Mr. Sommerfeld has proposed a theory which he wants to relate to that of Mr. Planck, although the only connection between them is that the letter h appears in both formulas, and that the same name of quantum of action has been given to the two very different objects which this letter represents.
The collision of electrons would not follow the same laws at all as of the complex bodies we know and are accessible to experience. When an electron encounters an obstacle, it stops all the faster as its speed is greater (if this law were applicable to railway trains, the braking problem would appear in a new light). And that applies to the production of X-rays. Cathode rays are moving electrons; these electrons stop when they meet the anticathode; this abrupt stop disturbs the ether whose vibrations produce the X-rays. Mr. Sommerfeld’s theory explains why the X-rays are all the more penetrating and “harder” as the speed of the cathode rays is higher. The higher this speed, the more sudden the stop, the more, therefore, the disturbance of the ether is intense and of short duration.