The first electrical hearing aids were carbon and were made possible by the invention of the carbon microphone and were made between 1900 and 1939. Carbon, which can be made to amplify electrical current, was used to provide some amplification prior to the invention of vacuum tubes. Later models often came with mechanical (carbon) amplifiers to try to boost the amount of amplification. The carbon microphone was originally invented for use in the telephone and had the properties of being able to modulate current. It is this property that made it useful in the early electrical hearing aids.
An example of a carbon hearing aid. This has an imitation tortoise shell casing.
Round dark red and black imitation tortoise shell carbon hearing aid with clip for attaching to clothing, and a slide for adjusting the sound (?) a cord split between a plug and a speaker (?).
Inscriptions & markings
"SONOTONE/REG U.S.P.A./292392/MADE IN U.S.A./ELMSFORD, N.Y." on bottom of unit. "SONOTONE" on plug. "PAT RE 21030" on speaker?