THE ACTIVE TYPE OF GYRO SHIP STABILIZER 147
and push it every time the pendulum starts to move toward the left from the right end of its path. After a few oscillations, the pendulum will be vibrating with a considerable amplitude of swing. The building up of the amplitude of swing is due to the pendulum's being acted upon by a series of separate torques occurring with the same period as the natural period of the pendulum and always in the same direction as the vibration of the pendulum. This is an example of resonance (Art. 26).
Again, while the pendulum is swinging and the gyro-wheel is spinning clockwise as before, pull the upper end of the gyro-shaft every time the pendulum bob starts to move toward the left from the end of its path and push it every time the pendulum bob starts to move toward the right from the other end of its path. This procedure causes the pendulum vibrations to become reduced to zero after a few swings. In these two cases, observe that the direction of the torque applied to the gyro-wheel that increases the amplitude of vibration of the attached pendulum is opposite the direction of the precession of the gyro-axle produced by the motion of the pendulum, and that the direction of the torque applied to the gyro-wheel that damps the amplitude of vibration of the attached pendulum is the same as the direction of the precession of the gyro-axle produced by the motion of the pendulum. Consequently, in the case of a gyro-wheel having two degrees of rotational freedom mounted on a pendulum in such a manner that the gyro-axle, the axis of precession and the axis about which the pendulum vibrates are mutually perpendicular to one another, the amplitude of vibration of the pendulum will be increased if the gyroaxle be rotated by an outside torque in the direction opposite to the precession, whereas the amplitude of vibration of the pendulum will be decreased if the gyro-axle be rotated by an outside torque in the same direction as the precession.
In the first case, the periodic gyroscopic torque L, Fig. 120, acting on the pendulum is in phase with the angular velocity of the pendulum. The pendulum is absorbing energy at each vibration and the amplitude of vibration increases (Art. 65). In the second case, the periodic gyroscopic torque applied to the pendulum and the angular velocity of the pendulum are in opposite phase. The pendulum is losing energy at each vibration with a consequent diminution of the amplitude of vibration.
93. The Sperry Ship Stabilizer. - The Sperry ship stabilizer is designed to neutralize each roll increment soon after if,-, inenn-