T. S. Taylor — Retardation of " Alpha RaysP 175 



The diminution in the range produced by a given sheet when 

 0*6 cm from the polonium, having been determined by the direct 

 method as previously stated, the diminution at any other dis- 

 tance was found from the curves in figure 3, which were plot- 

 ted on finely ruled co-ordinate paper, thus facilitating the 

 determination. The curves given in figure 3 were for a sheet 

 of gold (A in Table II), which cut down the range 0*635 cm 

 when it was 0'6 cm from the polonium. 



The method of making the determination can be illustrated 

 by finding the diminution in range produced by the sheet when 

 it was at a distance of l'8 cm from the polonium. In curve III 

 the abscissa, corresponding to the ordinate 1*8, which is the 

 distance of the sheet from the polonium, is seen to be 1*5. 

 The abscissa 1*5 in curve I corresponds to the ordinate 2*956 

 in the same curve. Thus the increase in the deflection of the 

 electrometer needle produced when the sheet is moved from 

 # 6 cm to l'8 cm from the polonium is the same as would be pro- 

 duced if the polonium with the sheet 6 mm above it were moved 

 from 3-0 cm to 2'956 cm from the ionization chamber. The total 

 range of the a particles when the sheet is l - 8 cm from the polo- 

 nium, is greater by [3'0-2;956] 0-044 cm than the total range 

 when the sheet is 0'6 cm from the polonium. In a similar man- 

 ner we find from curves IV and II this change is equal to 

 0'0I:2 cm . The mean value for the two determinations is then 

 0-0±3 cm . The barometer stood at 760-2 min when the curves 

 were plotted, and hence this change of the range when reduced 

 to barometer 760 mm is 



760-2 X 0-043 rt nJnpm 



=0-043 cm 



760 



Since there is this difference in the total range of the a parti- 

 cles in the two cases, it is evident that the sheet does not cut 

 down the range of the a particles as much when it is l'8 cm 

 from the polonium as when it is only 0'6 cm from the polo- 

 nium, and that the air equivalent of the sheet at l*8 cm distance 

 from the polonium is 0-043 cm less than at 0'6 cm . 



In this manner the air equivalent of the sheet was found 

 when it was at various distances from the polonium. In Table 

 I are given the values of these air equivalents for the A sheet 

 of gold corresponding to the distance of the sheet from the 

 polonium and the range that the a particle still had in air at 

 atmospheric pressure when it entered the sheet. The range 

 of the a particle in air at atmospheric pressure was found to 

 be 3'77 cm . This was the distance from the polonium to the 

 upper side of the gauze A when the ionization, due to the a 

 particles from the polonium, produced the first noticeable deflec- 

 tion of the electrometer needle above that due to the spon- 

 taneous ionization of the air in the chamber."* The spontaneous 



* If the distance had been measured to the middle of the ionization cham- 

 ber the range would be 3'87 cm , which is the value usually taken. 



