The paper is about the appearance of space charge in an ohmic conductor moving in a magnetic field, as pointed out in this journal by Lorrain (1990 "Eur. J. Phys." 11 94-8) and earlier by van Bladel (1973 "Proc. IEEE" 61 260-8). The phenomenon is reinvestigated here in the light of energy balance considerations, in the particular case of a cylinder rotating in a parallel uniform magnetic field. Part of the kinetic energy of the rotating body is lost in the Joule effect of the currents building up the space charge, but the greater part is converted to the electrostatic energy of the space-charge distribution. The findings of the paper are consistent with the usual account of the electrodynamics of moving media where the existence of space charge in good conductors is not recognized. The material of the paper can be taught at the end of second-year university lectures on electrodynamics. (Contains 4 footnotes and 1 figure.)