The Microphone
   The carbon-contact microphone was devised early in 1878 by Prof. David Hughes of the University in London. Its operation is based on the fact that the resistance between two pieces of carbon touching each other is a function of the pressure in the region of contact. A carbon rod, pointed at both ends, is held loosely in holes drilled in two other carbon rods. The circuit is completed with a battery and a magnetic earphone wired in series with the microphone. The slightest jarring of the system, such as the ticking of a pocket watch, serves to change the resistance of the circuit, and the diaphragm of the earphone moves.

   REFERENCE: Thomas B. Greenslade, Jr., "The Microphone", Phys. Teach., 15, 495-496 (1977)

                                 Hampden-Sydney College                                           St. Patrick's College, Maynooth, Ireland

The microphone below is in the Jack Judson Collection at the Magic Lantern Museum in San Antonio Texas. The 1900 Max Kohl catalogue describes it as a model of a micro-telephone station, particularly suited for demonstrations, and prices it at 40 Marks (about $16). Attached to the rear of the cedar diaphragm is the sensitive carbon element. Note the adjustable couple transformer on the left-hand side; the telephone receiver was connected to one side of this transformer.

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