In the preceding parts, I have mentioned several microphone (transmitters, in telephone lingo) types that were used in early telephone experiments. The first type, the one described by Borseul, was a make-and-break type transmitter, which was used by Reis. This type of transmitter is not very useful, and can hardly transmit understandable speech. The very closely related loose-contact transmitter, is the basis for the carbon microphone. Also related is the variable resistance transmitter used by Elisha Gray in his bid for the telephone. A needle was attached to the transmitter diaphragm, and the other end of the needle touched the surface of a conductive liquid in a conductive cup. When the diaphragm vibrated, the needle made more or less area in contact with the liquid, and the resistance of the circuit varied.
Read more…A brief history of electroacoustics, pt. 3:
Microphones