XI. A selective hot-wire microphone

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Abstract
The instrument described in the following paper provides:— (i) A convenient means of detecting a note of given pitch when other sounds are present; and (ii) A method of estimating the relative intensities of sounds of the same pitch. The idea which formed the starting-point for the construction of the instrument—viz., the placing of an electrically heated grid of fine platinum wire in the orifice of an otherwise closed vessel—was originally employed by one of us (W. S. T.) in the construction of a sound-detector for the use of Sound Ranging Sections in the British Army. In its original form, the detector was intended to respond to heavily damped aerial vibrations, such as those produced by the firing of guns. Further experiments, however, showed that the detector could be tuned to respond to any continuous sound of definite frequency by suitably choosing the dimensions of the vessel and its orifice.

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