How could spinning metal wheels help an organ make musical tones?

A classic Hammond tonewheel organ can produce a musical tone from a spinning metal wheel without striking the wheel like a bell. The rotation first creates a repeating electrical signal. An amplifier and speaker later turn that signal into the sound heard by the audience.

Each wheel has repeated high and low features around its edge and turns near a magnetic pickup. The pickup includes a magnet and a coil of wire. As the edge features pass, they change the magnetic flux through the coil—the magnetic field's contribution across its area. This change induces a small voltage, so a repeating mechanical pattern produces a repeating electrical waveform.

The signal's repetition rate depends on both how quickly the wheel turns and how many edge features pass during each turn. More features at the same rotation rate produce more repetitions per second. Different wheels and rotation arrangements therefore provide different tone frequencies without requiring one vibrating string for each.

Pressing a key connects selected signals into the instrument's sound path. Drawbars change the strengths of several components, altering the mixture and the tone's character. The amplifier supplies power to drive a speaker, whose moving surface disturbs the air. The wheel sets a signal pattern, the controls select and combine patterns, and the speaker makes that electrical pattern audible.