GYROSCOPE (from Gk. 70pos, gyros, circle+ ascareb, skopein, to view). The name given by Foucault to an instrument for the exhibition of various properties of rotation, and the composi tion of rotations first described by Bohnenberger in 1817. It differs from a top in having both ends of its axis supported. The invention is probably French or German, and in some of its forms it dates from about the end of the eigh teenth century, but no certain information can be obtained ns to the original inventor. Its ac tion is based on the principle that if a mass is set in rotation about its principal axis of inertia of greatest or least moment, it will continue to revolve about it, and unless extraneous force is applied the direction of the axis will remain unchanged. If, then, a mass of metal—as, for instance, a circular disk, loaded at the rim and revolving in its own plane—be made to rotate rapidly about its axis of greatest moment of in ertia", and if it be freely supported (in gimbals, like the box of a compass), the direction of its axis will be the same so long as the rotation lasts. It will, therefore, constantly point to the same star, and may be employed to show that the apparent rotation of the stars about the earth is due to a real rotation of the earth itself in the opposite direction. This application was made by Foucault in 1852, who set his gyroscope in rotation, and then with a telescope observed the apparent change in the plane of its rotation caused by the movement of the earth.
The most singular phenomena shown by the gyroscope are those depending on the composition of rotations. Any motion of a body which has one point fixed is of the nature of a rotation about an axis passing through that point. Hence, si multaneous rotations about any two or more axes, being a motion of some' kind, are equivalent to a rotation about a single axis. The effect, then, of impressing upon the frame in which the axis of the gyroscope is suspended a tendency to rotate about some axis, is to give the whole in strument a rotation about an intermediate axis; and this will coincide more nearly with that of the gyroscope itself, the greater the rate of its rotation. It is hardly possible to explain to the
non-mathematical reader the exact nature of the compound motion, but the rotation of the axis of a top round the vertical ( when it is not 'sleeping' in an upright position), and the precession of the earth's axis, are precisely similar phenomena. Thus, when the gyroscope is spinning, its axis being horizontal, a weight attached to the frame work at one end of the axis (Fig. A) makes the whole rotate about the vertical. Often the appa ratus is so arranged that the framework may be lifted by a string attached near one end of the axis (Fig. B), and the gyroscope will then revolve without falling. Its axis still projects horizontally from the string, but it revolves as a whole round the string. Various other singular experiments may be made with this ap paratus; and others, even more curious, with the gyrostat of W. Thomson (q.v.), which is simply a gyroscope inclosed in a rigid case. by which the ends of its axis are supported. When a gyrostat is made the bob of a pendulum under certain conditions, the plane of vibration of the pendulum turns, as in Foucault's cele brated experiment, but in general at a much greater rate.
The gyroscope is thoroughly discussed in a treatise by Gen. J. G. Barnard, published in 1858, and in the following standard works on physics: Wollner, Lehrbuch der Experimental physik (5th ed., Leipzig, 1894-95) ; let-Pfaundler, Lehrbuch der Physik (9th ed., Brunswick, 1886) ; and also in Poggendorff, Annalen, vol. xc. (Leipzig, 1878).