The instrument consists of two fine platinum wires mounted close together, and forming two of the arms of a Wheatstone bridge. These are heated by the current in the bridge. When a stream of gas moves in a direction perpendicular to the wires but parallel to the plane containing them, the leading wire is cooled, while the second wire, being shielded by the first, is not cooled as much, and may actually be heated on account of the air flowing past it being warmed by the first wire. A deflection of the galvanometer is obtained, therefore, which is reversed if the flow of gas is in the reverse direction. For low rates of flow of gas, the instrument is much more sensitive than the non-directional hot-wire anemometer. An explanation of the increased sensitivity is advanced.