;i2 



Prof. W. H. Bragg on the Influence of the 



In one experiment I used two thicknesses of tin-foil equi- 

 valent to about 3 cm. of air, and 16 thicknesses of Al foil 

 equivalent to about 1/5 cm. of air. When the Al foil was 

 next the Ea the reduction of range was less by '06 cm. than 

 when the Sn foil was next the Ra. The readings are given 

 in the following table and plotted in fig. 3. The foils w T ere 



Fig. 3. — Showing that the stopping-power of a pair of metal sheets 

 Al and Sn is altered by inverting the sheet*. 



reversed three times, so that the curve was found four times, 

 as recorded in columns I., II., III. and IV. 



Distance from 



I. 



II. 



III. 



IV. 



radium to 



Sn next 



Al next 



Sn next 



Al next 



ionization chamber. 



radium. 



radium. 



radium. 



radium. 



20 



495 



554 



487 



529 



2-1 



416 



479 



411 



464 



2-2 



327 



379 



324 



378 



2-3 



234 



283 



227 



279 



2-4 



163 



196 



155 



197 



2-5 



115 



137 



104 



137 



2-6 



89 



94 



75 



92 



27 



76 



69 



63 



67 



3-0 



63 



53 



45 



47 



3-3 



56 



45 



45 



43 



The figures in the first and third columns agree closely ; so 

 also do those in the second and fourth. Thus the errors of 

 experiment are much less than the effect observed ; this is 

 also obvious from the figure. 



Al and Au show the effect better than 

 results given in the following table were 

 Au sheet equivalent to about 3*4 cm. of 

 eight thicknesses of Al foil. They are 



The two curves are a whole millimetre apart 



Al and Sn. The 

 obtained with an 

 air and the same 

 plotted in fig. 4. 



