A plan and sectional elevation of the modern form of AnschÜtz compass are given in Figs. 50 and 51. The casings of the three gyros KLM hang by vertical stalks below a triangular spider A at the centre of which is affixed a float B immersed in a bowl C containing mercury. After the manner followed in the 1910 form, the float and all attached to it are centralised relatively to the bowl by means of a rod D fixed centrally to the cover of the compass. As before, this rod D is composed of a central core and a liner insulated from the core. The ends of the core and liner dip into two concentric mercury cups carried within the float in order that two phases of the three-phase current driving the gyro-motors may be transmitted through the core and liner of the rod D. The third phase is transmitted through the mercury and float by earthing the bowl C.

Matters are so arranged that the centre of flotation of the float B and its attached parts is above the centre of gravity of the floating parts. These floating parts—the spider A, the float B, the three gyros, and other items not yet mentioned—constitute the sensitive element, so that this element, as in the 1910 form, does not need the addition of a separate weight to provide the required pendulum action about a horizontal axis through the centre of flotation. Instead of there being but one such horizontal axis—the east and west axis EF of our models and diagrams—it is clear that the support of the sensitive element by means of a float provides the element with an east and west horizontal axis and an infinite number of other horizontal axes.

Attached to the spider A—and therefore forming a portion of the sensitive element—is a sheet metal annular casing of the section shown at EE or FF. The gyros are enclosed within this casing. Ventilating tubes and baffles G are provided at points on the casing between each pair of gyros. The compass card—in the form really of a ring—is attached to the casing at H. Regarding the casing and the spider A as the equivalent of the horizontal ring shown in our diagram (Fig.41), it will be noticed that the gyro casings are not fixed rigidly to it, but are really mounted on ball bearings surrounding their stalks, so that they may rotate about a vertical axis relatively to the rest of the sensitive element. As shown in the plan view, however, the casing of the gyro K is connected by two springs J to the rest of the sensitive element, so that in whichever way the gyro turns on the ball bearings it applies through one or other of the springs a force in the same direction to the annular casing, etc. The gyro is therefore substantially, though not actually, rigidly connected to the rest of the sensitive element, the springs being introduced to provide a yielding connection which will prevent the full force of a sudden turn of the ship from being thrown all at once on to the gyro. The gyros LM are similarly connected to the rest of the sensitive element, but in their case one pair of springs is made to serve both gyros by employing links and a bell-crank lever. The springs, it may be remarked, undoubtedly do play a part in the transmission of the directive force from the gyros to the card and in the avoidance of the quadrantal error. But their presence is not essential to the fundamental principle of action of the compass.

The damping system of the 1912 design is of a very simple nature, and represents a great improvement over the air blast method previously used. Although the wheels are not required to act as blowers, their casings are not exhausted of air. The casings, in fact, are perforated with four large holes on each side, the cooling effect of the circulating air being regarded as of more value in practice than the saving of power which would result if the wheels were run in an exhausted atmosphere.

The damping force is supplied by the weight of a body of oil contained within a trough N extending right round the foot of the annular casing containing the gyros. This trough, a circle as seen in plan, is blocked by eight bulkheads, one each below the north and south points of the compass card, and the others equally spaced round the trough. Through each bulkhead a short pipe passes, so that the oil in the trough may flow from one compartment to another. With the exception, however, of the north and south bulkhead pipes, which are quite free in the bore, the passage of the oil is restricted by means of a wire partially filling the bores of the pipes. By varying the size of wire used, the restriction to the flow of the oil from one compartment to the others, and therefore the rate at which the oil will flow when the sensitive element tilts, can be regulated to give the degree of damping required or to suit any change of viscosity in a fresh supply of oil.

The system in principle has much in common with the Brown method of damping. Should the compass card suffer an easterly deflection, the north point of the card will, as we know, tend to rise under the influence of the earth’s rotation, and will continue to rise until the turning moment applied by the deflected pendulum weight precesses the card back to the meridian, whereafter the north point of the card passing over towards the west will begin to fall towards the horizontal plane, and then, descending still farther, it will once more come back to the meridian. During this compound motion the oil in the trough flows backwards and forwards, accumulating below the south point of the card when the north point of the card is rising and gathering below the north point when the north point is falling. In other words, there is an excess weight of oil below the southern point of the card throughout the complete half-swing from east to west, the maximum excess occurring when the card is crossing the meridian. On the half-swing from west to east the excess weight of oil is below the northern point of the card, the maximum excess occurring, as before, when the card is crossing the meridian. The excess weight of oil at all times thus tends to increase the rise or dip of the north point of the card above or below the horizontal plane, whereas the pendulum weight at all times tends to diminish such rise or dip. Hence the excess weight of oil tries to precess the card in the direction opposed to that in which the pendulum weight is precessing it. The vibration of the card in this compass, as in the Brown design, is therefore damped by the generation of a counter-precessional tendency, and not, as in the early AnschÜtz and the Sperry designs, by precessing the sensitive element in the direction required to reduce the angle by which the pendulum weight is tilted away from the plumb line.

From what we have already said regarding the Brown system of damping, it will readily be inferred that there is no latitude error in the AnschÜtz 1912 compass. The damping force is equivalent to a reduction in the weight of the pendulum “bob,” and is not applied directly to reduce the tilt of the bob. The tilt of the pendulum weight required in north or south latitudes to provide the appropriate rate of westerly or easterly precession is not opposed by the damping force called into play by such tilt. Instead, the damping force merely makes the bob lighter, so that the tilt has to be carried farther before the effective weight of the bob can balance the tilting action of the earth’s rotation. The balance will be automatically struck when the moment of the effective weight of the bob is just sufficient to generate the required rate of westerly or easterly precession appropriate to the latitude.

