The invention relates to a thermocouple that is obtained by adding a system for the ohmic heating of the thermocouple pins or bars to the components of any conventional or passive thermocouple, said system consisting of an electrical resistor (17) embedded in a ring (16) made from the same material as one of the pins or bars (3) of the original thermocouple to which the ring is welded (20). The temperature of the thermocouple is increased, by means of ohmic heating, above that of the fluid for which the convection is to be characterised and, subsequently, the ohmic heating current is cut, resulting in a drop in the temperature of the thermocouple, the value of the convection coefficient being deduced directly from said temperature drop.