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XXXV. Efficiency of Production of X Rays from a Coolidge 

 Tuhe. By Sir Ernest Rutherford, F.R.S., and Prof. 

 J. Barnes, Ph.D.* 



IN the preceding paper we have given an account of 

 experiments which have been made to determine the 

 maximum frequency of the X radiation excited in a Coolidge 

 tube for different constant voltages supplied by a Wimshurst 

 machine. It was thought desirable to extend the experiments 

 to determine the efficiency of the production of X rays in the 

 Coolido-e tube for comparativelv high voltages. 



The question of the efficiency of the production of X rays, 

 i. e. the ratio of the energy of the generated X rays to that 

 of the exciting cathode rays, has been the subject of several 

 investigations. Wienf determined the energy of the X rays 

 generated in a platinum anticathode for a potential difference 

 of 58,700 volts, by measuring the heating effect of the 

 radiation. Similar experiments have been made by Angerer} 

 and Carter §. The latter observed that the efficiency increased 

 with the exciting voltage. In general, it was found that the 

 efficiency was of the order of 1/1000. 



Recently the question has been attacked under more definite 

 conditions by R. T. Beatty ||. Cathode rays of definite velocity 

 sorted out by a magnetic Held fell on a radiator. The generated 

 X rays passed out through a very thin window, and were 

 then completely absorbed in a cylinder 150 cm. long filled 

 with the vapour of methyl iodide. The total ionization of 

 the rays was thus measured, and the corresponding energy 

 deduced from general data. When characteristic radiation 

 was not excited, the energy of the X rays for an equal 

 number of exciting electrons was found to be proportional 

 to the fourth power of the velocity of the cathode rays. 

 Kaye^ had previously observed that the energy of X rays 

 under constant condition of excitation was approximately 

 proportional to the atomic weight A of the radiator. Beatty 

 finally concluded that the efficiency of production of X rays 

 by matter in general was given by the formula 



X-ray energy = ,.. 4>< ^ 

 catnode-ray energy 



* Communicated bv the Authors. 



+ WieD, Ann.d. Phys. xviii. p. 991 (1905). 



An^erer, Ann. d. Phys. xxi. p. 87 (1906). 



§ Carter, Ann. d. Phys. xxi. p. 955 (1906). 

 || Beatty, Proc. Roy. Soc. A. lxxxix. p. 314 (1913). 

 % Kaye, Phil. Trans. Roy. Soc. A.cciw p. 123 (1908; ; Proc. Roy. Soc. 

 A. lxxxi. p. 337(1908). 



