How was the hearing aid invented?

Hearing AIDS came out at the beginning of 19 century, and their shapes are generally trumpet-shaped. These devices are usually bulky, inconvenient to carry and not easy to use. 1923, Marconi company developed a new type of hearing aid, which uses vacuum tube to amplify sound and battery to supply power. However, this equipment is still inconvenient and weighs about 16 pounds. By the 1930s, people had smaller tubes and batteries, and hearing AIDS weighing about 4 pounds appeared and quickly put on the market. 1935, Edwin Stevens introduced the first hearing aid that can be worn on the head through Amplifox, weighing only 2.5 pounds. 1950 or so, with the advent of transistors, hearing AIDS have become smaller and smaller, and finally a small instrument that can be completely put in the ear has appeared.