of Production of X Rays from a Coolidge Tube. 367 



given by Beatty, it can be calculated that the energy assumed 

 to produce a pair of ions in methyl iodide corresponds to 

 110 volts. This is undoubtedly more than three times too 

 large, and the value of E = 7*6 x 10~ 5 A/3 2 is nearer the truth, 

 assuming the correctness of the other data involved. The 

 formula gives an efficiency for 96,000 volts of 4*1 xlO -3 , 

 which is about twice as high as that to be expected from our 

 experiments with a Coolidge tube after the probable cor- 

 rection for absorption has been made*. 



In these calculations no correction has been made for 

 reflexion or scattering of cathode rays by the tungsten. No 

 doubt the correction for this varies with the speed of the 

 electrons, and must be considerable for very high voltages. 

 In the absence of any definite data on this question, it seems 

 desirable, however, to give the efficiency of the conversion of 

 cathode rays into X rays under actual working conditions. 



The relations given by Beatty only apply to the " general " 

 or " independent " radiation from an X-ray tube. As pointed 

 out by Beatty, the efficiency rises rapidly when a characteristic 

 radiation is strongly excited. The low percentage value 

 obtained for the efficiency of a Coolidge tube for high voltages 

 is thus an indication that the radiation is mainly of the 

 " independent " type, and that the " K " characteristic 

 radiation is not so strongly excited as in the case of metals 

 of lower atomic weight. This is borne out by the difficulty of 

 detecting the presence of the " K " characteristic of tungsten 

 by absorption experiments, or by reflexion from crystals. 

 It is only in the case of radioactive substances that the 

 characteristic radiations of high frequency are strongly 

 excited. Ko doubt this is due to the ideal conditions of 

 excitation in this case, for the exciting electrons all come 

 from the nucleus of the atom. 



In the preceding paper we have shown that the voltage 

 required to excite the most penetrating rays in tungsten is 

 about twice that to be expected on the quantum theory, 

 indicating that about half of the energy of the exciting- 

 electron can be transformed into radiation. From the low 

 value of the efficiency at high voltage?, viz. about 1/500, it 

 is clear that, on the average, 1 electron only in 200 is efficient 

 in producing radiation. 



Universitv of Manchester, 

 July 1915. 



* Dr. Beatty has drawn my attention to the recent measurements of 

 Barkla (Phil. Mag. xxv. p. 838, 1913), in which he finds that the total 

 ionization in methyl iodide for cathode rays is T48 times that in air. 

 Using the amended data, Beatty 's relation becomes E = '51XlO~ 4 A/3 2 , 

 w r hich is in fair accord with our measurements. 



