  The Nyquist formula quantifies the thermal noise driven fluctuations of
voltage across a resistance in equilibrium. We deal here with the case of a
resistance driven out of equilibrium by putting it in contact with two
thermostats at different temperatures. We reach a non-equilibrium steady state
where a heat flux is flowing through the resistance. Our measurements
demonstrate anyway that a simple extension of the Nyquist formula to the non
uniform temperature field describes with an excellent precision the thermal
noise. For a metallic ohmic material, the fluctuations are actually equivalent
to those of a resistance in equilibrium with a single thermostat at the mean
temperature between the hot and cold sources.
