Franklin's inspiration for the instrument came during a visit to London in 1757, when musical glasses were very much in fashion. Performances of works by Gluck and others were done on drinking glasses of various sizes arranged in a row on a board and filled with varying amounts of water, so that each glass had a different pitch. The challenge to the inventor was to create a new instrument capable of more tones that would be easier to play. The Armonica was the result.
What Franklin came up with was an instrument that had 37 glasses of different sizes and thicknesses that are near hemispheres in shape and require no water to achieve the three octaves. Instead of standing upright, these glasses are placed sideways, mounted on an iron spindle that is run through the centers of the glasses so that the rim of one glass overlaps, but does not touch the next.
Franklin came up with a foot treadle to turn the spindle rotating the glasses that the performer wet with a sponge and then played, pressing the fingers against the rims to create the melodies. The action could be compared to caressing meat that turns slowly on a spit. The treadle has disappeared on Franklin's instrument, which he bequeathed to his daughter Sarah's husband, Richard Bache.
The glasses on the rod were placed in a handsome wood case, the original of which is believed to have been made according to Franklin's instructions by Charles James of Purpool Lane, London. The mahogany-veneered case is rounded on top and embellished with brasses.Armica.
It stands on graceful cabriole legs with pad feet. According to Carl Van Doren, nothing that Franklin "wrote between 1757 and 1762, no experiment undertook and carried out, absorbed him more happily than his musical invention."