By means of a small heating coil, the temperature of a thermocouple junction is made to oscillate regularly between two temperatures T1 and T2. Simultaneously the junction makes about one contact every two seconds with the surface being measured. A short-period galvanometer in the thermocouple circuit produces on a moving photographic film a serrated sine curve whose serrations vanish once for every half-oscillation at a point that corresponds to the surface temperature T3, provided T3 lies between T1 and T2. This mode of measurement minimizes disturbances due to air currents. Alternatively, use of a thermocouple with two adjacent junctions is proposed. A simple but accurate portable thermostat for maintaining reference junctions temperatures somewhat higher than room temperature is also described.