I remember reading a reference to someone seeing a performer in the 1880s who had something hung from a cord around his neck, behind his banjo. The banjo was louder than the norm for stage instruments, and the observer wondered why. He watched closely as the performer went off-stage, and saw that it was a mirror, hanging on the cord, held against the chest by the banjo. It wasn't long before the resonator-ed banjo appeared. I assume that the same principle was applied by many different players/makers from Minstrel Show days on up till commercial banjos started to appear with resonators.

Tone rings have their own story.

Bob Clayton