These are based on the telephone principle; however, Alexander Graham Bell had nothing to do directly with their development. These appeared first in limited quantities in a table model about 1899, but in wearable and practical instruments beginning only in 1902. Carbon aids were popular through the 1940's. Most of these used a rather large 3-volt or 6-volt battery but did not have enough power to assist those with more than a moderate hearing loss.
This was a hearing aid used by priests during confession. The user would hold the receiver (attached by the cord) to his ear and hold the rest of the device toward the speaker. Made by the Confessional Hearing Aid Company of Port Washington, New York.
1920s Carbon Hearing Aid
Carbon (that is, electric) hearing aids were a decade old and becoming popular, but non-electric devices were still used by many. Carbon aids required a single (but large) 3-volt or 6-volt battery.
Made about 1905. There are four microphones in a wooden case. Measures 11.3 x 45.4 x 5.1 cm. Used for listening in groups, such as in church.