[pound] absorption OF THE DIFFERENT TYPES OF BETA RAYS 59 



recorded in Table IV, and it may be seen from the curve in Fig 7 

 that after a field corresponding to ten amperes was exceeded, the 

 ionisation approached a limiting value which indicated that for 

 magnetic fields excited by currents of ten amperes, and greater, the /5 

 rays were all turned aside and the y rays alone were left to enter 

 the chamber. The maximum ionisation due to the fi, /3 secondary, 

 Y, Y secondary, and that due to natural causes is given by the 

 ordinate of the initial point of this curve. With the ^interpretation 

 given above the ionisation due to the Y; Y secondary, and that due 

 to any radiations from the metal forming the walls of the chamber 

 may be taken to be represented by the point on the curve correspond- 

 ing to the highest field. The difference in the values of these two 

 ionisations gives a value for the maximum conductivity impressed 

 upon the air by the (3 rays, and by the secondary radiations excited 

 by them in the tinfoil. 



In Table V is given the deduced values of the maximum 

 ionisations which were due to fi and secondary rays from similar 

 sets of measurements for different thicknesses of tinfoil at the bottom 

 of the chamber. The curve drawn in Fig. 8 is plotted with ordinates 

 representing the values of these maximum yS and /i secondary 

 ionisations as recorded in the fourth column of this table, and with 

 abscissae representing the corresponding thicknesses of tinfoil. 

 From this curve it is clear that the maximum conductivities produced 

 by the ft and the reflected ft secondary rays reached a limiting value 

 when the tinfoil sheets attained a thickness of .24 mms. and for still 

 greater thicknesses remained constant. 



Summarising all the results obtained with tinfoil it would then 

 appear: — That when ft rays from radium are allowed to impinge on 

 sheets of tinfoil a maximum reflected secondary radiation is obtained 

 when the tinfoil attains a thickness of .24 mms., and further that a 

 thickness of 2.5 mms. of tinfoil is sufficient to absorb not only the 

 transmitted secondary rays excited by ft rays, but also the whole of 

 the primary radiation itself. 



This result, however, while giving definite information regarding 

 a lower limit to the thickness of tinfoil requisite to absorb primary 

 ft rays gives only an upper limit to the thickness necessary to absorb 

 the transmitted secondary radiations produced by such rays. In 

 order to obtain a lower limit to the thickness of tinfoil required to 

 absorb the transmitted ft secondary radiation alone which is excited 

 by ft rays, it would be necessary to modify considerably the arrange- 

 ment of the apparatus used in making these measurements. 



