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XI. A Selective Hot-Wire Microphone. 
By W. S. Tucker, D.Sc., A.R.C.Sc., and E. T. Paris, M.Sc. ( Land.). 
a 
Communicated by Prof. H. L. C allendar, F.R.S. 
"Received November 2, 1920,—Read January 20, 1921. 
Contents. 
§ Page 
1. Introduction. 389 
2. Description of the Microphone.391 
3. Electrical Connections.394 
4. Sharpness of Tuning of the Microphone. ... 396 
5. Sensitivity. 400 
6. The Resistance Changes in the Wire Grid.404 
7. Experiments on the Measurement of Sound.411 
8. The Effect of Tilting the Microphone.416 
9. Some Observations of Distribution and Intensity of Sound made with the Hot-Wire 
Microphone.•...423 
10 . Summary . 429 
§ 1. Introduction. 
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 other¬ 
wise 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 vibra¬ 
tions, 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. 
The tuned instrument is highly selective in its action. It is very sensitive when 
used to detect low-pitched sounds, but its sensitivity is diminished for the higher 
* British Patent No. 13123 of 1916, and No, 8948 of 1918 ; United States Patent No. 269902 of 1919. 
VOL. CCXXI.—A 592, 3 H 
[Published March 3, 19-1. 
