Maximum Frequency of X Rays from a Coolidge Tube. 349 



Attempts were made to test whether the absorption curves 

 •could be analysed into components corresponding to definite 

 characteristic radiations. Kaye * has made numerous ex- 

 periments of this kind to analyse the radiation from different 

 anticathodes, and obtained indications that by the use of 

 suitable absorbers the radiation could be analysed into 

 definite components. While it was not difficult to express 

 the absorption curve for any particular voltage with con- 

 siderable accuracy by the sum of three exponentials, the 

 values of the constants changed with voltage, and it was 

 concluded that such an analysis, apart from showing the 

 presence of the "L" characteristic, had no physical meaning. 

 It may yet be possible, as Kaye has suggested, to analyse 

 the radiation as a mixture of two or more characteristic 

 radiations, but before this can be accomplished it will be 

 necessary to carry out a large number of experiments on the 

 absorption of: the radiation by different materials. It 

 -appears to us, however, unlikely that the absorption curves 

 •can be completely expressed as the result of a small number 

 of characteristic radiations each of which are absorbed 

 exponentially. 



Experiments with Induction-coil. 



It was not found feasible to examine the absorption 

 curves of the radiation excited by the Wimshurst above 

 115,000 volts. In order to carry the experiments still 

 further, a large induction-coil which gave a 20-inch spark, 

 operated by a Sanax break, was used. The gap between 

 the sparking spheres was set to the potential required, and 

 the current through the coil carefully adjusted so that an 

 occasional spark passed. In many cases, a spark-gap ending 

 in fine needle-points was used in parallel with the sparking 

 spheres, to test whether there was any sensible alteration of 

 the potential required to spark across the spheres, owing to 

 possible alteration of their surface by the passage of the 

 preliminary sparks. 



The current through the heating-coil was adjusted so 

 that the intensity of the radiation was about the same as that 

 •excited by the Wimshurst, and the absorption curves in 

 aluminium determined as before. 



It was anticipated that the radiation excited by the variable 

 voltage of the induction-coil would on the average contain 

 a larger proportion of softer radiation than the radiation 

 excited by the Wimshurst. To our surprise, however, we 

 found that the curves for equal intensities of radiation were 

 * Kaye, 'X Rays,' pp. 121-123, Longmans, Green & Co., 1914. 



