418 



Mr Beatty, The production of Cathode Particles 



Hence given the actual curve we find the part due to cathode 

 ionisation by drawing through the origin a line parallel to the 

 straight portion of the curve, and subtracting its ordinates from 

 those of the actual curve. This process gives the curve showing 

 the increase of cathode ionisation up to a certain pressure. 



The pressure at which the ordinate of the cathode curve is half 

 the length of the maximum ordinate gives the pressure at which 

 half the cathode particles starting from L reach D. 



Knowing the distance between D and L (1 cm.), the tempera- 

 ture of the room, and this critical pressure, we can now calculate 

 the thickness of the layer of air at 170 mm. pressure and 15° C. 

 tempei'ature, which would absorb one-half of the energy of the 

 cathode particles starting from L (Table I, column 1). 



Table I. Air. 



If we assume that Ge~''^ represents the amount of energy 

 which gets through a thickness x (in cms.) of air at normal 

 pressure and temperature we can now calculate \, the coefficient 

 of absorption of the cathode particles by air. For when 



•7 

 ^ d 



The values of \ are given in column 2. 



Further, since the experimental curves can be split up as 

 shown in fig. 2, we can determine the ratio of the ionisation 

 caused by the emerging cathode particles to the ionisation due to 

 the X- radiation in the layer of air between D and L. For the 

 sake of uniformity we shall measure this X-ionisation when the air 

 is at a pressure of 760 mm. 



This ratio has really a very simple meaning. Barkla and 

 Sadler* have shown that if we take any two substances which 

 do not give a homogeneous radiation under the stimulus of a 



* Earkla and Sadler, Phil. Mag., May 1909, p. 751. 



