Patent CA-2043974-A1 (CA)
Title: Device for the elimination of the anomaly deviation in magnetic compasses
Inventor: BARTLETT JOHN F
IPC: G01C17/38
Filed: 1991-06-05
Abstract: Abstract This invention relates to the construction and use of the magnetic compass in general and, in particular, to the intro-duction of a new and unique device into the card and magnet system of that instrument, in order to render it more accurate, reliable, and convenient to the user.   The purpose of the device is to eliminate the troublesome anomaly present in all navigational magnetic compasses that are mounted in vessels having ferrous metals and/or electrical cir-cuits in their construction, more particularly if they happen to be in the vicinity of the magnetic compass, which many of them are. This anomaly is known as Deviation, the introduction of errors in the magnetic compass caused by interference, at at the compass magnet, mainly by stray magnetic fluxes emerging from partially-magnetised ferrous metal objects and/or DC wir-ing. These objects can be either fixed or movable, the effect being to draw the compass magnet away from its normal lay, the North Magnetic Meridian, from which all navigational calcula-nions derive.    As the compass magnet is attached to, and rotates with, the compass card, false readings of bearing are the result, and the accuracy of the magnetic compass, which is its most important characteristic, is lost. If the offending items are in a fixed position in relation to the compass, and reasonably close by, the error will vary. This will be in 2 greater or lesser amount and of an East or West orientation, as the vessel changes its course. If one or more of the movable items is removed, any calculations for Deviation  made earlier will no longer be valid and the error becomes an unknown quantity.    There is, at present - and this condition has existed for seven hundred years - neither the means nor the mechanism for the adjustment of the compass for this anomaly. An exception to 1.     this will be found in the magnetic compass of the warship which, being constructed entirely of steel, contains all of the ele-ments for the most violent degrees of Deviation errors, counters with the most complete array of correctors to overcome these errors. This array is greatly increased by self-adjusting corr-ectors when de-gaussing gear is fitted to the vessel to protect it from magnetic mines.  Smaller vessels, such as private pleasure craft, fishing trawlers, landing craft, and the like, and including aircraft, are compelled, for reasons of economy, to make do with simple compensators. These are used for adjusting the compass  to the vessel, but are limited, in fact, to the Deviation at the Card-inal points. From this meager information it is necessary to interpolate the Deviation error around the compass card, and to construct a Deviation Table. These figures have to be taken into account, with Variation, when plotting a course by conversion from Magnetic to True, by which most navigation is done.   As stated in the first paragraph of this Abstract, the sole objective of this inventive idea is the elimination of the last factor in the calculation - Deviation.