A Wheatstone‐bridge arrangement of capillary tubes carrying a flow of inert gas is described, with which the resistance to flow of a capillary tube serving as a thermometer may be measured. The resistance is shown to be proportional to the productT&eegr;, for constant mass rates of flow in the viscous regime. Gas‐viscosity thermometers have relatively open and linear scales, and are usable over wide temperature ranges. With helium gas, for example, the productT&eegr;varies strongly and smoothly with temperature from below 1° to above 1500°K and probably much farther. Experimental tests of Al2O3capillary thermometers in a high temperature furnace are reported. Some of the thermometers were reasonably stable at 1788°K.