Different Types of Beta. Rays. 143 



o£ tin and lead under the same conditions showed an exact 

 coincidence. This peculiarity in the behaviour of the 

 aluminium screen was investigated at considerable length, 

 and was finally shown by some experiments which are 

 described later in Section VI. to be due to the action of 

 the secondary rays excited on the far side of the thicker 

 aluminium screens by the 7 rays entering the chamber. 



Table XIII. 



Eemarks. 



Saturation Current. 



Thickness of aluminium | 

 over the top of chamber J 



mm. 



o-o 



mm. 

 0-0065 



mm. 

 0-28 



mm. 

 1-184 



mm. 

 3-41 



mm. 

 4-73 



mm. 

 8-14 



I. Max. /3+/3 secondary 1 

 + y -j- y secondary + [ 

 natural ionization J 



365 



396-5 



287-5 



793 



20-3 



13-5 



12-2 



II. y + y secondary + ] 

 natural ionization j 



12*6 



10-2 



11 



12-3 



13-1 1 12-3 11-7 



III. Max./3+jS secondary 1 

 ionization J 



3524 



386-3 



276-5 



67-0 



7-2 



1-2 0-5 



IV 



332-4 



386-3 



2765 



670 



6-7 



0-7 o-o 









In Table XIII. there is given in row I. the maximum satu- 

 ration-currents in the chamber due to the ft and ft secondary, 

 y and 7 secondary radiations, and that due to natural causes 

 tor the different thicknesses of aluminium-foil ; in row II. 

 saturation-currents due to the 7 and 7 secondary radiation 

 and that due to natural causes ; and in row III. the maximuum 

 ionizations due to the ft and ft secondary radiations deduced, 

 as explained in Section III. A, from the Tables above and 

 their corresponding curves. On looking at the figures given 

 in row III. of this table, it is seen that there is apparently a 

 /3-ray ionization of "5, or about one-seventh of one per cent, 

 of the greatest /3-ray ionization in the chamber when the top 

 of the chamber is covered by 8" 14 mm. of aluminium. This 

 conductivity, however, represents really a 7-ray effect, due, 

 as said before, to the thickness of the aluminium used, and 

 should be deducted from the last three of the numbers given 

 in row III. of the table. These corrected values of the 

 maximum ft and ft secondary ionizations are given in 

 row IV., and a curve (fig. 14) is plotted from these values. 

 An examination of this curve makes it evident that a thickness 



