Maximum Frequency of X Rays from a Coolidge Tube. 355 



radiation, a thickness of 3 mm. of lead affords practically 

 complete protection against the rays. Through a thickness 

 of 4 mm. it would be difficult to detect the ionization even 

 when the bulb was strongly excited at 125,000 volts. 



Discussion of the Results. 



It has been shown that the penetrating power in aluminium 

 of the X rays from a Coolidge tube reaches a maximum at 

 142,000 volts, and that no sensible alteration has been ob- 

 served when the voltage is raised to 175,000. The maximum 

 value of the absorption coefficient fju in aluminium is "39. 

 Remembering that the value of jjl for the penetrating rays 

 from radium C is '115 in aluminium, it is seen that the rays 

 from the Coolidge tube have only about 3/10 of the pene- 

 trating power of the gamma rays from radium C. The 

 radiation from the Coolidge tube is, however, slightly more 

 penetrating than some of the rays from radium B, for which 

 fi has been found to be "51. From the variation of frequency 

 with voltage, it would appear that the frequency of the 

 radiation reaches a natural limit, probably controlled by the 

 frequency of the " K " characteristic radiation of that 

 element. This and other points are very clearly brought 

 out by comparison of the radiations from the Coolidge tube 

 at different voltages with the y rays emitted by radium B. 

 The latter has been carefully analysed by H. Richardson *. 

 By examining the absorption in aluminium, the rays were 

 found to consist of two components, for which //, = 40 

 and *5 ; the former radiation, which is easily absorbed, 

 undoubtedly corresponds to the " L " radiation of radium B. 

 The analysis of the radiation was carried still further by 

 determining the absorption curve for lead. In addition to 

 the " L " characteristic, the rays were found to consist 

 of three components, for which //, in lead was 46, 6, and 

 1*5 (cm.)" 1 . Since the most penetrating radiation of the 

 Coolidge tube gives a value //, = 23 for lead, it is clear that 

 the radiation from the Coolidge tube is more penetrating 

 than one of the main components of the radiation from 

 radium B, but is far less penetrating than two other com- 

 ponents. In the following table are given the wave-lengths 

 of the chief lines observed in the radium B spectrum by 

 Rutherford and Andrade |, and the absorption coefficients 

 in aluminium and in lead to be probably ascribed to the 



* H. Richardson, Proc. Rov. Soc. A. xci. p. 396 (1915) ; Rutherford 

 & Richardson, Phil. Mag. xxv. p. 722 (1913). 



f Rutherford & Andrade, Phil. Mag. xxviii. p. 263 (1914). 



2 A 2 



