gyroscope

A gyroscope is a rotor (a wheel or disk) spinning rapidly about its symmetry axis, usually mounted on gimbals that allow the axis of rotation to orient freely in space. The core physical property of a gyroscope is that its angular momentum — a large vector along the spin axis — resists changes imposed by applied torques. A torque perpendicular to the spin axis does not cause it to tip over; instead it causes the axis to precess slowly in a direction perpendicular to both the torque and the axis.

The gyroscope was turned into a scientific instrument by Léon Foucault in 1852, who used it to make the Earth's rotation visible: the axis of a freely-suspended spinning flywheel maintains its orientation in an inertial frame, so the Earth's rotation beneath it becomes directly observable. Foucault coined the name — from Greek gyros (circle) + skopein (to see).

In the twentieth century gyroscopes became navigational instruments of central importance. Elmer Sperry's 1908 gyrocompass aligned its axis with true (geographic) north regardless of the magnetic field, and became standard equipment on large ships. Aircraft use gyroscopic attitude indicators, directional gyros, and turn coordinators to tell pilots their orientation. Modern inertial navigation systems — in submarines, commercial aircraft, missiles, and spacecraft — use sets of orthogonal gyroscopes (often solid-state, using ring lasers or fibre-optic loops) to continuously track orientation by integrating the rotations they measure.