Thought I'd share an article on the in's and outs of Magnetic Deviation. Note most of this stuff is from the 1700's!See: Magnetic Deviation: Comprehension, Compensation and ComputationIn the following extracts, note the term hard iron refers to iron with magnetic hysteresis (magnetic field is retained after initial magnetizing field is turned off), as in a ship's hull, vs. soft iron without magnetic hysteresis (magnetic field is not retained after initial magnetizing field is turned off), as in the ferrite core of a transformer.The sections I found most interesting are:1. Other Sources of Magnetic Deviation

There is another effect not taken into account in this chart, the distortion due to heeling of the ship, i.e., the leaning of the ship from wind as well as the transient rolling of the ship. Smith derived equations for all that as well in his manual [Gray presents a nice summary of it all]. Heeling produces a maximum deviation when the ship is heading north or south, and no deviation when heading east or west (although the needle will have less directive force to north in this case). The complementary pitching action of the ship, being more transient that heeling, does not produce a significant difference in deviation on average. Another transient effect found in practice, the Gaussin error (not Gaussian error), is a time lag in magnetic change with heading change of about 2 minutes due to opposing magnetic fields in the soft iron created by eddy currents (by Lenz’s Law). Of greater concern is the retentive error, or the tendency to retain residual, subpermanent magnetism in the hard iron that is accumulated as the ship maintains a set course for a long time (say, several days) while being hammered by waves, an effect that can last from hours to more than a day after a heading change. This certainly required some experience and a good bit of art to reckon in the past.Add to this the changing effects on magnetic deviation from variable quantities of ammunition on board, varying turns on cable reels, attached boats and nearby ships, personal effects such as watches and belt buckles, stowing of the anchor chain, lightning strikes, the heating of smoke stacks and exhaust pipes, and so on. Newer aluminum boats, for example, don’t provide magnetic shielding of sources below deck, and Barber relates that in one new aluminum cutter the compass deviation obediently tracked the generator speed. With all these effects it’s not surprising that at one time a magnetic compass was often placed high on the mizzenmast for a sailor to climb to take readings, a very effective solution in calm seas but a problematic one when a bobbing ship magnified the needle bounce and swing! (The construction of a compass that would minimize needle swing due to the motion of a ship was also a long-running debate, with the version by William Thomson—later Lord Kelvin—in his popular commercial binnacle eventually losing out to liquid-filled models.)The direction of permanent magnetism of hard iron is related to the direction that the ship was facing when it was built; the compass needle will be attracted to the part of the ship that was south of it during construction. Smith held that an iron ship should be built with its head in a north-south direction, and preferably south. The effect is due to the alignment of magnetic domains in the iron with the external magnetic field of the Earth while being worked and pounded. In fact, Gilbert had created magnets by hammering iron rods laid in a north-south direction as part of his demonstration that the Earth acts as a mostly dipole magnet. But this initial permanent magnetism doesn’t last, and in some cases over half of a ship’s original permanent magnetism is lost in the course of its first year of use. And while the permanent magnetism of a ship is fairly constant after that point, any collision or repair of the ship will alter that permanent magnetism, requiring a new set of measurements and corrections to be applied.

2. Compensating for Magnetic Deviation

The magnetic deviation of a ship is typically corrected, even today, by components located in the binnacle holding the compass. Permanent magnets are positioned to compensate for the permanent magnetism of the ship. A vertical soft iron bar (the Flinders bar) is also located near the compass to counter the effects of the vertical component of the Earth’s magnetic field. These correct for the semicircular forces. Soft iron spheres on a rotating base serve to correct for the quadrantal forces, but their positions have to adjusted for the magnetic latitude.Compass Binnacle:The spheres also help negate magnetic deviations from heeling of the ship. There are also adjustable permanent magnets included to overcome these heeling effects. A permanent magnet mounted vertically directly beneath the compass does not have any effect when the ship is upright, but will correct for heeling error as the compass needle dips a bit in the lean. These permanent magnets also need to be adjusted with latitude.Finally, there are current-carrying coils in the binnacle that are energized to counter the effects when the ship activates its degaussing coils to elude mines that trigger on the magnetic fields of passing ships.Occasionally the net effect of magnetic deviation on an uncompensated compass completely negates the magnetic effect from the Earth, and the compass has no preferential direction at all, or only a weak one that makes observations uncertain. For this reason compensation is usually preferable to simply adding a correction in degrees from a table or diagram. I might add that Thomson once said that the chances were 50-50 that the navigator would get the sign wrong in calculating a compass correction [barber]. Also, the equations given earlier assume a magnetic deviation of less than about 20° in order that B and C can be expressed as simple arcsine functions, and so a certain amount of ship correction is generally needed in an iron ship to ensure this.For centuries, long compass needles (say, up to 15 inches) were thought better for higher magnetic strength (true enough) and better stability in rough seas (not true at all). But it happens that there are sextantal deviation terms in 3ξ′ for these long compass needles due to their response to the permanent magnet compensators, and octantal terms in 4ξ′ due to the interaction of these needles with their magnetic images in soft iron compensators. Smith early on had developed a rule for compass card needles, that when there are two needles they should be placed with their ends on the card at 30° to each side of the ends of the north-south diameter of the compass (the lubber line), and when there are four needles they should be placed with their ends at 15° and 45° from the ends of the diameter (30° apart). In this way there are equal moments of inertia parallel and perpendicular to the compass axis, which eliminates wobbling of the card But twenty years later, in analyzing ship data exhibiting higher-order effects, Evans and Smith discovered that small needles arranged in just the way he had prescribed exhibited less sextantal and octantal deviation than one short needle—and exactly zero mathematically! It was "a happy coincidence" according to Smith, and this justified moving the compensating permanent magnets and soft iron correctors much nearer this type of compass for more accurate elimination of the semicircular and quadrantal deviations [see Lyons for proofs].Because of the calculations required for tabulating the magnetic deviation for all magnetic or compass courses for a location at sea are time-consuming and error-prone, much work was done to create graphical ways of plotting the values for all courses from a few measurements taken at the location. The result could also be used by navigators to read the magnetic deviation quickly and easily while at sea. These graphical calculators are the subject of Part II of this essay.

Cheers,- jahman.

One of my professors was a navigator in the navy and talked about how they navigated with those balls and finders bar. He also had some cool pictures of degaussing stuff being built in Hawaii.

This long ago sailor learned to correct a chart heading to a binnacle reading with the rhyme-"True Virgins Make Dull Company" or TVMDC.True Heading +- Variation = Magnetic Heading +- Deviation = Compass CourseAnd if puzled as to whether to add/subtract variation- "East is Least"(minus), "West is Best"(plus)AR

This long ago sailor learned to correct a chart heading to a binnacle reading with the rhyme-"True Virgins Make Dull Company" or TVMDC.True Heading +- Variation = Magnetic Heading +- Deviation = Compass CourseAnd if puzled as to whether to add/subtract variation- "East is Least"(minus), "West is Best"(plus)AR

Yeah same thing for airplanes. Except you have to add the wind in there.

Yeah same thing for airplanes. Except you have to add the wind in there.

Similarly at sea you have to add any currents. While motoring down to Yucatan on a shrimp boat many moons ago (with Loran C and Loran charts - no direct readout of LAT/LONG!), you'd be amazed at the drift correction you're putting in when you're doing 12 kn and the Gulf Current is doing 3.5 kn 45º off your course :-)Cheers,- jahman.