3 o2 SCIENCE PROGRESS 



out by an X-ray tube depends approximately on the current 

 through the tube, while the hardness or penetrating power is 

 a function of the potential difference applied. 1 The character 

 and output of the radiation thus depend greatly on the pressure 

 at which the discharge passes : a low pressure means a large 

 electric intensity in the tube, fast cathode rays and hard Rontgen 

 rays. 



Emission of Rontgen rays. — Rontgen, in 1896, found that the 

 rays from an anticathode of platinum are more intense than 

 from one of aluminium in the same tube. The writer obtained 

 (1907-8) a relation between the atomic weight of a metal used 

 as anticathode and the intensity or quantity (measured by an 

 ionisation method) of the hard rays it emits — ra} r s which have 

 passed through about 2 mm. of aluminium. The result is graphi- 

 cally shown in fig. 1, and, as will there be seen, the intensity 

 of radiation and atomic weight increase together : the two are 

 indeed roughly proportional. It is interesting to note that the 

 rays from all the metals are, at this stage, of the same quality — 

 very little change in the relative intensities is produced by 

 increasing the thickness of the aluminium screen. Thick 

 screens of other metals yield much the same sort of curve, 

 modified slightly in the neighbourhood of the atomic weight 

 of the radiator. As the potential on the tube rises, the heavy 

 atoms increase their intensity values somewhat relative to 

 those of the lighter atoms. 



The rays from the different radiators were also measured 

 when they were cut down only by a very thin aluminium 

 window fixed in the Rontgen-ray tube. The order of the 

 intensities— which is not that of the atomic weights — agrees 

 with that derived by Starke for the relative numbers of 

 cathode rays returned from the surfaces of different metallic 

 reflectors. 



Adams 1 experiment. — As Birkeland was the first to show, in 

 1897, the different cathode particles in a discharge produced by 

 an induction coil have a wide range of velocities, and under the 

 influence of a magnet yield, owing to the varying deviations, 

 a visible " magnetic spectrum " on any phosphorescing surface. 

 J. M. Adams, at Harvard University (May 1907), has made a 

 neat application of this, to show that a hard X ray is associated 



1 The matter is however more complicated than this. See, for example, 

 Duddell {Rontgen Society's Journal, July 1908). 



