Tonewheel

A tonewheel is a simple electromechanical apparatus for generating electronic musical notes. The tonewheel assembly consists of a synchronous AC motor and an associated gearbox that drives a series of rotating disks. Each disk has a given number of smooth bumps at the rim; these generate a specific frequency as the disk rotates close to a pickup assembly that consists of a magnet and coil. As each bump in the wheel approaches the pickup, it temporarily concentrates the magnetic field near it, and thus strengthens the magnetic field that passes through the coil, inducing a current in the coil by the process of Electromagnetic induction. As the bump moves past, this concentrating effect is reduced again, the magnetic field weakens slightly, and an opposite current is induced in the coil. Thus, the frequency of the current in the coil depends on the speed of rotation of the disk and the number of bumps. Typically, the coil is connected to an amplifier through a network of switches, contacts, resistor banks, and transformers which can be used to mix the fluctuating current representing the note from one coil with similar currents from other coils representing other notes. A single fundamental frequency can thus be combined with one or more harmonics to produce complex sounds. Tonewheels were first developed for and used in the Telharmonium and later in the original Hammond organs.