A tachometer is a device that measures the angular speed of a rotating shaft in terms of revolutions per minute (RPM). The Extech 461750 Tachometer features surface wheels that measure contact surface speeds of rotating (10 to 9,999 RPM) and moving (3 to 6,560 ft./minute; 1 to 1999. 9 m/min) objects. Complete with 9V battery and surface wheels.