Some few words of description fall to be made regarding certain mechanical and electrical features of the principal types of gyro-compass—features, that is, which, not being primarily connected with the gyroscopic behaviour of the compass, have not so far been mentioned in our discussion of the general theory of the device.

A sectional diagrammatic view of the early form of AnschÜtz compass is given in Fig.44. This compass, although it is now obsolete by reason of the fact that no provision was made in its design for the quadrantal error, is still, we think, of interest from other than the historical point of view.

The gyro-wheel is shown at A. The axle is carried in bearings inside the casing B. This casing is attached to a tubular stalk C, which is fixed to a head D carrying the compass card R. This system of parts as a whole is floated in mercury Q contained inside a circular steel bowl K, the flotation being secured by means of a hollow steel ring S joined to the head D by a dome-shaped member E, which is perforated all over for the sake of lightness. As the float S is completely immersed in the mercury, the exact level to which it will sink can be delicately settled by controlling the form of the bore inside the head D. The central position of the float inside the bowl is controlled by means of a steel stem T fixed at its upper end to the centre of the glass G covering the card and pointed at its lower end to dip into a cup containing mercury which is fixed centrally inside the stalk C. The stem T is insulated, and is enclosed within a tube, the upper end of which is also attached to the glass G and the lower end of which is splayed out to dip into a second mercury cup surrounding but insulated from the first. Through this tube and its mercury cup one phase of a three-phase current is led to the motor driving the gyro-wheel from the terminal F. The second phase reaches the motor from the terminal H by way of the stem T, while the third phase is transmitted through the bowl, the mercury Q, and the float. The bowl is provided with knife-edge bearings at L, whereby the whole system shown is mounted within two gimbal rings providing the external longitudinal and athwartship axes. The outer gimbal ring is supported inside the binnacle by means of springs, the attachments of these springs, as well as the gimbal supports, being insulated on account of the method adopted for transmitting the third phase of the current to the gyro-motor. The gyro-axle runs on ball bearings. The driving motor comprises a stator carrying the windings and fixed inside the casing B and a rotor forming a rigid part of the gyro-wheel itself.

It will be seen that this design of compass does not explicitly reproduce the horizontal axis EF, nor the pendulous weight S of our simple model. The float is, however, free to tilt within the bowl in any direction, so that the system of suspension is practically equivalent to the provision of an infinite number of horizontal axes EF. As regards the apparent absence of the pendulous weight, it is to be noted that the centre of gravity of the floating system is below its metacentre, and therefore that the pendulum effect is exactly reproduced. The damping details are not shown in the engraving, but they are substantially as we have described them previously.

In view of the high speed, 20,000 revolutions per minute, at which the gyro-wheel runs, giving a stress in the rim of about 10 tons per square inch and a peripheral velocity of 340 miles an hour, it is of interest to note that during a test to destruction the wheel did not fail until it was being driven at a speed involving the supply to it of five times the normal driving power. A special motor generator had to be built to enable this test to be carried out. Even the normal motor driving the gyro-wheel had to be specially designed, for no motor capable of running at 20,000 revolutions was at the time commercially available, while the small space within which it had to be accommodated rendered the question of temperature rise a very serious one. It was found, it may be added, that in designing the motor the usually accepted magnetic constants for the iron in the motor did not hold good with a periodicity as high as 333 cycles per second.

The flotation of the sensitive element in mercury is a simple and convenient method of obtaining a practically frictionless support for the vertical axis—the axis HJ of our elementary model. The absence of friction arises from the facts that the drag of the mercury on the parts of the sensitive element with which it is in contact is proportional to the velocity with which the parts move through it, and that this velocity is always extremely slow. If the mercury is freshly distilled, there is, it would appear, substantial absence of frictional drag. But in course of time dust, oil, etc., may collect on the surface of the mercury, and may introduce a drag on the sensitive element of sufficient amount to interfere with the accuracy of the compass readings. The suspension of the vertical axis by flotation has, however, the advantage that no external gimbal rings need be provided to give the sensitive element three degrees of freedom in all configurations. The sensitive element, as we have said, is virtually provided with an infinite number of horizontal axes, so that within the bowl itself there is always a longitudinal and an athwartship axis about which the sensitive element may turn. The external gimbal axes provided in this compass permit the bowl to remain horizontal when the ship rolls or pitches; they are not essential gyroscopically. A similar remark applies to the later form of AnschÜtz compass in which the sensitive element is also floated in mercury.