1. A device for measuring or evaluating a characteristic of a heat flux exchanged between a first medium and a second medium, the device comprising a first thermoelectric element and a second thermoelectric element, the first and second thermoelectric elements being electrically connected in series and thermally connected in series, the first thermoelectric element being of a first type and the second thermoelectric element being of a second type different from the first type, the first and second types being such that the first thermoelectric element and the second thermoelectric element polarize with opposite polarities under the effect of the heat flux having a given direction and a given sign.