Abstract: A movable permanent magnet (38) is moved by a rotating rig (29, 30) driven by a spiral thermometer (27). This magnet is designed to activate an interrupter (43) of an electric circuit housed in a sonic detecting-warning device (44) capable of rotating on the periphery of a housing 11. The circuit also comprises a battery (45) and a buzzer (46). When the magnet (38) rotates past the interrupter (43) it causes the interrupter to close, thereby actuating the buzzer (46). The angular position of the detector (44) is selected according to the temperature at which the warning signal is to be given.