Various output heaters were extruded with acetylene black-filled platinum-catalyzed silicone rubber. The resistivity-temperature behavior of extruded heaters exhibited a positive-temperature coefficient (PTC) effect without any negative-temperature coefficient (NTC) effect. Resistivity and thermal reproducibility of the extruded heaters were investigated during heating and cooling cycles at an applied voltage of 220 V. These heaters initially showed poor reproducibility of resistivity during the repeated cycles and this effect increased significantly as the acetylene black content decreased. PTC effect and electrical reproducibility were improved significantly during the thermal ageing process. (C) 2004 Wiley Periodicals, Inc.