62 BELL SYSTEM TECHNICAL JOURNAL 



mission loss. This filter may be located in the base of the mounting 

 or in a container fastened to the back of the microphone. 



Aging of granular carbon may result from changes in the contact 

 surface caused either by mechanical abrasion or overheating due to 

 excessive contact potentials. Aging is usually accompanied by an 

 increase in resistance and loss in sensitivity. Care should therefore 

 be exercised in the use of the carbon microphone that it is not sub- 

 jected to unnecessary vibration which would cause the granules to 

 move relative to one another and abrade the surfaces. The use of 

 abnormally high voltages should also be avoided. 



The quality of transmission obtained with the double Initton carbon 

 microphone compares favorably with that secured with a condenser 

 microphone. The carbon microphone also requires less amplification. 

 There is, however, one characteristic which limits its use, namely 

 carbon noise. The level of the noise is much higher than that due to 

 thermal agitation within the carbon granules ^^ and appears to be 

 caused by heating at the contacts between the granules. A certain 

 amount of gas is contained in the pores in the contact surfaces. When 

 current passes through the button, a sufiicient increase in contact 

 temperature takes place to cause a portion of this gas to be driven off 

 and produce the non-periodic changes in resistance which give rise 

 to carbon noise. 



In conclusion it may be stated that the condenser and carbon types 

 of microphones have been developed to a point where there is little to 

 choose between them from the standpoint of quality of transmission. 

 The design from a mechanical standpoint has also been carried to a 

 point where little difficulty should be experienced in their use if reason- 

 able precautions are exercised. Although requiring less amplification 

 than the condenser microphone the extent to which the carbon micro- 

 phone is used at present is limited by the higher noise level obtained. 

 The condenser type of microphone has therefore been adopted for 

 most of the recording work in the sound picture field. 



'* "Thermal Agitation of Electricity in Conductors," J. B. Johnson, Physical 

 Review, July 1928, pp. 97-109. 



