File:Tone variator from Max Kohl Catalogue, rack and pinion design.png

Tone variator in which the bottom of the resonator is not displaced by a spiral, but by rack and pinion; in these, an excentrically-operated pointer is used to indicate the frequency on the scale.

Source: Kohl, Max. Physical Apparatus / Vol. II. Apparatus and Supplies for General Use. Introduction to Physics. Mechanics. Wave Theory. Acoustics. Optics. Heat. Meteorology. Cosmology. Chemnitz, Germany. p 446.