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XXXIV. Maximum Frequency of the X Rays from a Coolidge 

 Tube for Different Voltages. By Sir Ernest Rutherford, 

 F.R.S., Professor J. Barnes, Ph.D., and H. Richardson, 

 M.Sc* 



IX the course of last year, Mr. C. Gr. Darwin began an 

 investigation in the University of: Manchester to 

 examine the relation between the velocity of cathode rays 

 and the frequency of X rays excited by them in different 

 radiators. The cathode rays were generated by the electric 

 discharge in a suitable vacuum-tube, and by means of an 

 adjustable magnetic field rays of definite speed were allowed 

 to fall on a radiator. It was the intention of Mr. Darwin to 

 examine the frequency of the X rays initially by measuring 

 their absorption in aluminium, and if possible by direct 

 reflexion from crystals. Some difficulty was experienced 

 in obtaining a sufficiently good and constant vacuum, 

 and measurements were interrupted by the departure of 

 Mr. Darwin to the seat of war. The experiments were 

 continued by Mr. H. Richardson, but in the complicated 

 apparatus employed it was found difficult to keep the vacuum 

 sufficiently constant when a discharge was passed. 



As soon as the Coolidge tube was put on the market, it 

 was recognized that it afforded a much more convenient 

 method of attacking a part of the main problem and over 

 a much wider range of voltage. As is well known, the 

 discharge in the very perfectly exhausted Coolidge tube is 

 mainly carried by the negative electrons liberated from a 

 tungsten spiral heated to a high temperature by means of 

 the electric current. The anticathode is of tungsten, and 

 the exhaustion in the tube employed in the present experi- 

 ments was so perfect that the tube maintained, with suitable 

 precautions, 175,000 volts across its terminals without any 

 obvious breakdown of the vacuum. Since tungsten is of 

 atomic weight 184, and atomic number 74, the X radiation 

 from the Coolidge tube corresponds to that emitted from a 

 heavy element ; but it is to be expected that the frequency 

 of the X radiation for a high voltage should he somewhat 

 less than from the ordinary X-ray tube with a platinum or 

 platinum-iridium anticathode, since fhe atomic weights of 

 iridium and platinum are 193 and 195, and atomic numbers 

 77 and 78 respectively f. 



* Communicated bv the Authors. 



t See Moseley, Phil. Mag. xxvii. p. 703 (1914). 



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