of Production of X Rays from a Cooiidge Tube. 365 



the velocity of the cathode rays as a fraction of the velocity of 

 light. It is seen from the last column that X//3 4 is sensibly 

 constant. Substituting the value of A = 184 for tungsten, the 

 values of X to be expected from this equation are 2'9, 4*8, 9*1 

 for 48,000, 64,000, and 96,000 volts respectively in place of 

 *86, 1*4, 3*0 observed experimentally. The observed values 

 are about one third of the values calculated on Beatty's 

 relation. 



Correction J or Absorption in Bulb. 



In our calculations, however, we have measured the total 

 ionization produced outside the bulb, and have not corrected 

 for the absorption of the rays by the wall of the bulb and by 

 the air and other absorbers in the path of the rays. Special 

 measurements showed that the wall of the bulb where the 

 rays issued was '5 mm. in thickness. The absorption in glass 

 for equal thicknesses is about the same as for aluminium, 

 and there will not be much error in taking the absorption of 

 the rays before entering the ionization vessel as equivalent 

 to '6 mm. of aluminium. Comparing the relative absorption 

 of air and aluminium for soft radiations, *6 mm. of aluminium 

 is equivalent to about 8 mm. of water. Until experiments 

 are made of the total radiation from tungsten under conditions 

 that the absorption of radiation in escaping from the tube 

 is a minimum, it is difficult to make more than a rough 

 estimate of this correcting factor for absorption. From a 

 consideration of the absorption curves in water, it seems 

 probable that the correcting factor for the total ionization is 

 at least 2 for 48,000 volts, and may he somewhat greater. 

 It is to be expected that this factor would be somewhat less 

 for the higher voltages. Making this correction, it is seen 

 that the results for tungsten are in very fair agreement with 

 Beatty's relation, even though it is extrapolated over a wide 

 range of atomic weight and voltage. 



Energy of the X Rays. 



Knowing the current due to complete absorption of the 

 radiation, the energy of the radiation can at once be deduced 

 if die average energy required to produce a pair of ions in 

 air is known. The value of this important quantity can best 

 be deduced from the total ionization current produced bv the 

 absorption of a single alpha particle of known energy. 

 Geiger found that a single alpha particle from radium C gave 

 rise to 2\37 x 10 5 ions in air each of charge 4*65 x 10" lb e.s. 

 units, ?. e. a quantity of electricity 3*67 x 10" 15 e.m. units. 



