Electric charge and magnetic flux on rotating black holes in a force-free magnetosphere

The electric charge on rotating black holes is calculated to be similar to BJ in the force-free configuration of Ghosh, with a horizon flux of similar to BM2. This charge is gravitationally weak for B similar to 10(15) G, so that the Kerr metric applies. Being similar to the electric charge of a magnetar, both electric charge and magnetic flux should be, in sign and order of magnitude, continuous during stellar collapse into a black hole. Extraction of the rotational energy from newly formed black holes may proceed by interaction with the magnetic field.