Retardation of a Particles hy Metals. 



Table I.— Gold No. 5. 



Weight per unit area = '002186 grm. per sq. cm. 



Pressure 774 mm. Temp. 17° C. 



187 



Beading on 



Emergent 



Mean reading 



on V 



for end of range. 



Air- 



scale K. 



range. 



equivalent. 





cm. 





cm. 



Zero. 



• • • 



8-845 



... 



0-58 



0-08 



8-525 



•320 



100 



o-ao 



8-460 



•385 



1-50 



100 



8-422 



•423 



2-00 



1-50 



8-400 



•445 



3-00 



250 



8-367 



•478 



4 00 



350 



8-335 



•510 



4-74 



4-24 



8-326 



•519 



The first column gives the readings on scale K. From 

 these readings the values of the distance between the foil F 

 and the zinc-sulphide screen Z are deduced and entered in 

 column 2 under the heading of emergent range. The 

 emergent range is thus the distance through which the 

 a particles have to travel alter leaving the foil before com- 

 pleting their range. Column 3 gives the means of readings 

 on vernier V for several determinations of the end of the 

 range. From these, the values of the air-equivalents are 

 calculated and entered in column 4. Two or three sets of 

 observations were made for each foil, and the means obtained 

 from a smooth curve drawn through the actual observations. 

 In some cases a source was used consisting of RaC deposited 

 on a small metal disk, so that the observations could be 

 extended to higher ranges. The mean values obtained are 

 given in the following table (p. 188) and shown diagramma- 

 tically in fig. 2 (p. 189). The results are expressed throughout 

 in terms of the emergent range as defined above. The in- 

 cident range can be obtained by adding to this the air- 

 equivalent in the particular position. The air-equivalents 

 and emergent ranges are reduced in all cases to 76 cm. 

 pressure and 15° C; for it has been well established by 

 many observers that the range of an a particle in any gas is 

 proportional to the density. 



It will be at once seen from the curves that the varia- 

 tions of air-equivalent are quite considerable and specially 

 noticeable for low ranges. In particular, the variation of 

 air value in the cases of aluminium and mica is almost 

 entirely confined to the last two centimetres of emergent 

 range. It may be noted in passing, that this variation is 



