Abstract: The glow plug comprises a tubular metal sheath (5) closed at the tip, in which an electric spiral (7) is placed as embedded within a powder (6) of electrically insulating material.The filament of the initial length (L.sub.1) of the spiral, from the connection location with the current feeder (4), is coated with high electric conductivity material, the resistance of which has a very high positive temperature coefficient with respect to that of the filament.For the filament an alloy of Fe--Cr--Al is advantageously used and for the coating a Ni-based conductive material.Thus, the end length (L.sub.2) of the spiral acts as sheath heating element, while the initial coated length (L.sub.1) acts as supply current control element.Application: preheating plugs for diesel engines of motor vehicles.Advantages: reduced cost of the glow plug due to construction simplicity. Possibility of varying the electric and thermal characteristics of the glow plug by simply varying the dimensions of the conductive coating.

Abstract: A glow plug for preheating the combustion chamber of a diesel engine includes a tubular metal sheath (5) closed at an end to define a tip and in which an electric heating element spiral (7) is embedded within a powder (6) of an electrically insulating material. The spiral (7) is connected by one end to the sheath tip and and at its other end to a current feeder (4). The filament (8) of the spiral (7) is advantageously made of a Fe-Cr-Al alloy and is coated for an initial length (L.sub.1) of the spiral (7), from the connection location thereof with the current feeder (4), with a high electrical conductivity material (9), preferably nickel (Ni) having a very high positive temperature coefficient of resistance with respect to that of the filament (8). The uncoated end length (L.sub.2) of the spiral (7) acts as a heating element while the initial coated length (L.sub.1) acts as a control resistance element for the supply current upon increase in the sheath temperature.