ResonantLive Sound Archive

Equipment file · Telephone transmitter

Carbon-button transmitter

Thomas Edison · 1877–1878

Evidence status · documented

Identity and core specifications are documented; some historical fields remain unavailable.

Thomas Edison's transmitter placed a button of compressed lampblack carbon beneath the diaphragm, so sound pressure varied its resistance and a battery supplied most of the signal power. Filed in 1877, the patent was not granted until 1892 (US 474,230). Western Union marketed the design before selling its telephone business to the Bell interests in 1879.

Introduced
1877–1878
Category
Telephone transmitter
Intended use
Boosting transmission with a pressure-sensitive carbon button
Production
Marketed by Western Union, later made under Bell and Edison patents

Equipment photographs · 2

Carbon-button microphone capsule from a Western Electric telephone

1 / 2 · Carbon-button microphone capsule from a Western Electric telephone

Carbon-button microphone capsule from a Western Electric telephone·Gerry Ashton·GFDL

Why it mattered

  1. 01

    Carbon's pressure-sensitive resistance became the basis of nearly all telephone transmitters for a century, and of the first public-address microphones.

  2. 02

    It fixed the weakness of Bell's transmitter and set off a decade of competing carbon designs.

Engineering record

Operating principle
A compressed lampblack button beneath the diaphragm; pressure changes varied its resistance and the battery current in the line[1]
Patent
US Patent 474,230, filed 1877 and granted 1892, covering a speaking-telegraph system with a carbon transmitter[2]

Original pricing

No reliable period price located

2026 equivalent

Not calculated without a reliable original price.

Public price evidence was not sufficient to publish a defensible original figure.

Use in the field

Used on Western Union telephone lines before Western Union sold its telephone business to the Bell interests in 1879[1]

Manuals & original documents

No authoritative manual or original document has been located for this profile.

Sources

  1. 1Telephone TransmitterThomas A. Edison Papers, Rutgers University · archive
  2. 2Carbon Telephone Transmitters - 1: The Edison Transmitter (1877)samhallas.co.uk · reference

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