II.

Gyroscopic Inertia

The rigidity in space of a gyroscope is a consequence of Newton's first law of motion (see Mechanics), which states that a body tends to continue in its state of rest or uniform motion unless subject to outside forces. Thus, the wheel of a gyroscope, when started spinning, tends to continue to rotate in the same plane about the same axis in space. An example of this tendency is a spinning top, which has freedom about two axes in addition to the spinning axis. Another example is a rifle bullet that, because it spins or revolves in flight, exhibits gyroscopic inertia, tending to maintain a straighter line of flight than it would if not rotating. Rigidity in space can best be demonstrated, however, by a model gyroscope consisting of a flywheel supported in rings in such a way that the axle of the flywheel can assume any angle in space. When the flywheel is spinning, the model can be moved about, tipped, or turned at the will of the demonstrator, but the flywheel will maintain its original plane of rotation as long as it continues to spin with sufficient velocity to overcome the friction with its supporting bearings.

Gyroscopes constitute an important part of automatic-navigation or inertial-guidance systems in aircraft, spacecraft, guided missiles, rockets, and ships and submarines (see Guided Missiles; Rocket; Submarine). In these systems, inertial-guidance instruments comprise gyroscopes and accelerometers that continuously calculate exact speed and direction of the craft in motion. These signals are fed into a computer, which records and compensates for course aberrations. The most advanced research craft and missiles also obtain guidance from so-called laser gyros, which are not really inertial devices but instead measure changes in counterrotating beams of laser light caused by changes in craft direction. Another advanced system, called the electrically suspended gyro, uses a hollow beryllium sphere suspended in a magnetic cradle; fiber-optic systems are also being developed. The remainder of this discussion deals with the conventional gyro.