Abstract: A vaporizer for anaesthetics or narcotics comprises a housing having an interior liquid anaesthetic reservoir and an evaporation chamber above the reservoir. An inlet into the housing divides so that there is a vaporizing flow passage over and through a coiled vaporizing tube wick in an annular passage down to the liquid anaesthetic, then upwardly in a central passage and past a regulatable annular passage to a discharge passage. The inlet also includes a bypass flow through a passage which joins an annular space above the vaporizing chamber portion above the anaesthetic liquid and back to the discharge passage. A control device is located in the second or bypass passage and comprises an annular trough-shape body having a closed bottom wall which is of a material of a high coefficient of thermal expansion. A hollow cylinder is positioned in the trough-shape annular body and forms a close fit therewith and has a low coefficient of thermal expansion relative to the trough-shape body.