Particles by Homogeneous Rontgen Radiations. 323 



This distance was 5 mm. when soft radiations were used. 

 RR was then placed in position in the vessel, E was placed 

 on top of it and connected to the secondary electroscope. 



The lid was heated and put in position after the screws 

 had been connected to earth. 



Method of Experimenting. 



When the bulb was in action the air between E and R 

 was ionized and a charge communicated to E. The pressure 

 of the air inside the vessel was varied, and the ionization in 

 the space ER was measured at different pressures (the 

 primary electroscope being always used to standardize the 

 ionization). 



The sources of this ionization are twofold : — 



(1) Ionization due to Rontgen radiations alone. This has 

 been shown by Crowther * to vary directly as the pressure 

 of the air. 



(2) Ionization due to cathode particles emerging from the 

 silver leaf. The amount of ionization due to this source will 

 remain constant as long as the pressure is great enough to 

 absorb all the particles. When the pressure is lower some 

 of the particles will reach E before being absorbed, and the 

 ionization will decrease. 



Hence, given the actual curve, we can find the part due to 

 cathode ionization by drawing through the origin a line 

 parallel to the straight portion of the curve, and drawing the 

 curve whose ordinates are got by subtracting the ordinates 

 of this line from those of the actual curve. 



Fig. 6 (PL V.) shows how the ionization due to the 

 cathode particles is deduced from the actual curve. 



The pressure at which the ordinate of the cathode curve is 

 half the maximum ordinate gives the pressure at which half 

 the energy of the cathode particles which start from R 

 reaches E. 



Knowing the distance between R and E, the temperature 

 of the room, and the -critical pressure, we can now calculate 

 the thickness of the layer of air at 760 mm. pressure and 

 15° C, which would absorb one half of the energy of the 

 cathode particles starting from R. 



Further, we can easily determine from the curves the 

 ratio of total ionization due to the cathode particles which 

 emerge to the ionization due to Rontgen radiations in the 

 layer of air between R and E (e. g. PN/QN, fig. 6), and also 

 how the total number of ions made by the cathode particles 



* J. A. Crowther, Roy. Soc. Tree. A. lxxxii. 1909, p. 103. 



