A Cosmic Battery Reconsidered

We revisit the problem of magnetic field generation in accretion flows ontoblack holes owing to the excess radiation force on electrons. This excess forcemay arise from the Poynting-Robertson effect. Instead of a recent claim of the generation of dynamically important magneticfields, we establish the validity of earlier results from 1977 which show onlysmall magnetic fields are generated. The radiative force causes the magnetic field to initially grow linearly withtime. However, this linear growth holds for only a {\it restricted} timeinterval which is of the order of the accretion time of the matter. The large magnetic fields recently found result from the fact that the lineargrowth is unrestricted. A model of the Poynting-Robertson magnetic field generation close to thehorizon of a Schwarzschild black hole is solved exactly using GeneralRelativity, and the field is also found to be dynamically insignificant. These weak magnetic fields may however be important as seed fields fordynamos