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