D O C . 2 9 2 F E B R U A R Y 1 9 2 0 2 4 3 Ehrenhaft’s work. To me, the matter apparently stands thus—that, on the one hand, the current state of theoretical physics calls for the assumption of the electron on the other hand, Ehrenhaft’s observations are incompatible with it, grounded on cur- rently accepted laws for falling motion and motion in an electric field. It is easily possible that the electron will carry the day. Then, however, Ehrenhaft’s experi- ments show that the laws assumed hitherto will have to be modified for very small particles, which likewise is an important finding, even though it probably would not have the importance of proving that there existed quantities of electricity smaller than the electron. The situation is similar for negative photophoresis as well, it seems to me. The detection of this surprising fact remains an important accom- plishment of experimental physics, even if it should be wholly attributable to the radiometric effect and the like.[2] For all known radiometric effects are—if my knowledge is not too sketchy—rotations. In any event, by demonstrating a linear motion toward the light, this experiment has run ahead of the theory, regardless of how theory will accommodate it. Besides that, Ehrenhaft’s research has brought to light some other important things as well, for ex., securing determination of the size of tiny particles experimental proof of the Schwarzschild-Debye maximum for light pressure.[3] I am very much obliged to you for your reference to J. Franck. Although my at- tention had already been drawn to him, I did not have any time to immerse myself in his papers, which are somewhat far afield for me, and was therefore not in a po- sition to evaluate him against other experimental physicists.[4] Only now do I come to the main purpose of my letter. You characterized your letter as strictly confidential yet I would need a letter that I could make use of with the faculty. This you will gather from the following sketch of the state of affairs, which I likewise request be handled in strict confidentiality. Here we have three regular professors of physics. Two of them are connected with larger institutes (1st Phys. Inst., Ernst Lecher 2nd [Phys. Inst.], Franz Exner). The third chair (for theoretical physics, Gustav Jäger) is not connected with any laboratory.[5] Prof. Ehrenhaft works in the 1st Phys. Institute. His relationship with its director Lecher is not a good one, which explains Ehrenhaft’s many work con- straints. Exner is retiring upon reaching the age limit his is the position to be filled. Of most urgency in the new appointment, it seems to me, is the exigency that we obtain an institute director who knows how to direct experimental investigations on a grand scale. For this neither the no-longer young Lecher, nor Jäger, who is chiefly a theoretician, is suited. An appointment from Germany is presently scarcely prac- ticable, owing to the financial and currency-exchange conditions. Thus, one is led to consider the following gentlemen: Ehrenhaft, furthermore the Professors
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