The science, art and practice of compass compensation became a serious endeavor when wooden ships began to incorporate iron and iron fittings into the hull and rigging. Gun barrels in the lower decks had always been problematic in wooden ships but it was not until the formation of the Liverpool Compass Committee with grants from the British government and private donors that real investigation became possible.
The Admiralty Manual for the Deviations of the Compass was finally published in 1862 and there were multiple editions after this date as more information was gathered when larger iron ships were built and voyaged greater distances, particularly on the newer and lucrative trade routes to the southern hemisphere. The safety of crews, passengers and cargoes became an issue when dramatic changes in magnetism and large increases in compass deviation were shown to be the cause of tragic groundings and loss of life in and around the British Isles and Ireland.
The practice of compass adjusting has changed little in the 150 years since the first volume on the mathematics and methodology first went to print. WWI and WWII saw dramatic increases in the number of compass adjusters in need of training both in the UK and US as Merchant Fleet tonnage ballooned to support the war effort and the allied Navies grew to massive numbers to protect trade and convoys. It was in these times that a particular class of destroyer or frigate or indeed battleship, would undertake a voyage from the San Diego ship building yards to the magnetic equator to determine the amount of soft iron needed. From this investigation, the length of Flinders Bar and size and distance from the needle system of the D-Correctors or Soft Iron Spheres, were standardized for a class of vessel.
An excellent text on compass adjusting taught during WWII is the publication by the NGIA and is as relevant today as it was then. An equally compelling text giving the history and method of adjustment in the Royal Navy is contained in Lectures on Compass Adjustment [formerly given to the Navigating Officers of the Royal Navy (1906)] by Captain W R Martin who was the instructor in Compass Adjustment at the Royal Naval College, Greenwich.
What can you expect from a Compass Adjustment on your vessel?
Interrogate the interactive compass deviation card which shows a set of deviations that the compass adjuster has observed on 8 equidistant points of the compass. These are shown graphically as a curve of deviation and from these deviations the 5 Approximate Coefficients A, B, C, D & E are calculated. The adjuster will compensate the errors, and in doing so the curve and Coefficients will be reduced and the directive force at the compass will be restored almost to the same value of the earth's horizontal force at that location.
As a final check, the vessel is swung through 360 degrees and the compass needle will now point Magnetic North or very close-to on all points of the compass; it is now up to the Master to ensure that the deviation is checked at regular intervals as required by AMSA.