ARTICLES 581 



from the general radiation spoken of above. The only differ- 

 ence is that all the lines of a K series appear together, always 

 with the same relative intensities ; and the energy of the 

 electron must be that which is associated by the same equa- 

 tion with a frequency just a little greater than that of the 

 7-line (see fig. 5). See above, p. 577. 



All these measurements and confirmations combine to 

 give a simple and striking picture of some aspects of the radia- 

 tion process. We see that moving electrons excite X-rays, 



g tf jfl 2223 24 25 M f ** fcf 30 



Kilo/olts 



Fig. 5. 



In fig. 5 the process recorded is the same as in fig. 4, except that the narrow ranges of frequency 

 represented by the four curves include in one case the a-ray of rhodium and in the other the |3-ray. More- 

 over, the anticathode is of rhodium and not of tungsten. The figure is freely drawn from Webster's results. 

 Note how in the two special cases the curves suddenly break away from the normal form, because when the 

 roltage has reached 23-3 kilovolts the a, or the /3-ray lines are respectively excited. This voltage corresponds 

 to a frequency just above the y-line. The real voltage of the a-line — that is, that which is given by Ve = hv — 

 is about 20 kilovolts, and that of the /3-line is a little less than 23 kilovolts. 



and that X-rays set electrons in motion ; and always, in these 

 occurrences, there is a fundamental condition to be satisfied. 

 The energy of the electron and the frequency of the radiation 

 are always proportional to each other. It is possible to define 

 the motion of an electron by the wave-length which it can 

 just excite. We might talk of a yellow electron, or an ultra- 

 violet electron. 



It is true that when a plate is struck by a stream of elec- 

 38 



