A Brief History of the Hearing Aid

1890 The electric hearing aid is patented by Alfonzo Miltmore. These devices make use of a carbon dust microphone, an earphone and a very large battery pack yet are very unreliable in efficiency due to humidity, dust particles, and the movements of the wearer. The carbon microphone consists of carbon particles in between a fixed electron plate and a vibrating electron. The current flowing between the electrons varies depending on the pressure exerted on the vibrating electrode by the carbon particles providing amplification.
1898 The first electric aid, the Akoulathon, reaches production.
1900 The carbon ball is invented by Hutchison and Kelley, greatly improving the reliability of carbon microphones. These hearing aids utilize a microphone, an earphone, an amplifier (the vacuum tube is located here), and two batteries, one to warm the vacuum tube filament and one to amplify.
1934  The vacuum tube hearing aid is introduced and produced by multiple English manufacturers.
1947 The transistor is invented and quickly adapted to the hearing aid. The transistor, along with advances in battery technology, make the devices much smaller, cheaper and energy efficient.
1959 Nicholas & Clark make an in-the-canal (ITC) device.
1962 The Zeta noise blocker is invented by Linear Technology. This device incorporates circuitry to remove environmental noise from speech.
1980-1985 Many circuits are introduced that make use of analog sound processing.
1987 A joint project between the University of Wisconsin and Nicolet Instrument Cooperation produces the first digital hearing aid.