ADVERTISEMENT. The object of the GENERAL APPENDIX to the Annual Report of the Smithsonian Institution is to furnish brief accounts of scientific discovery in particular directions; reports of investigations made by collaborators of the Institution; and memoirs of a general character or on special topics that are of interest or value to the numerous correspondents of the Institution. It has been a prominent object of the Board of Regents of the Smithsonian Institution, from a very early date, to enrich the annual report required of them by law with memoirs illustrating the more remarkable and important developments in physical and biological discovery, as well as showing the general character of the operations of the Institution; and this purpose has, during the greater part of its history, been carried out largely by the publication of such papers as would possess an interest to all attracted by scientific progress. In 1880 the secretary, induced in part by the discontinuance of an annual summary of progress which for 30 years previous had been issued by well-known private publishing firms, had prepared by competent collaborators a series of abstracts, showing concisely the prominent features of recent scientific progress in astronomy, geology, meteorology, physics, chemistry, mineralogy, botany, zoology, and anthropology. This latter plan was continued, though not altogether satisfactorily, down to and including the year 1888. In the report for 1889 a return was made to the earlier method of presenting a miscellaneous selection of papers (some of them original) embracing a considerable range of scientific investigation and discussion. This method has been continued in the present report for 1911. 110 THE GYROSTATIC COMPASS.1 [With 3 plates.] By H. MARCHAND. The gyrostatic compass may be looked upon as one of the most interesting inventions made during recent years. The gyroscope is familiar to all. Nor are we ignorant to-day of the fundamental laws which govern it. The great physicist Foucault first completely formulated them as the result of his profound researches on the subject. The first of these laws is that a gyroscope with perfect freedom of movement—that is, the power to move in any direction, and free from the action of gravity-will tend to maintain the initial position given to it. The second law is that if a gyroscope has only two degrees of freedom, in such a way that it can undergo displacement in two planes only, it must, if subject to the action of gravity, and provided that it is not at the poles of the earth, tend to place itself so that its axis is parallel to that of the earth and accordingly will indicate the direction of true north. A system of this kind is free from the errors which affect the magnetic compass, and therefore the idea of taking advantage of it for navigation must have early attracted the attention of investigators, especially as the general use of steel in the construction of vessels entails grave difficulties in the use of magnetic instruments. Formerly the means at hand were insufficient for constructing a satisfactory, practical instrument, and so, during the time of Foucault, and even much later, the numerous scientists who approached the problem did not meet with much success. A German investigator, Dr. Anschütz, has recently succeeded in constructing an instrument on gyrostatic principles which is practical. In 1900 he commenced the study of a gyroscope with perfect freedom of movement. Later, however, in 1906, he abandoned that for one having only two degrees of freedom. Even with the latter conditions the problem required great nicety. A grave difficulty comes from the fact that such a device is affected, under ordinary circumstances, not only by the rotation of the earth but also by all the forces to which it is subjected because of the rolling 1 Translated by permission from Cosmos, Paris, New Series No. 1385, Aug. 12, 1911, pp. 181–184. of the ship. Therefore, in order to get a good result, it was necessary for the compass to have a very great gyrostatic resistance, so termed, opposing energetically any force tending to change the direction of its axis of rotation, and that the friction of its bearings should be made as small as possible. A consequence of the latter condition, however, would be that the gyrostat would come to its normal position only after a relatively long time, oscillating to and fro about that normal position. Meanwhile it would be subject to new perturbations. Accordingly one great desideratum was to provide some device for lessening these oscillations; at first, Anschütz tried for this purpose a second gyrostat; later he developed a much more simple and efficient method. His gyrostatic compass was tried on the steamer Deutschland in 1908 and has since been used in the German Navy. It has just been adopted by the English Navy, and other navies are also trying it. Let us consider its principles: We know that a gyroscope once started tends to maintain its axis in an invariable direction, and that if any force is applied tending to change this direction, precessional movement takes place, which displaces the axis perpendicular to the direction of the disturbing force. Such being the case, let us imagine a gyroscope, inclosed in an appropriate box, suspended from a float which rests in a liquid bath in such a manner that the gyroscope is perfectly free to swing in any direction like a pendulum which is at rest; the center of gravity of the system is below the metacenter; the gyroscope is mounted at the lowest point possible. Because of its weight the axis of the gyroscope tends to maintain itself, as well as the whole attached mechanism, in a horizontal position. Let us set the gyroscope disk rotating. In the past such rotation. could be effected only by rough and very unsatisfactory means; now we have a much more advantageous method at our disposal. We may, for instance, drive it by a little three-phase motor fed by means of fine conducting wires so that the rotation may be kept up indefinitely. As soon as the gyroscope disk is in rapid rotation with its axis horizontal, then if this axis is not in the plane of the terrestrial meridian, the rotation of the earth will tend to alter the axis from its original position. The gyroscope tends to respond, but, restricted by its weight, which forces the axis to remain horizontal, it will undergo only a horizontal displacement. This leads it to take a north and south direction, because as long as its axis is not parallel to that of the earth, the cause of this movement is still effective, so that if it is sufficiently free to move, it will indicate true north. Plate 1 shows a model designed to show experimentally this action. It consists of a small gyroscope, driven by a small three-phase elec tric motor, and entirely free to turn in any direction were it not for two small springs, which serve to represent the attraction due to gravity and tend to keep the gyroscope's axis tangent to the surface of the sphere upon which it is mounted. This apparatus may be slipped along a large movable metallic circle representing a meridian. When the springs are detached the axis of the gyroscope tends to take an invariable direction and to depart from it only because of the inevitable friction which is present at the pivots. However, once these springs are attached, if the meridian circle is moved to another place on the globe, then the axis will change so as to come into the plane of the circle, one of its extremities being directed toward the upper pole of the circle. According to the direction of the gyroscope and that of the circle, one or the other end of the axis moves so as to point to this upper pole. In the Anschütz compass, of which figure 1 is a sectional and plate 2 a perspective view, the gyrostat is similarly driven by a three-phase electric motor; the float from which the gyroscope is B ST -S suspended is a hollow FIG. 1.-Anschütz gyrostatic compass. Vertical section. A, three bowl of steel partially immersed in a mercury bath contained in an annular box, also of steel. To the top of this float is fixed the phase alternating-current motor in box B (the rotor of the motor serves as the gyrostat); R, compass card; S, hollow annular steel float, floating in the mercury Q and counterbalancing the weight of the moving system; K, compass box, suspended as usual by gymbals; S, T, special suspension, made of two insulated conductors for leading in two branches of the three-phase current, the third circuit being made through the floating ring and the surrounding mercury. compass card. The north-south line of the card coincides exactly with the direction of the axis of the gyrostat. The small motor which rotates the gyrostat disk is so constructed that its stationary part is built on the box of the gyrostat. The electrical connections to the exterior are made for two of the circuits through small cups of mercury. The third circuit passes through the mercury bath containing the float and then through the case itself. The rotor is rigidly built on the gyrostat disk. It is of one piece, spindle and all, and made of nickel steel. It is provided with ball bearings of extra-hard steel and makes 20,000 revolutions per minute. The axle is of the Laval type or "flexible axle." Its great 38734°-SM 1911-8 |