PURPOSE: To maintain an optimum and stable light flux level at all times by forming a temp. gradient in the neighborhood of the melting point of amalgam as extending in the longitudinal direction of an amalgam accommodation part, and arranging amalgam in the accommodation part so as to be continuous alongside this temp. gradient.
CONSTITUTION: Amalgam 11 is adhered to the inner surface of a glass thin tube 10 wide spreadly and continuously. The ambient temp. of the amalgam 11 is largely influenced by the heat from an electrode 9, and a high temp. part 11H is formed in the amalgam portion near this electrode 9 while a low temp. part 11L is formed in the amalgam portion remote from the electrode 9. In steady lighting, for ex., at an atmospheric temp. of 25°C, the high temp. part 11H shall be at 102°C while the low temp. part 11L at 92°C, the temp. difference being 10°C. Setting of temp. difference being possible in this manner at a short amalgam distance is because the amalgam 11 is put widely in close contact with the inner surface of the glass thin tube 10, and the heat radiating effect of the tube 10 is greatly reflected in proportion to the thermal conductivity of the amalgam 11 alloy. As a result, the Hg vapor pressure of the lamp concerned is restricted with its lowest value.