D O C U M E N T 2 7 7 O C T O B E R 1 9 2 8 2 7 3 277. To Max von Laue [Berlin,] 3 October 1928 Dear Laue, I shall be very happy if you would send me offprints of your handbook articles.[1] But with your interpretation of the paradox, you didn’t quite hit the target.[2] It is in fact not a paradox, but instead only an unprejudiced question taking account of the well-known circumstance that the Lorentz-force approach is a rela- tively poorly tested hypothesis of Maxwell’s theory, which itself cannot be derived from the field equations. The electrodynamic forces that act on the radial charge currents in a magnetic field have nothing to do with the phenomenon in question. For one could just as well imagine those currents to be due to slip rings at the periphery of the disk. The disk need not even be in a magnetic field. I suggest changing the setup as follows: iron core —rotatable ring with leads [slip rings] for a charge current. The ring can be slit, in order to avoid ind.[uction] currents. If the ring is charged (with a charge ), then a torque will act upon it if the flux F through the iron core changes over time. The torque is prop.[ortional to][3] . If, in contrast, changes with time, then according to Lorentz, there is no force. If both and F vary, then the integral is in general nonzero, even for cyclic processes. This is [the situation] according to Maxwell-Lorentz. Other hypotheses are cer- tainly possible. For example, one could begin with the equations and multiply them by , obtaining for example[4] .  dF dt -   dF dt ------dt- 2  x2  ----------- -   =   x  ---------  x  --------   x  ------------------   x  1 2   2  x  --------- –     x  ---------  =