The roller comprises a conductor (42) coiled about an insulating resin bobbin (41), within a conducting outer roller (30). The conductor coils pass through grooves (46) or between projections provided on the surface of th bobbin to prevent contact between respective coils. Flange like projections on the bobbin or sliding spacers also prevent the outer roller from contacting the conductor if the outer roller or bobbin deform. The bobbin or core member may have a central hole to facilitate cooling and a processor programmed to prevent excessive cooling. The gap between the roller and coil is preferably 3 - 8 mm.
