1 7 4 D O C U M E N T 1 7 0 A P R I L 1 9 2 8 170. To Wander J. de Haas Berlin, 11 April [1928] Dear De Haas, First of all, my touching and emotional lament that the dear lady didn’t find me at home[1]—that was my bitter bad luck, and it wasn’t the end of it. I arrived back from the trip with an abominable heart problem, with a rapid, cringingly weak pulse, and I will have to lie in bed endlessly.[2] However, I am not writing to you about all these things, but instead about the damned electron moment. I namely be- lieve that I have really found a method of detecting it, if it exists at all. 1) Can we produce an electron moment at ordinary temperatures? Now, why not? The incandescent filament produces an electron vapor pressure. This has to come to equilibrium with the walls of the bulb, leaving aside the electrostatic field. The electrons collide frequently with the walls, but they can’t stick to them forever. They give up their high velocities and become an ordinary gas. 2) Through the wall of the bulb, an electrically insulated iron needle projects into its interior. It becomes negatively charged up to a certain limiting potential, because of the electrons, and [that potential] depends on the temperature of the electron gas. If the needle is now magnetized, then its potential will change by a small amount, and the additional voltage should be readily detectable using an electrometer—around V, presuming that magnetic moment [of the electrons] indeed exists. What do you think of all this? Everything depends on whether the damned elec- trons stay mobile along their racetrack. With warm greetings to the entire holy family and the Ehrenfest gang,[3] your A. Einstein vacuum bulb incandescent filament 1 1000 ----------- -