A SELECTIVE HOT-WIRE MICROPHONE. 
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(3) That the overtones are not in harmonic relation ; and 
(4) That the dimensions of the resonator need only be small compared with the wave¬ 
length of the sound to be observed. 
The simplified theory of such resonators is due to Rayleigh,* who showed that the 
number of vibrations in the resonant note is given by 
where V is the velocity of sound in the gas in the neck, S is the volume of the reservoir, 
and c is a quantity depending on the shape and dimensions of the orifice and called 
(from an electrical analogy) the “ conductivity ” of the orifice. 
§ 2. Description of the Microphone. 
The complete microphone, comprising the Helmholtz resonator with the platinum 
wire grid suitably mounted in the neck, is made for convenience in three separate parts :—- 
(i) The platinum wire grid, mounted in a circular mica plate. 
(ii) The “ holder,” which includes the neck of the resonator and the necessary 
contact-pieces and terminals for carrying current to the grid ; and 
(iii) The “ container,” or reservoir. 
A short description of each of these three parts will now be given. 
(i) The Platinum Wire Grid.— Fig. 1, A, shows one form of the grid. It consists of a 
circular plate of thin mica 4 cms. in diameter, in the centre of which is cut a circular 
hole 0 -65 cm. in diameter. A number of small pin-holes are punched at the edge of 
* Rayleigh, * Theory of Sound,’ vol. II., p. 174. 
3 h 2 