The arrangement of the three gyros at the corners of an equilateral triangle and the general form given to the annular casing and the rest of the sensitive element results in the distribution of the mass of the sensitive element in a very uniform manner around the vertical axis. There is no excessive concentration of the mass towards the east and west plane, and as a result it is unnecessary to add compensator weights to this compass in order to avoid the effects of centrifugal force during quadrantal rolling.

The gyro-wheels are made of a special quality of nickel steel, and are mounted on axles of the de Laval type—that is to say, they are tapered and made of very small diameter (about 0.15 in. at the parallel ends)—in order that they may yield a little should the centre of gravity of the wheel not be truly coincident with the centre line of the shaft. The wheels are 5 in. in diameter, weigh 5 lb. 2 oz., and run at 20,000 revolutions per minute. The motors are of the squirrel-cage type with the rotor windings fixed to the wheels inside a recess concentric with the axle. The field coils are fixed relatively to the gyro casing. It is of interest to note that when the gyro-wheel is being run up to its full speed—an operation taking about five minutes to complete—the axle passes through three critical speeds. These speeds are approximately 7000, 11,000, and 14,000 revolutions, and are believed to be associated, the first with one end of the axle, the second with the other end, and the third with a combined action at both ends. During the period of running up the gyros the starting current is, of course, heavier than the current taken to drive the wheels at the top speed, and as a result a considerable temperature is developed in the wheels and their casings. When, however, the gyros have been running for some time at the top speed the temperature drops, and throughout the compass remains fairly constant at about 150 deg. Fahr. The viscosity of the damping oil, on the constancy of which the constancy of the damping force depends, is therefore less affected by external atmospheric changes of temperature than might be expected. The oil used is a mineral one. It serves not only to damp the vibrations of the card, but also to lubricate the gyro-axles. To this end, as shown in Fig.51, pipes are led down from each end of each axle to dip into the oil trough, the flow of oil being induced by means of wicks inside the pipes.

The method adopted for the transmission of the readings from the master compass to the repeaters is of considerable interest. The bowl C containing the mercury and the float is surrounded by two semi-cylindrical strips PQ of silver-plated brass. At one of the gaps between these strips the two abutting edges are faced with platinum. The gap between these platinum faces is 0.11 in. in width, and into it there is inserted, as shown in the plan view, a platinum-iridium ball R, measuring 0.095 in. in diameter.

On the switchboard serving the compass there is a reversible motor, two of the windings of which are constantly connected to a generator—the same generator as serves the gyro-motors. The contact ball R is connected to the third phase of the generator, while the two strips PQ are connected to the third winding of the reversible motor, this winding being duplicated in such a way that the motor revolves in one direction or the other, according as the circuit is completed at the ball R through the strip P or the strip Q. A commutator is mounted on the axle of the reversible motor, and from it current is distributed to the motors operating the repeaters and to the “follow-up” motor S (Fig.51). The latter motor is geared to the shaft carrying the bowl C, and when started up by the reversible motor turns the bowl in the direction required to restore the ball R to the middle of its slot, and so break the connection with the strip P or Q. Thus when the ship’s course is altered the bowl tends to rotate with the ship, but the ball R is mounted on the sensitive element, and therefore maintains its position. Contact is thus established between the ball and one of the strips PQ, the reversible motor is set rotating in the appropriate direction, and current is distributed to the “follow-up” motor S to rotate the bowl relatively to the ship until the ball R is once more lying midway in the gap. The tendency of the bowl to rotate with the ship is thus counteracted; the action of the “follow-up” motor practically results in the bowl being held in constant relationship to the sensitive element substantially as though it were part thereof. Simultaneously the cards of the repeaters are prevented from rotating with the ship, so that virtually they, too, act as if rigidly connected to the sensitive element, without, however, any frictional drag being thrown from them on to the sensitive element.

As illustrating the refined construction of the entire compass, the design of the ball contact may be noticed. The ball is carried at the end of a tapered spiral spring. It is free to rotate on the spring end, but is prevented from moving axially thereon. The spring end is provided with a button. The ball is drilled out and beaded over the button. To ensure good electrical contact at all times between the ball and the spring a drop of mercury is carried inside the ball between it and the button. Should the ship turn very suddenly the ball may spring out of the gap and be dragged across the face of one or other of the strips PQ. It is for this reason that these strips are silver-plated.

The repeater compasses are provided not only with an ordinary card graduated from 0 deg. to 160 deg., but also with an inner dial which makes one revolution for an alteration of 10 deg. in the ship’s course. This dial is graduated to 1/10 deg., and permits very small departures from the set course to be immediately noticed and corrected. An elaboration of the same idea is provided in the multiple repeater of the Brown compass. In this repeater the inner dial is the ordinary 360 deg. card. The outer annular dial makes four revolutions for every complete turn of the ship. With the ship sailing due north the graduations on the outer dial are numbered from 0 to 45 round the east half of the dial, and from 360 to 315 round the west half. The numbers, however, are not marked on the dial itself, but on the edges of discs seen through slots in the dial. As the ship turns from the north towards the east, the discs on the west side of the dial are successively rotated one stage as the south end of the lubber line passes over them, so as to exhibit numbers forming a continuation of the numbers on the east side of the dial. The outer magnified dial is thus of itself sufficient for navigational purposes.

In the AnschÜtz equipment arrangements are made for attaching an azimuth mirror to the repeater dial for the purpose of providing an artificial horizon during the taking of bearings. A separate gyroscopically stabilised artificial horizon device, such as is sometimes to be found on board ships, is thus rendered unnecessary.