressure. The source of rays was a preparation of polonium 

 mounted within the brass plug " B," upon the end of a screw 

 of one millimetre pitch. The brass plug was insulated from 

 the screw by means of an amber washer under it. By means 

 of the screw, the polonium could be moved toward or away 

 from the left end of the box forming the outer wall of the 

 ionization-chamber, and its position determined at any time. 

 A brass cap, having a small hole of \ millimetre diameter 

 and 8 millimetres long in it, was fitted over the brass plug 

 " B " containing the polonium. This cap served to limit the 

 beam of emergent rays such that they would not strike the 

 walls of the ionization-chamber before they had readied 

 the end of their path. The ionization-chamber, &c. were 

 enclosed in a metal case, as shown in figure 1. The metal 

 case was always connected to earth. 



Determination of Capacity. 



The capacity of the measuring system was determined 

 by the method outlined by Campbell * for measuring small 

 electrostatic capacities. A special auxiliary air-condenser 

 was constructed to be used in the determination of the 

 capacity of the measuring system. This capacity consisted 

 of a brass plate 10 centimetres long by 7 centimetres wide 

 within a brass box 12 centimetres long, 8 centimetres wide, 

 and 1 centimetre high. The brass plate was insulated from 

 the box by means of an amber plug through which a small 

 rod supporting the plate was inserted. The entire box was 

 enclosed in a metal case connected to earth. The box was 

 insulated from the case, however, by means of hard rubber 

 supports. The inner plate was always connected to the inner 

 coating of the measuring system during the determination of 

 the capacity. 



* Campbell, Phil. Mag. vol. xxi. pp. 42-45, January 1911. 



