When a hot-wire is exposed to an air-stream near an industrial area, it collects a deposit consisting of particles of soot and tar. The deposit, by changing the heat-loss from the wire, can decrease the sensitivity by as much as 20%. Even in cleaned air, the useful life of a hot-wire may be no more than 2 hours. It has been found that repeated measurements of turbulent velocity, under the same conditions but using different wires, differ from one another by 4% or more. A technique has therefore been developed for cleaning hot-wires, so that a single wire can be used repeatedly; the sensitivity then remains constant to within 2%. Making use of this technique, measurements of turbulent velocity have been reproduced to within ½%.