Beam powers and black hole masses of 48 extended radio sources are combined to obtain lower bounds on the spins and magnetic field strengths of supermassive black holes. This is done in the context of the models of Blandford & Znajek (the "BZ" model) and Meier; a parameterization for bounds in the context of other models is suggested. The bounds obtained for very powerful classical double radio sources in the BZ model are consistent with black hole spins of order unity for sources at high redshift. The black hole spins are largest for the highest redshift sources and decrease for sources at lower redshift; the sources studied have redshifts between zero and two. Lower-power radio sources associated with central dominant galaxies may have black hole spins that are significantly less than one. Combining this analysis with other results suggests that the maximum values of black hole spin associated with powerful radio galaxies decline from values of order unity at a redshift of 2 to values of order 0.7 at a redshift of zero, falling roughly as, while lower power radio sources have spin values that range from about 0.1 to 0.8. These black hole spin values decrease if the data are considered in the context of the Meier model rather than the BZ model.
black hole physics - galaxies: active - galaxies: nuclei