Abstract: A device, for assuring selective commutation of one or the other of two electric measuring circuits to a single electric exploitation circuit as a function of the temperature gradient present between two distinct points of reference, has a first thermocouple placed at the first point of reference; a second thermocouple placed at a second point of reference, connected in opposition to the first thermocouple in order to deliver a differential electro-motive force (.DELTA.e) of which the value and the sign are a function of the value and sense of the gradient; and commutation means sensitive to this electro-motive force and fed solely by the energy produced by the two thermocouples. The commutation means includes a relay fed by the differential electro-motive force.