tonewheel

thumb|Simplified diagram of how a tonewheel works thumb|Goldschmidt tone wheel (1910), used as an early beat frequency oscillator

A tonewheel or tone wheel is a simple electromechanical apparatus used for generating electric musical notes in electromechanical organ instruments such as the Hammond organ and in telephony to generate audible signals such as ringing tone. It was developed by Thaddeus Cahill for the telharmonium c. 1896 and patented in 1897. It was reinvented around 1910 by Rudolph Goldschmidt for use in pre–vacuum-tube radio receivers as a beat frequency oscillator (BFO) to make continuous wave radiotelegraphy (Morse code) signals audible.