Thermostatically controlled electric immersion heater for aquariums

In an immersion heater for an aquarium including, within a vertically disposed waterproof elongated transparent tubular envelope, an electric heating element in the lower portion of said envelope and an adjustable temperature regulating unit in the upper portion of said envelope controlling operation of the heating element, the temperature regulating unit including both a bimetal strip responsive to the temperature of said envelope and axially movable adjustment means operable from the exterior of said envelope for varying the rest position of said bimetal strip and thus the operating temperature of the device, the improvement wherein said adjustment means includes in combination

• a threaded pin (13) having an end resting against the bimetal strip (7) and rotatable by means extending through the upper end of said envelope to the exterior thereof,
• a cylindrical body (11) rotatable with said threaded pin and provided on its outer surface with a helical line (12) running over the entire length of said outer surface and dividing it into portions,
• a screen (18) within said envelope adjacent said cylindrical body and visible from the exterior of said envelope, said screen having a vertically disposed longitudinal slot (19) through which a short portion of said helical line (12) is visible,
• at least one graduated scale alongside said slot indicating temperature values which, for each angular position of said regulating unit (3) and, therefore, for each axial position of its end in contact with the bimetal strip (7), correspond to an operating temperature of the device as indicated on said scale by that portion of the helical line (12) visible through the slot (19).