The Brown compass is the invention of Mr. S.G. Brown, F.R.S., of North Acton, with Professor John Perry, F.R.S., in association as technical adviser and co-patentee. It has been evolved after five years of laborious experimental work, and is claimed to be the only British gyroscopic compass so far constructed and applied on board ship.

In Fig.47 we show the Brown compass as seen looking forward on a vessel steaming due north. The south end of the gyro-axle is pointing towards the reader. In Fig.48 it is shown as it would be seen when looking forward on a vessel steaming due west. The north end of the axle is pointing to the right in this view. In the line engraving (Fig.49) we are supposed to be looking forward on a vessel steaming due south. The north end of the axle is therefore pointing toward us. In the half-tone engravings the compass is shown removed from its external gimbal rings. The axis A (Fig.47), it will be understood, is the athwartship axis of the external mounting. It is mounted on ball bearings within a couple of small brackets which by means of four screws are attached to the inner gimbal ring, as shown in Fig.49. This inner ring, in turn, is carried on a longitudinal axis inside an outer gimbal ring, which, finally, is hung on springs from the binnacle. The frame B, within which the sensitive element proper is mounted, is very much as we have represented it to be in our previous diagrams, except that it swings pendulum-wise on the axis A—or the other axis of the external mounting—not because of the attachment to it at its lowest point of an actual bob weight, but because its centre of gravity is designedly below the axis of suspension, and because at its lowest point it carries a casing containing an electric motor and other adjuncts possessing a degree of weight.

As shown best in Fig.49, the frame B supports a vertical ring C carrying the compass card, and within which the casing D containing the gyro-wheel is mounted on a horizontal, east and west, knife-edge axis. The easterly end of this knife-edged axis—towards the left in the engraving—has associated with it the nozzle and divided air box, whereby, as we have explained, the pressure of the air blast generated inside the casing by the spinning wheel is transmitted by open and crossed pipes respectively to the oil-damping bottles E and the oil control bottles F.

What we regard as the most interesting non-gyroscopic feature of the Brown compass is the manner in which the vertical ring C is supported within the frame B. The ring at its highest point is formed with a vertical trunnion, which at its top is guided within a ball bearing fixed in the central boss of the frame B. Three slip rings lower down on the trunnion acting in conjunction with three mercury contact rings attached to the frame serve to conduct three-phase current to the sensitive element. At the lower side the vertical ring is also provided with a trunnion, which is of the nature of a foot-step bearing. In reality, however, the trunnion is never allowed to touch the bottom of its bearing, for it—and with it the vertical ring, the compass card, and the whole of the sensitive element—is forced, by means of a supply of oil delivered below it, to vibrate up and down through about an eighth of an inch at the rate of some 180 times a minute. Before the trunnion can fall to the foot of its bearing it gets a fresh kick up by the oil, so that no actual metal-to-metal contact is established. The oil supply is drawn from and returned to the reservoir G by a pump driven from an electric motor H. The slight rapid vibratory motion thus communicated to the sensitive element has no appreciable effect upon the ease or accuracy with which the compass card may be read. On the other hand, it is sufficient to relieve the frictional resistance to turning at the upper and lower trunnions of the vertical ring in accordance with the well-known fact that if we overcome friction in one direction we overcome it also in the direction at right angles to the first.

A second mechanical feature of the Brown compass of outstanding interest is the method adopted for transmitting the indications of the master compass card to the repeater compasses. The problem to be solved in this matter is to establish connection with the master compass card in a manner which shall impose no frictional or other drag on that card. To this end in the Brown compass the air blast generated by the spinning wheel is called upon to fulfil a function additional to those to which we have already referred. The air blast is delivered not only from the eastern, but also from the western end of the horizontal axis of the casing. It is thus directed through a nozzle J against the face of a contact maker K containing a pair of disc-like plungers working in balanced connection within two cylinders. Opposite each plunger is a slot, and when no movement has to be transmitted to the repeater compasses the air blast is directed against the blank wall between these two slots or enters each slot in equal proportion. Thus the plungers being balanced, neither makes contact. If, however, the ship’s course is altered, the contact maker K, being attached to a ring L beneath the lubber ring, moves with the ship, while the nozzle J remains stationary with the sensitive element. Thus the air blast exerts unequal pressures on the plungers, and one of them is forced back to make contact. As a result, a step-by-step electric motor M carried on the frame B and in mesh through reduction gearing with a rack on the ring L is started up and turns the ring in the direction required to make the two slots again divide the air blast equally. When this neutral position is reached the motor is cut out. As in the Sperry compass, the follow-up motion is open to a little hunting movement as a result of the momentum acquired by the parts. Ignoring this hunting movement, the ring L and the contact maker K are thus driven to remain stationary in fixed relationship with the sensitive element no matter to what extent the frame B may move with the ship when the course is altered. The relative movement between the ring L and the frame B is communicated to the repeater compasses in an electrical manner. The details of the electrical transmission are outside the ambit of this discussion, but we may say that the movements communicated to the repeater compasses are derived from a distributor on the compass switchboard, this distributor being arranged to operate in exact synchronism with the follow-up ring L. It is claimed for the Brown system of transmission between the master and the repeater compasses that even should the whole repeater system break down the correct action of the master compass will not thereby be affected.