Electret transducers applied to the telephone

Successful application of electrets as telephone transmitter and less successful application as receivers are discussed. Amplifier gain and maximum input noise are derived from electret performance and system requirements. Simple corona charging is sufficient to produce stable fluorocarbon electrets with a useful life from 10 to 20 years. Two design approaches to compensate slackening of the electret diaphragm due to creep are described.