134 Mr. V. E. Pound on the Absorption of the 



B. Measurements on Reflected Rays. 



A series of measurements was also made on the secondary 

 radiation produced at the front side of sheets of tinfoil when 

 /3 and 7 rays were allowed to fall on them. One layer of 

 aluminium-foil '0065 mm. in thickness was placed over the 

 opening at the top of the chamber, and layers of tinfoil of 

 increasing thickness were placed over the opening at the 

 bottom. In these experiments, the capsule containing the 

 radium bromide was placed vertically above the ionization- 

 chamber, so that the rays, after passing between the poles of 

 the magnet, impinged directly on the thin sheet of aluminium- 

 foil forming the upper wall of the ionization-chamber, and 

 after passing through it, traversed the air in the chamber 

 and then impinged on the tinfoil at the bottom. As the 

 magnet was excited by increasing currents, the /3 rays were 

 more and more deflected until all were swept aside by the 

 field and y rays alone entered the chamber. 



Table IV. 

 Ionization by j3 reflected Secondary Rays from Tinfoil. 





Saturation Currents (arbitrary scale). 





Different thicknesses of Tinfoil. 



Current 

 in magnet. 























I. 



II. 



III. 



IV. 



V. 



VI. 



VII. 



VIII. 





mm. 



mm. 



mm. 



mm. 



mm. 



mm. 



mm. 



mm. 





00 



0-0081 



00196 



0-039 



0-0784 



0-1568 



0-6272 



1-96 



00 



1086 



1121-9 



1165 



1198 



1225 



1230 



1234 



1230 



0-85 





826-4 



858-2 



864-6 



881-2 



891-4 



898-4 



'920 



09 



821-4 

















2-5 



267-4 



230-3 



224-1 



294-8 



225-7 



226-3 



2321 



2502 



45 



103-5 



104-5 



95-8 



98-2 



91-2 



95 



950 



86-4 



675 



68-0 



72-3 



64-9 



64-9 



57-5 



61-8 



62-5 



573 



100 



58-9 



61-4 



53-9 



55-4 



47-6 



51-5 



50-8 



47-7 



15-0 



54-2 



54-7 



49-1 



48 : 7 



43-9 



47-3 



46-7 



43-5 



220 



51-8 



50-4 



46-3 





43-4 



44-9 



44-9 



41-0 



235 









46-0 











28-5 





48-5 



44-7 





42-6 



43-2 



43-0 





29-5 









44-7 











In column II. Table IV. is given a typical set of these 

 measurements, and a typical curve plotted from them is 

 shown in fig. 7. The values obtained with the complete set 

 of reflectors used are recorded in columns I. to VIII. of 

 Table IV. : and it may be seen from the curve in fig. 7 that 

 after a field corresponding to ten amperes was exceeded the 

 ionization approached a limiting value which indicated that 



