The 461891 is a contact Tachometer. Surface wheels measure contact surface speeds (10 to 9999 rpm) of rotating objects. The memory button holds last reading and recalls min/max readings display. Complete with wheels for linear surface speed (0.2 to 6560 ft/min, 0.05 to 1999.9 m/min) or RPM measurements. Comes with cone tip, flat tip and spare wheel for surface speed measurements, four AA battery, carrying case and user manual.