4422 HYDROGRAPHIC MANUAL PaGE 340 



If the axis of a spinning gyroscope is in an east-west direction, as the earth rotates the 

 axis will gradually tilt with reference to the horizontal. 



Foucault's general law is that any freely suspended spinning body ''tends to swing 

 around so as to place its axis parallel to the axis of any impressed forces, and so that its 

 direction of rotation is the same as that of the impressed forces." This principle is 

 utilized in the gyrocompass. The rotation of the earth and the force of gravity furnish 

 the forces which make the gyrocompass indicate true north. 



Another characteristic of the gyroscope is called i^recession. If a force is applied 

 to a spinning gyroscope, tending to change the direction of the axis, this force will not 

 only be resisted but the gyroscope will turn slowly about an axis perpendicular to the 

 one about which the force is being applied. 



The precession characteristic is utilized to make the gyrocompass move to and settle 

 on true north. A U-tube, partly filled with mercury, is mounted perpendicular to 

 the plane of the gyroscope. As the earth rotates and tilts the gyroscope, if its axis is 

 not in the plane of the meridian, the mercury flows in the U-tube to the low side. The 

 force of gravity then exerts a downward force on that side, which causes the axis of the 

 gyroscope to precess toward the meridian, where the tilt is corrected. This is a con- 

 tinuous effect, tending to keep the axis of the gyroscope in the plane of the true meridian 

 once it has placed itself there. 



4422. The Gyrocompass Equipment 



The gyrocompass is an electrically operated compass which is unaffected by mag- 

 netism and which automatically points very nearly to true north continuously. Small 

 corrections for variations due to latitude, course, and speed are made semiautomati- 

 cally. Once the latitude and speed corrections are set, no readjustments for changes of 

 latitude of less than 3° or for changes of speed of less than 3 knots need be made. 



A typical equipment consists of (a) a master compass, (6) a steering repeater, (c) 

 bearing repeaters, (c?) a course recorder, and often (e) a gyro pilot. Tfie master com- 

 pass is the principal unit; it indicates true north; the other units are auxiliaries for 

 special purposes. 



(a) The master compass is not used to steer by, nor is it generally installed in the pilothouse. 

 It should be in a clean well-ventilated compartment, where heat is not excessive. For convenience 

 and easy access it is generally located near the pilothouse. Advantages may be gained by having 

 it near the center of gravity of the vessel. The lubber's line of the compass must be parallel with 

 the fore-and-aft line of the vessel. 



The master compass consists essentially of a gyroscopic wheel, mounted to spin on a horizontal 

 axis and rotated electrically at high speed. The gyrocompass card is attached al ove the wheel and 

 mounted with its 0° to 180° line in the same plane as the axis of the gyroscope and so that all move- 

 ments of the wheel in azimuth are transmitted to the card. 



A transmitter, to operate the repeater compasses, is attached to the frame which carries the 

 lubber's line and is driven from a gear attached to the compass card so as to reproduce any move- 

 ment of the card. The repeaters contain step-by-step motors through which the transmitter actuates 

 them. Any number of repeaters can be operated. 



The whole is housed in a binnacle for protection. 



A detailed description of the construction and parts of the master compass is contained on 

 pages 34 and 35 of Bowditch. 



(6) The steering repeater resembles a compass in appearance, but it is a dummy, its card being 

 synchronized with the master compass so as to repeat the movements of the compass card in the 

 master compass. It is located near the steering wheel, mounted on an adjustable bracket so that 

 its position can be changed to give a full view of the card from any position desired. All navigating 

 is done from repeaters of which there may be several, one at each steering station. 



