Mr Townsend, Secondary Rontgen Rays. 



223 



when the plates are 15 millimetres apart a number of ions equal 

 to 142 x n are produced between the plate, where n is an unknown 

 multiplier. 



Of these 55 x n are produced within one millimetre of the 

 brass, 26 x n in the second layer one millimetre thick, 28'5 x n in 

 the third layer three millimetres thick. After five millimetres 

 the number per millimetre becomes constant, and the fourth and 

 fifth layers, each 5 millimetres thick, contain 32 x n per millimetre. 



It is evident therefore that the metal emits a radiation which 

 is rapidly absorbed by the air in contact with it. At atmospheric 

 pressure the effect of this radiation is hardly appreciable at 

 distances greater than five millimetres from the surface. In the 

 present case we may consider that the effect of the direct rays is 

 to produce an ionization equal to 32 x n per millimetre of air 

 traversed, and that the rest of the ionization is due to secondary 

 radiation. 



If we subtract 32 x X from the values of N in Table III. we 

 obtain the effect of the secondary radiation. Thus taking the last 

 column we can find numbers jDroportional to the ionization pro- 

 duced by the secondary rays from the two aluminium plates A and 

 B. Dividing these numbers by two we obtain the effect due 

 to one aluminium plate, and the results can be used in order to 

 correct for the secondary rays given out by the plate B in the 

 other cases. 



The results of these calculations are shown by the curves, 

 Fig. 3. The ordinates represent the total ionization produced by 



80- 



Fig. 3. 



the secondary rays within a distance X of the surface. The 

 straight line P gives the corresponding effect of the primary rays 

 which excite the secondary radiation. 



16—2 



