Page:Elementary Text-book of Physics (Anthony, 1897).djvu/67

§ 47] If a body in rotation about its axis of greatest moment of inertia be acted on by a couple tending to change the direction of rotation, that is, to give it an angular acceleration about another axis at right angles to this, the various reactions which arise from the changing directions of the parts of the body which are in motion occasion other couples which oppose the action of the applied couple. That is, the rotating body possesses a certain stability due to its rotation. This effect is illustrated by the instrument called the gyroscope and by the common top.

The construction of the gyroscope can best be understood by the help of the diagram (Fig. 19). The outermost ring rests in a frame, and turns on the points $$a, a_{1}$$. The inner rests in the outer one, and turns on the pivots $$b, b_{1}$$, at right angles to the line of $$a, a_{1}$$. Within this ring is mounted the wheel $$G$$, the axle of which is at right angles to the line $$bb_{1}$$, and in a plane passing through $$aa_{1}$$. At the point $$e$$ is fixed a hook, from which weights may be hung. It is evident that if the wheel be mounted on the middle of the axle the equilibrium of the apparatus is neutral in any position, and that a weight hung on the hook $$e$$ will bring the axle of the wheel vertical, without moving the outer ring. If, however, the wheel be set in rapid rotation, with its axle horizontal, and a weight be hung on the hook, the whole system will revolve with a constant angular