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