Abstract: Disclosed herein is a physiological, electrically heated, thermostatically controlled sensing unit of the type used in the medical field for measuring blood contents, such as oxygen. A typical unit comprises a housing for application to the skin of a living being, an electric heater element within the housing, and a temperature-responsive switching circuit, also accommodated within the housing. The switching circuit automatically disconnects a power supply to the heater element when the skin temperature exceeds an adjustable predetermined threshold level. The power supply for the switching circuit is directly connected to the power supply for the heater element within the housing.