[ 647 ] 



LXT. The Variation of Ionizing Power with the Velocity of 

 Cathode Rays. By J. L. G-lasson, B.A., B.Sc, 1851 

 Exhibition Scholar of Adelaide University *. 



IT is highly important that we should know the number of 

 ions a cathode ray makes in going unit distance in a 

 gas, and how this number varies with the velocity of the 

 cathode ray. The only experiments which have hitherto 

 been made on this subject are those of Durack. He f 

 reached the conclusion that one cathode ray moving with a 

 velocity of 4 X 10 9 cm. per. sec. makes *4 pairs of ions per 

 cm. in air at a pressure of 1 mm. of mercury. Later j 

 using beta rays from radium whose velocity he estimated 

 at 2*3 to 2'8 x 10 10 cm. per sec. he found that the number of 

 pairs of ions made per cm. in :iir at 1 mm. pressure w;is 

 •17. Thus increasing the velocity 6 times has decreased 

 the number of ions produced 2\ times. However, in both 

 his determinations there are several sources of error. His 

 rays were in neither case homogeneous. In his first series 

 he used the Lenard rays which had come through an 

 aluminium window, and which must have been fairly com- 

 pletely scattered by it. The beta rays from radium which 

 he used are known to range in velocity from 2 x 10 10 to 

 nearly 3 x 10 10 cm. per second. 



Moreover, in neither series did he take any adequate 

 precautions to prevent reflexion of the rays by the electrodes 

 used, nor did he apply any correction factor for this reflexion. 



The present experiments were therefore undertaken to 

 eliminate these sources of error. 



In order to get a homogeneous bundle of cathode rays of 

 known velocity an apparatus similar to that described by 

 Mr. Whiddington § was used. AB (fig. 1) represents the 

 section of a solenoid 13 cm. long, wound in 3 layers on a 

 brass tube of 3*70 cm. external diameter. The ends of the 

 tube are flanged and are closed by glass plates waxed on so 

 as to make an air-tight joint. The ravs enter the solenoid 

 by a small side tube E soldered on to the main brass tube ; 

 into E the discharge tube which is shown at D is fitted. 

 The rays are deflected through a right angle by the magnetic 

 field and leave the solenoid through a similar exit tube F, 



* Communicated bv Sir J. J. Thomson, F.R.S. 



t Phil. Mag. [6] vol. iv. pp. 29. 



| Phil. Mag. [6] vol. v. pp. 550. 



§ Proc. Hoy. Soc. July 1911. 



2 U 2 



