tone variator

German psychologist William Stern invented the tone variator in 1897 to study human sensitivity to changes in pitch, going beyond the traditional psychophysical research of studying the sensitivity to differences in discrete tones. The instrument consists of an adjustable brass resonator, which is supplied with a constant flow of air across the opening at the top. Turning a graduated cam on the front of the apparatus raises or lowers a piston in the bottom of the resonator, changing the volume of its interior, thus altering the sounded pitch over a continuous range.