AbstractsEngineering

The use of a heated fine wire as a delicate detector of fluid velocity and pressure has been known for many years. King, in an intensive series of experiments and theoretical examination, developed the hot-wire anemometer and indicated the linear relation between the heat lost by a hot wire in a moving stream of air and the square root of its velocity, at high velocities. The hot-wire microphone, a sensitive means of detection of acoustic waves was brought into prominence by Tucker and Paris. With grids of fine platinum wires mounted in the necks of suitable resonators they were able to detect, selectively, low frequency sound waves which were quite inaudible. In their method, a grid of wires raised to high temperature above ambient by the passage of d.c. current was exposed to the resonantly-amplified air-particle velocity, thereby effecting average and periodic cooling, producing average and periodic resistance changes. The former was measured with a Wheatstone net, the latter produced a periodic voltage which was amplified and measured with a tuned galvanometer. Also developed by them was an empirical relation which indicated that the heat lost by the grid varied as the square of the velocity of unidirectional air, at very low velocities.[...]