178 Mr. D. C. H. Florance : A Study of the ionization 



coefficient of the ft rays in aluminium (between *2 and 

 *1 mm.) for any pressure was approximately 19 cm. -1 , 

 whereas in the usual experimental arrangement the value 

 is 14. 



The Effect of the Final Plate on the Amount of 

 Emergent ft Radiation. 



The 7 rays passing into the ionization-chamber were 

 always approximately of the same intensity. The ionization 

 between the plates at atmospheric pressure should be mainly 

 dependent on the nature of the plates. Since at atmospheric 

 pressure the ionization currents were extremely small, 

 another experiment was performed to see how the quantity 

 of emergent ft rays depended on the material of the plates. 



Fig. 5 illustrates the experimental arrangement. The 

 ionization-chamber (kindly lent by Mr. Moseley) was con- 

 structed of Al foil supported on knitting-needles. It was 



Fiff. 5. 



Radium 



TO ELECTROMETER 



/O/V/ZAT/C/V 

 VESSEL 



LEAD 



ORDER OF PLATES 



C M Fe Sn Pb Fb 



Distance between radium 

 and ionization vessel 60 cms. 



|l|l|— l| TO 



EARTt 



about 7*3 cm. cube. This ionization-chamber rested on thin 

 glass tubes. Against the side of the ionization-chamber 

 nearest the radium was arranged the following series of 

 plates : — 



Plate ... Carbon. 



| 



Aluminium. 



Iron. 



Tin. 

 20 



Lead. 



Lead. 



Thickness 1 | .« 

 in mm. J j 



1 



34 



21 



43 



92 



Between the plate of lead 9'2 mm. thick and the vessel 

 there was jnst sufficient space to admit one of the other 

 plates, the ft rays of which were to be examined. The 

 7 rays always passed through the same quantity of material ; 

 hence, in each case the effect observed was chiefly due to 



