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



that to be expected for the component of the shortest wave- 

 length for the " K" characteristic radiation of that element. 

 On the other hand, the experiments of Kaye have shown 

 that a radiation can be obtained from aluminium at 30,000 

 volts, which is much more penetrating than the *• K " 

 characteristic of that element. Results of a similar kind 

 have been obtained by Rawlinson *, who found that at about 

 50,000 volts he obtained a radiation from nickel for which 

 yL6 = 6*9, while the value of /i for the " K " characteristic is 

 148 (cm.)" 1 . Such results show that the highest frequency 

 to be obtained from these atoms is much greater than that of 

 the K radiation. It is possible, however, that the radiation 

 of higher frequency may represent the octave or still higher 

 harmonic of the fundamental mode of radiation which is 

 represented by the " K " characteristic. A close analysis 

 of the frequency-voltage curves of the radiation from 

 different elements should throw much light on this question. 

 Arrangements have been made to continue experiments, 

 such as have been made for tungsten, for a number of other 

 elements. 



We have already drawn attention to the fact that, even 

 with the very high velocity of projection of the beta rays 

 from radium B and radium 0, no frequencies have been 

 observed much higher than those to be expected for the 

 " K " radiation. 



Some years ago, Whiddingtonf made a number of 

 experiments on the minimum voltage required to excite 

 the " K " characteristic of a number of light elements. 

 Assuming, as seems probable, that it is necessary to excite 

 the shorter wave-length of the two main components of the 

 Ai K " radiation before the characteristic appears, the voltage 

 required on the simple quantum theory to excite the radia- 

 tions is given in the following table, taking the values for the 

 beta component of the " K " radiation found by Moseley. 



It is seen that for all metals except aluminium the voltage 

 required to excite the " K " radiation is distinctly higher than 

 that expected on the simple quantum theory. Such a result is 

 to be anticipated if the relation between frequency and voltage 

 is of the same general form as that observed for tungsten. 



We have seen that the radiation from the Coolidge tube 

 was first detected at 10,300 volts. It seems probable that 

 this corresponds to the voltage required to excite the 

 "Li - " characteristic of tungsten. Assuming that the line 



* Rawlinson, Phil. Mag. xxviii. p. 274 (1914). 



t Whiddington, Proc. Roy. Soc. A. lxxxv. p. 323 (1911). 



