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Do what I did. Look up HEARING AID HISTORY. Then look for images. Remember, the telephone had been invented long ago and was in service.

With the phone came all the makings for most of a hearing aid. There was a pickup microphone and a speaker. It was all 12 or 24 volt DC gear so you could put a decent paste or dry cell in the box with it for power.

All you needed was a simple amplification circuit, and you could do that "electronically" (by the time tubes came around) or "electrically" with a simple linear induction circuit ~ couple of wire coils of different sizes, one size hooked up to the speaker, another size hooked up to the microphone, and one or the other hooked up to a permanent magnet, or maybe even an electromagnet.

Just about anything would work.

I suppose you could have done this with different size cans, a good stiff cord, and a fair sized room where no one was walking around running into the cord.

This hearing aid was no more complicated than the Q-Multiplier sold to boost receptivity in a shortwave radio (way back when).