340 Sir E. Rutherford, Prof. Barnes, and Mr. Richardson : 



General method of the Experiment. 



The primary object of the experiment was to determine 

 the maximum frequency of the X rays emitted from a 

 Coolidge tube excited by different constant voltages. Since 

 the penetrating power of X rays in a light element like 

 aluminium increases regularly and rapidly with the fre- 

 quency, an estimate of the maximum frequency of the 

 radiation can be made by determining the absorption in that 

 metal of the "end" radiation from the tube, i. e. the ab- 

 sorption of the most penetrating rays present when the 

 rays of smaller frequency have been almost completely 

 absorbed. 



It has been known for some time that the absorption 

 -coefficient /j, in aluminium of the characteristic X radiation 

 from different radiators is given approximately by //, =k\ 5/2 

 where X is the average wave-length of the radiation *. This 

 relation has been recently examined in detail by W. H. 

 Bragg and Pierce f by determining the value of jjl in 

 .aluminium for individual spectrum lines of definite fre- 

 quency, and found to hold fairly accurately over the limited 

 range employed, viz. for wave-lengths between '49 X 10~ 8 

 .and '6l5xl0~ 8 cm., i. e. for radiations which are reflected 

 from rock-salt between angles of 5° and 6°. There seems to 

 be no doubt that this relation will hold very approximately 

 for the much shorter wave-lengths contained in the more 

 penetrating 7 radiations emitted by radium B and radium C 

 It can be deduced from Bra org's results as a mean of the 

 measurements on the silver fi and palladium /3 lines that a 

 radiation of wave-length \ = 5xl0 -9 cm. his an absorption 

 coefficient /j, = 5'6 (cm.) -1 in aluminium. If the above 

 relation between absorption and frequency for aluminium 

 holds, the 7 radiation from radium C, which has the value 

 ^ = 0*115 in aluminium, corresponds to a radiation of 

 \=l'06xl0~ 9 cm., which should be reflected from rock- 

 salt at an angle of 1° 5'. By far the strongest line in the 

 7-ray spectrum of radium C was found by Rutherford and 

 Andrade J to be reflected at an anode of 1° from rock-salt. 

 There is thus a very fair accord between experiment and 

 calculation when the relation is extrapolated over a wide 

 range in the value of /jl, viz. nearly 50 times. We may 

 •consequently assume with confidence that the relation 

 fi — kX bl2 holds very approximately over the whole range 



* See Owen, Proc. Roy. Soc. lxxxvi. p. 426 (1912). 

 T ' X rays and Crystal Structure,' by W. H. & W. L. Bragg, 

 ■pp. 180, 181. 



% Rutherford and Andrade, Phil. Mag. xxviii. p. 263 (1914). 



