A general view of the Sperry compass as removed from the binnacle is given in Fig.45. This view depicts the compass as it would be seen looking forward on a vessel steaming towards the south-west. At A is shown the compensator weight on the north side of the sensitive element, and at B the ballistic gyro is to be seen. The ring C is the inner of the two external gimbal rings, and carries on an athwartship axis D the spider E, from which the sensitive element is suspended. At F may be seen one of the bearings whereon this ring is swung on a longitudinal axis within the outer gimbal ring. The outer ring, in turn, is hung on springs attached to the inside of the binnacle. The speed and latitude corrector dials are to be seen directly above the bearing F, while behind them the inclined cosine ring or course corrector ring is visible. The phantom ring is shown at G. This ring in section is channel-shaped, and in the view hides the vertical ring within which the casing of the gyro-wheel is journalled on the horizontal east and west axis. It is to be carefully noted that the lugs H, by which the compensator weights are attached to the sensitive element, are not fixed to the phantom ring, but pass through easy-fitting holes therein, and are directly secured to the vertical ring inside. The bail weight lies within the phantom and vertical rings, and can be seen at J, the tracks for the excentric pin, one on the bail and the other on the wheel casing, being shown at K. The stirrups L are fixed to the ends of the bail, and carry the pins whereon the bail is swung from the phantom ring. The bail latitude corrector dial, whereby the bail is allowed to assume the tilt appropriate to each latitude without disturbing the horizontality of the gyro-axle, is visible behind the stirrup. The horizontality of the axle is indicated on a level attached at this point to two brackets springing from the wheel casing.

Of the non-gyroscopic mechanical details of this compass, chief interest probably centres in the method of providing the vertical axis about which the sensitive element may turn relatively to the spider E in a substantially frictionless manner. In Fig.46 we give a diagram of the compass as we would see it looking aft on a ship steaming due south. It will be seen from this diagram that the phantom ring, shown in black section, is extended at the top to form a horizontal flange. On this flange the angular divisions of the compass card are engraved. The phantom ring is also formed with a central hollow stalk whereby it is hung on ball bearings within the central boss of the spider. A torsionless wire composed of several strands of steel is secured at one end to a cap fixed over the top of the stalk, and at the lower end is secured to a ball-borne pin forming part of the vertical ring. Diametrically below, the vertical ring and the phantom are united by a pivot pin. The gyro case, as we have explained before, is carried on a horizontal axis within the vertical ring, while the bail is swung on the phantom. The phantom and all within it may be regarded as the sensitive element, the constant alignment of which in the north and south direction gives the device its utility as a compass. The spider carrying the lubber ring and all outside it move round the wire-and-pivot-pin vertical axis with the ship when the course is altered. When such a movement occurs the absence of any direct mechanical connection of a rigid nature between the phantom ring on the one hand and the rest of the sensitive element on the other would tend to result in the phantom ring following the movement of the spider, binnacle, and ship rather than in its remaining stationary with the wheel, casing, and vertical ring. The suspension wire would thus be twisted. As we have previously explained, however, the phantom ring is formed just below the card with a circular rack with which there meshes a pinion driven through reduction gearing by a small electric motor—the azimuth motor—attached to the spider. The starting and stopping of this motor and the direction of its rotation are controlled automatically by means of gold-rimmed wheels on two trolley heads fixed to the vertical ring and certain gold-faced contactors on the phantom. Thus when the ship’s course is altered the azimuth motor is started up in the direction required to take the twist out of the suspension wire and bring the phantom into alignment with the rest of the sensitive element. When this alignment is reached the motor is automatically cut out. In actual use the phantom overshoots the exact position of alignment by a very small amount, bringing the reverse contactors into action and starting the motor in the reverse direction. The phantom, in fact, oscillates about the position of alignment with the sensitive element through, roughly, a quarter of a degree. This small vibration is transmitted to the repeater compasses elsewhere in the ship, and is of practical value in that its visibility is some assurance that the master compass is working correctly.

It will be seen, then, that the absence of friction about the vertical axis of the Sperry compass is secured by suspending the sensitive element within a member—the phantom ring—the frictional drag on which is eliminated by driving it by power in such a way that it follows all the movements of the sensitive element relatively to the supporting spider in an automatic and substantially instantaneous and dead-beat manner.

The repeater compasses are operated electrically from the master compass through a transmitter and a pinion meshing with the circular rack on the phantom. The pinion is journalled on a pin depending from the lubber ring, and therefore transmits to the repeaters not only the relative movement between the phantom and the binnacle, but, as we have before explained, any movement of the lubber ring relatively to the binnacle which may be made to correct the reading of the master compass for the latitude and north steaming errors. In this way the repeater compasses always indicate true north both for course-setting purposes and for the purpose of taking bearings on passing objects. It is regarded as outside the scope of this discussion to describe the mechanical features of the repeater compasses. It may, however, be remarked that the fact that they may be fitted in any number and in any position, and that automatic course recorders and similar devices may also readily be linked up with the gyro-compass, forms a very strong reason in itself for recommending the adoption of the gyroscopic rather than the magnetic compass on board ship.