Abstract: For use in a swirl-producing recombustion chamber of a Diesel engine, a glow plug comprises a shell which can be screwed into an engine block so as not to protrude into the precombustion chamber and a cylindrically shaped heating head attached to a plunger which is received in the shell and can be electromagnetically or hydraulically moved relative to the fixed shell. The heating head can be protruded from the shell so as to extend into a central region of the precombustion chamber when preheating of air is necessary but, at other times, can be retracted almost entirely into the shell so as not to obstruct violent swirling of air in the precombustion chamber.