FIG. 77.—SOME OF THE FORCES DEVELOPED IN A GYROSCOPE WHEN ITS PLANE OF ROTATION IS SUBJECTED TO ANGULAR MOTION
Some idea of the nature of these forces and why they give rise to precession may be understood by reference to the diagram, Figure 77. Here we have a disk with a heavy rim turning on the axis X, X′. At A, B, C and D are four particles whose flights we are going to consider. Suppose the wheel to be at rest; then if X, X′ is tilted in the direction of the arrows x x′, the wheel will turn about the line Y Y′; D will move forward toward D′, and B backward toward B′, but A and C will remain where they are. Now, suppose, the wheel to be revolving clockwise, or in the direction A, B, C, D, then the particle A will pursue a spiral course that will bring it to B′, and C will pursue a spiral course that will bring it to D′. However, particle D will have an irregular course, as indicated by the dotted line, starting first to move forward and then curving back toward A. The same will be true of B, except in the reverse direction. The course of particles D and B is, therefore, materially different from that of A and C. Now, the particle D will resist being deflected from its course and will develop an opposing force represented by the arrow d. A moment later this is reversed as the particle bends back toward the axis Y Y′, and we may represent the new force by the arrow d′. It may be proved that the force d′ is more powerful than that of d. The particle A in the meantime exerts a force opposing its deflection, which is represented by the arrow a. On the other half of the wheel there are similar but opposite forces, b, b′ and c. The sum of these forces gives the wheel a tendency to turn about the axis Z Z′. To avoid complicating our diagram with too many arrows, we had better refer to a new diagram (Figure 78) which shows only the resultant of the forces developed. The application of the forces x x′, which would have turned the wheel on the axis Y Y′, had it been stationary, have resulted in the development of forces z z′ at right angles to x x′, tending to turn the wheel about the axis Z Z′. Now, if we go through the same processes of reasoning as before, it will be evident that the forces z z′ will result in a third set of forces y y′ at right angles to z z′ tending to turn the wheel about the axis Y Y′. The forces y y′ exactly balance the forces x x′, and hence the wheel does not turn about the axis Y Y′ in response to the original forces, but starts instead to revolve slowly about the axis Z Z. Because the forces x x′ and y y′ balance each other, there is no fourth couple developed and hence no opposition to the forces z z′.
FIG. 78.—DIAGRAM EXPLAINING PRECESSIONAL MOVEMENT OF A GYROSCOPE
The gyroscope was used as a toy ages ago. The top, which is one form of gyroscope, was a favorite plaything of ancient Egypt. But although known these many centuries, it is only in the past few years that any real effort has been made to set the top to work. Because it persists in maintaining its plane of rotation it has proved most useful on submarine torpedoes to control the rudder and hold the torpedo on a true course to its target.
THE GYROSCOPE AS A COMPASS
Another most important use for the gyroscope is found in the submarine itself. The needle of a magnetic compass is kept pointing north by action of the magnetic lines of force which surround this earth. Whenever a large mass of iron is placed near the compass the magnetic field is distorted and the compass needle is deflected from the true north. On modern steel vessels the compass has to be carefully corrected by using iron masses to counterbalance other disturbing masses. However, in a submarine the whole shell of the vessel is of steel and the magnetic lines of force flow along this shell. The compass needle is virtually insulated from the terrestrial magnetism by the surrounding steel hull. But, fortunately, the gyroscope may be used as a compass and it is in no way affected by magnetism. Once the gyroscope is set spinning with its axis pointing to the North Pole of the heavens it will continue to point in that direction no matter how devious a course the vessel may pursue. If pointed in some other direction, the precessional forces set up by the rotation of the earth will turn it due north. As the vessel rolls or pitches, disturbing precessional movements are likely to be set up. These are overcome by special mechanism, so that the gyroscopic compass may now be depended upon as a perfectly reliable instrument.