Abstract: A glow plug has a sheath heater, a heating coil and a control coil. The sheath heater has a sheath tube having a closed leading end. The heating coil is disposed adjacent to the leading end of a projection of the sheath tube beyond a main metal shell. The control coil is connected in series to the rear end of the heating coil and arranged to be heated by the heating coil to raise the electrical resistance to control the supply of electric power to the heating coil. The projection of the sheath tube has a substantially uniform outer diameter in the axial direction and the outer diameter is between 3.0 mm to 4.4 mm. Therefore, a temperature rising characteristic at the surface of the leading end of the sheath tube has a peak temperature TP in an initial stage of energization and saturates at a temperature not higher than the peak temperature TP.

Abstract: A sheathed heater comprises a heater disposed in a sheath tube by way of an insulation powder and is connected in series with a heat resistor and a current regulation resistor. The current regulation resistor is made from a cobalt-copper alloy which contains a copper component in the range from 1.0% to 14% by weight.