[pound] absorption OF THE DIFFERENT TYPES OF BETA RAYS 61 



at the bottom, while the single sheet of aluminium foil .0065 mms. 

 in thickness covered the top. As before, the saturation currents in 

 the chamber were taken as the magnetic field deflected the ft rays 

 farther and farther from the opening of the chamber, and the values 

 of the saturation currents are given in Table IX. From these it will 

 be seen that with the lead reflectors of different thicknesses the 

 saturation currents were practically the same when magnetic fields 

 of sufficient strength to deflect all the ft rays were applied. In order 

 to ascertain the maximum ionisation for the various lead reflectors 

 due to the ft ray effect, the mean of the readings obtained with 

 the high fields was taken as representing the conductivity due to 

 the Y radiations, that due to the secondary radiations excited by these 

 in the reflectors, and also that due to the so-called natural ionisation. 

 This me*an was subtracted from the maximum ionisation obtained 

 with each of the reflectors before the application of a magnetic field 

 and the differences which are recorded in column IV of Table X, and 

 represented graphically by the Curve A in Fig. 11 were taken to 

 represent the ionisations produced in the chamber by the primary 

 ft rays and by the secondary rays excited by them in the lead reflec- 

 tors. From a consideration of those values and of the form of the 

 curve in Fig. 11, it is evident that a maximum secondary radiation 

 due to the impact of ft rays on the lead reflectors was obtained with a 

 thickness of .16 mms. of this metal. 



From these results then it is clear that the secondary radiation 

 emitted by the front side of a lead plate, which the ft rays from 

 radium fall do not come from a depth of the metal greater than .16 

 mms. It is also established by the results that a plate of lead .9 mms. 

 in thickness will completely absorb all the primary ft rays from radium 

 as well as all the secondary radiation excited by these rays in the lead 

 plate. 



V. Experiments on the Absorption and Reflection oj ft Rays by 

 Aluminium Foil. 



A. Measurements on transmitted rays. 



A series of readings was also made with a number of different 

 thicknesses of absorbing layers of aluminium foil over the top of the 

 chamber, in order to find the minimum thickness of aluminium 

 necessary to stop the ft rays. The bottom of the chamber was closed 

 by the same sheet of aluminium foil .0065 mms. thick, used throughout 

 these experiments. As before, the first series of measurements was 

 taken without any cover over the top of the chamber and this series 

 is given in column I of Tables XI and XII. The results obtained 

 with layers .0065, ..28 mms., 1.184 mms., 3.41 mms., 4.73 mms., and 



