GYROSCOPE, (from the Greek guros, a circuit, skopio, I see), scientifi cally, any freely suspended rotating body; tech nically an instrument making use mechanically of forces developed by rotation and the compo sition of rotations. The gyroscope is simply a manifestation of the laws of inertia. Its inven tion is ascribed to Jean Bernard Lion Foucault (1819-68), whose famous experiments with pendulum and gyroscope proved and measured the diurnal motion of the earth. The applica tion of the gyroscopic principle, however, was made many years previously to Foucault's ex periments, and the instrument in some of its forms originated probably in Germany or Prance, toward the end of the 18th century. A form of the instrument is popular as a toy, in the familiar gyroscopic top.
The construction of the gyroscope is such that the axis of rotation can be made to point to some star in the sky. Then, as the heavy disc whirls round, it is found that the axis continues to point to the moving star, though, in consequence of this, apparently altering its direction relatively to bodies on the earth. If, again, the axis be pointed to the celestial pole, which is fixed, no alteration in its position relative to bodies on the earth takes place.
The following lucid exposition of the prin ciples governing the action of the gyroscope is given by Dr. S. Tolver Preston in an article on Mechanics of the Gyroscope' repro duced from °Technics" in the Scientific Ameri can .Supplement of 8 Oct. 1904: °According to the Newtonian system of dy namics (a system which is now universally recognized and accepted), the velocity of a particle can only be increased in any given direction by the application of a force acting in that direction; conversely, its velocity in a given direction can only be diminished by the application of a force acting in an opposite direction. The magnitude of the applied force is proportional to the rate of increase or de crease of the velocity of that particle.
*Let to suppose that a series of equally heavy particles are arranged around the circumfer ence of the circle in Fig. 1. These particles may be supposed to be rigidly connected one with another, the whole being connected by massless spokes, with an axle passing through C, the centre of the circle; this axle being at right angles to the plane of the paper. This arrangement constitutes an ideal flywheel and may be considered typical of an ordinary gyro scope disc.
*Let the flywheel be set in rotation in the direction indicated by the arrow. The prob lem before us is to determine the nature of the forces which must be applied to the rotating flywheel in order to deviate the axis of rota tion. Let us suppose that the flywheel, while still rotating at a uniform velocity about its axle, is constrained in addition to turn about the line A B, at right angles to the axle. Look ing in the direction A B, let the flywheel turn about that line in a clockwise direction, so that the side L moves downward through the plane of the paper, while the side R moves upward through the same plane. The particles at e and p being, at the given instant, on the axis of rotation A B, will possess no velocity of rotation about that axis. So far as it con cerns other particles, their velocities of rotation about A B will be proportional to their per pendicular distances from that line. Sixteen equidistant particles on the circumference of the circle have been indicated. The rotational velocities of these particles, about the line A B, will be proportional to the respective perpendiculars let fall on A B.
*In a certain interval of time the disc will complete a revolution about its axle. In one sixteenth of this interval of time, the particle a will move round the circle so as to attain the position previously occupied by the particle b. In doing so, the particle a will acquire the velocity previously possessed by the particle b, i.e., its velocity about the axis A B will be diminished, since b is nearer than a to the axis A B. The diminution of velocity will of course be proportional to a D, where b D is a line drawn from b perpendicular to C a. But since the velocity of the particle a, in a direction passing vertically downward through the plane of the paper, is diminished as the partigle moves from a to b, this particle must have been acted upon by a force directed vertically upward through the plane of the paper, and propor tional to a D. This force is indicated by a small circle containing a dot at its centre. The dot indicates the pointed end of an arrow sup posed to be directed vertically upward through the paper; while the diameter of the small circle is drawn proportional to a D, or to the magnitude of the force.