of the shell the application of a metallic layer 7 is essential to prevent a plasma from forming here the metalization can be applied simply to the interface between the ceramic 4 and the shell 3 independently of the chamber 6 fig4 a to 5b illustrate additional arrangements that allow the behavior of the chamber to adapt automatically to the temperature conditions of the spark plug so as further to improve the u201c non linear u201d regulation of the thermal index of the spark plug particularly to make it behave like a very hot spark plug when the engine is still cold and like a warm spark plug when the engine is hot particularly under heavy load as indicated in these figures the chamber 6 may contain an expansion piece 8 9 capable of opening or closing its inlet to hot gases when the temperature is low the expansion piece is contracted and opens the passage to the hot gases which supply a stream of heat accelerating the operation of the spark plug once the spark plug has reached its operating temperature the piece is expanded and closes the passage to hot gases thus the spark plug reaches its thermal equilibrium at a lower temperature than if the chamber had.