Overview
Spintronic ammeters indicate the speed of current by the pitch of sound they make. The more current, the higher the pitch.
Detailed description
The pitch of sound generated by the spintronic ammeter is directly proportional to the current running around it. A small wedge attached to a diaphragm slides over a bumpy surface. As the wedge moves up and down over the bumps, it generates sound in the diaphragm.