The invention of the microphone and telephone led to the development of the first electronic hearing aid in the 19thCentury. In 1898, Miller Reese Hutchison developed the Akouphone, in which a carbon transmitter amplified weak signals with an electric current.
Soon after, in 1920, vacuum-tube hearing aids were developed. They converted sound into an electric signal and amplified it at the receiver.
The next electric breakthrough came with the invention of the transistor in the 1940s, which replaced vacuum-tubes with smaller components that required less power and gave an even clearer signal. Transistors allowed hearing aids to become truly portable, and with new materials like silicon, the behind-the-ear hearing aid became the basis for most modern aids.