56 BELL SYSTEM TECHNICAL JOURNAL 



This t\j)c (){ button was relatively insensitive and shortly after its 

 introduction the suggestion (Hunnings, 1878) was made that the 

 space between the diaphragm and the fixed electrode be "partially 

 filled with puKerized engine coke," '"* in order to increase the number 

 of contact points and render them more susceptible to the forces 

 developed by the motion of the diaphragm. When at its best the 

 Hunnings transmitter was fairly efiicient but at times was erratic in 

 its performance due in part to the nature of the microj^honic material. 

 In 1886 P2dison ^^ proposed the use of granules of hard coal Avhich had 

 been heat treated. This was an important advance, for carbon made 

 from anthracite coal is used not only in the microphones which are 

 being considered in this paper but in commercial telephone trans- 

 mitters as well. 



As in the case of the condenser microphone, the displacement of the 

 diaphragm of the carbon microphone must be substantially constant 

 at all frequencies if uniform response is to be obtained. In the early 

 microphones of the carbon type, diaphragm resonance introduced 

 rather prominent irregularities in response. Air damped stretched 

 diaphragms offered one solution of this problem. During the World 

 War instruments of this type were developed and applied to the 

 problem of locating airplanes. In 1921 double button stretched 

 diaphragm microphones were made available for use with the public 

 address equipment installed for the inaugural address of President 

 Harding and the excercises at Arlington on Armistice Day.^^ The 

 carbon microphones employed in sound picture recording are of the 

 stretched diaphragm double button type. The electrical output 

 from this type of microphone is not only of substantially uniform 

 intensity over a wide frequency range but due to the "push-pull" 

 arrangement of the buttons is comparatively free from harmonics. 

 A typical example of the present day carbon microphone is shown in 

 the photograph, Fig. 7. F'ig. 8 is a cross-sectional view of the same 

 type of microphone. 



The diaphragm is made from duralumin. .0017 inch in thickness and 



is clamped securely at its outer edge. The clamping surfaces are 



corrugated and emery cloth gaskets are provided to prevent slipping. 



The stretching of the diaphragm is done in two steps. The initial 



stretching ring is first advanced by means of six equally spaced screws 



until the diaphragm is smooth and free from irregularities. The inner 



or final stretching ring is then adjusted to a position which gives the 



1^ "Beginnings of Telephony," F. L. Rhodes, p. 79, 1929. 

 «U. S. Patent Xo. 406,567, 1889. 



"> "Public .Address Svstems," 1. W. (".rtn-n and |. I'. .Maxtield, .1. /. E. E. Journal, 

 .\pril 192.^ pp. .U7-358. 



