The primary component of a unique method of sound generation, invented by Laurens Hammond, which produces musical tones by means of an electromagnetic tone generator. The tonewheel is approximately two inches in diameter and contains a series of notches (“teeth”) around its rim. The tonewheel revolves at the end of a magnetized rod. The teeth generate variations in the magnetic field, inducing an electrical voltage, which is amplified and sounded as a fundamental musical tone.

Most Hammond organs contain 96 tonewheels divided into eight groups. The number of teeth on a tonewheel determines the pitch. Octave “0″ wheels have two teeth; octave “1″ contain four; octave “3″ have eight; and continuing in powers of two to 128. The highest octave tonewheels have 192 teeth (rather than 256, which machinery wasn’t capable of cutting at the time). In most Hammonds, the last five tonewheels of the upper octave are not cut, have no pickups, and are included only for mechanical balance.