From separate taps to a steady tone
A toothed wheel turns sound into a sequence of regularly spaced impacts. As each tooth passes the edge of a card, it flicks the card and makes a brief tap. When the wheel turns slowly, the taps are far enough apart to hear one by one. As it speeds up, they arrive too quickly to distinguish, and the ear hears a continuous tone instead.
The pitch depends on how often the teeth strike the card. A wheel with more teeth, or one spinning faster, produces more impacts each second and therefore a higher tone. That gives the experimenter a way to change the sound in a controlled, repeatable manner rather than relying on an instrument that simply plays a note.
Turning a sound into a hearing test
Savart could vary the wheel’s speed and ask what a listener could hear. The wheel supplied a sound whose pitch could be shifted by changing its rotation; the listener’s response showed where that sound became audible or no longer distinguishable. In effect, the apparatus linked a mechanical setting to a listening experience.
That link mattered because hearing is difficult to measure by asking someone to describe it. A repeatable tone gives the test a reference: the same wheel setting can produce the same pattern of impacts. The spinning wheel made pitch adjustable and testable by turning motion into sound.
