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Equipment file · Telephone transmitter

Liquid transmitter

Alexander Graham Bell · 1876

Evidence status · documented

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

Alexander Graham Bell's liquid transmitter used a needle attached to a diaphragm and dipped in acidulated water, so that sound moving the needle varied the circuit's resistance. It is described in his US patent 174,465, 'Improvement in Telegraphy', filed 14 February and granted 7 March 1876. Bell used a version of it on 10 March 1876 for the first intelligible transmission of speech by wire, to Thomas Watson.

Introduced
1876
Category
Telephone transmitter
Intended use
Converting speech into a varying current for voice telegraphy
Production
Hand-built experimental apparatus by Bell and Thomas Watson; never mass-produced

Equipment photographs · 3

Liquid transmitter. 10th of March, 1876. Replica

1 / 3 · Liquid transmitter. 10th of March, 1876. Replica

Liquid transmitter. 10th of March, 1876. Replica·CodeName47km·CC BY-SA 4.0

Why it mattered

  1. 01

    It showed for the first time that a continuously varying current could carry intelligible speech, the principle behind every later microphone.

  2. 02

    It was soon abandoned as impractical, but the variable-resistance idea led straight to the [[edison-carbon-button|carbon transmitters]] that made amplified speech possible.

Engineering record

Operating principle
A metal needle on a speaking diaphragm dipped into acidulated water in the circuit; vibration changed the needle's depth and so the resistance, producing a current that followed the sound wave[1][2]
Patent
US Patent 174,465, 'Improvement in Telegraphy', filed 14 February 1876, granted 7 March 1876[1]

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 by Bell on 10 March 1876 in his Boston laboratory for the first known transmission of intelligible speech over a wire[3]

Manuals & original documents

Sources

  1. 1US174465A - Improvement in TelegraphyGoogle Patents / USPTO · patent · 1876
  2. 2A. G. Bell Liquid Transmitter, Patent 174,465Smithsonian National Museum of American History · archive
  3. 3Who is credited with inventing the telephone?Library of Congress · reference

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