Range of a. Particles from Radium C and Thorium C. 539 



§ 2. Apparatus. 



The apparatus used in the present investigation was 

 similar in its essentials to one described by Bragg *. At one 

 end of a cylindrical glass vessel A (PI. XVII. fig. 1} was fixed 

 an ionization chamber B, at the other the source of « rays C. 

 The distance between B and C was kept constant and the 

 pressure of the air varied. 



The a rays from C passed through an antechamber formed 

 by the wire gauzes b x and b 2 into the ionization chamber 

 proper, formed by b 2 and a copper plate b 3 . A flanged 

 copper disk b 4% waxed to the end of the vessel A, served to 

 support the plate and gauzes as shown in fig. 1. The plate 

 b 3 was connected to an electrometer and the gauze b 2 to —80 

 volts, ebonite insulation being used throughout. The disk 

 6 4 and gauze ^ were connected to earth, together with a 

 thin copper tube b 5 fixed close to the inside of the glass 

 vessel to protect the insulated plate b z from stray electro- 

 static effects. 



The purpose of the antechamber was of course to prevent 

 the entry into the ionization chamber of ions formed in the 

 body of the vessel A: The antechamber and ionization 

 chamber were each 1 mm. deep, and the potential gradient 

 in each was about 800 volts per cm. Although the electric 

 field was parallel to the paths of the u particles, it appeared 

 from the results of Moulin f that this potential gradient 

 should be sufficient to ensure saturation, especially as the 

 air pressure in the vessel was never more than 17 cm. of 

 mercury. 



The source of a rays was a polished brass or nickel disk C, 

 threading into the end of a rod supported by the glass tube 

 D. This tube fitted into the end of the vessel A by means 

 of a ground glass joint, thus facilitating the changing of the 

 source. A brass ring was slipped over the source to cut off 

 a particles from its edge, while an aluminium screen, of 

 thickness just sufficient to absorb the a particles, could bo 

 moved from without the vessel by a magnetic device so as to 

 prevent the a rays from entering the ionization chamber. 



The solid angle of the beam of a rays entering the chamber 

 was limited by the lead diaphragm E, 2 mm. thick. The 

 diameter of the opening was 1*0 cm., that of the source l'O 

 cm., and the distance between them 2C) cm. It will easily be 

 seen that the longest and shortest paths possible to a particles 

 ■entering the ionization chamber differed by less than one 



* Bragg-, ' Studies in Radioactivity,' p. '^o (London, 1912). 

 f Moulin, Ann. de Cliim. et th Phys. xxii. p. 73 (1911) 



2 2 



