What is "tonewheel"

n. A mechanical apparatus which converts the spinning movement of an electric motor into electronic musical notes. Used principally in Hammond organs.

A tonewheel or tone wheel is a simple electromechanical apparatus for generating electric musical notes in electromechanical organ instruments such as the Hammond Organ. It was invented around 1910 by Rudolph Goldschmidt and was first used in pre vacuum tube radio receivers as a beat frequency oscillator (BFO) to make continuous wave radiotelegraphy ( Morse code) signals audible.