Talk:Flinders bar

Is it correct to refer to the Flinders Bar as a "soft iron magnet"? The Handbook of Compass adjustment published by the US Department of Geospatial-Intelligence www.nga.mil (at pages 2 and 15) refers to the need to ensure there is no residual magnetism in the Flinders bar. Perhaps more correct to call it a "soft iron bar"?

Pgtee (talk) 13:20, 15 April 2008 (UTC)

My Google searches support "soft iron bar", especially this interesting article on the web site of the Royal Geographical Society of Queensland quoting pages 411-413 from "Matthew Flinders: Navigator and Chartmaker" by G.C. Ingleton, Genesis Publications. (Which, incidently points out that the term "Flinders Bar" wasn't common till Lord Kelvin included one in his 1879 patented design for a compass binnacle.) - Will change article accordingly.GraL (talk) 18:14, 11 July 2008 (UTC)

The article on the flinders bar states: "The Flinders bar is used to counteract the vertical magnetism inherent within a ship...where deviations caused by this inherent magnetism are negated by the use of horizontal (or quadrantal) correctors." For more clarity and technical accuracy, I recommend restating that point as follows: "The Flinders bar is one of several devices used to counteract or compensate for a vessel's magnetic compass deviation, the magnetic compass error induced by the inherent magnetism in a vessel's iron or steel hull and/or steel equipment (e.g., engines or other machinery). Objects aboard the vessel that are made of magnetic materials (e.g., large iron or steel objects), or that have inherent magnetism, can distort the Earth's magnetic field in the vicinity of the magnetic compass and cause the error called deviation. Deviation varies with the vessel's heading as the vessel's local influence on the Earth's magnetic field varies with the angle of the vessel's net magnetic influence relative to the local direction of the magnetic lines of flux of the Earth's magnetic field. The Flinders bar is used to compensate for the vertical component of the vessel's magnetism. Other compass compensating devices include soft iron balls (typically about the size of softballs on the average sized binnacle) mounted on brackets on either side of the binnacle. These are called quadrantal spheres, or "Kelvin's balls." In addition, inside the binnacle (or inside some marine compasses) are a horizonal, "east-west," bar magnet and a horizontal "north-south" bar magnet. The position relative to the compass card of each of these devices can be adjusted by a compass adjuster when "swinging ship" or "swinging the compass" to minimize the vessel's deviation or magnetic influence on the compass.

The next paragraph states: "Where the deviation from a compass point cannot be counteracted..." The way this was stated is not exactly correct. Deviation, one of several types of magnetic compass errors, is not said to be from a "compass point" as stated, but rather, the deviation is the angular difference between the vessel's compass heading, i.e., as indicated by her magnetic compass, and the magnetic heading, which is the heading relative to magnetic north (the north pole of the Earth's magnetic field), without any local influence on the compass by the vessel's hull or equipment. Therefore, I recommend that this point be explained thus: "Where the deviation of a vessel's magnetic compass cannot be fully compensated for by means of the various compensating devices described above, Flinders bar, heeling error magnets, quadrantal spheres, and horizontal magnets, a deviation card (table) or graph (called a Napier diagram) is produced by the process of swinging the compass as mentioned above. In this process, the vessel is turned ("swung") to various directions that can be determined to be true. Applying the known local magnetic variation (the angular difference at any given geographic position betweem true north and magnetic north) for the vessel's geographic position, the vessel's magnetic heading is found, i.e., what the compass heading would be without the vessel's influence. Comparing the magnetic heading to the actual compass heading yields the deviation for that heading. The table or graph thus produced gives the vessel's remaining deviation for various magnetic courses or headings to be applied to a desired magnetic course when determining a compass course to steer in order to steer that desired magnetic course (called "uncorrecting"). The table also gives the deviation for various compass courses in order to determine what the magnetic course or heading is for any given heading or direction indicated by the compass (called "correcting"). When correcting, i.e., determining a compass course that corresponds to a given magnetic course, easterly deviation is subtracted from, or westerly deviation is added to, the magnetic course to find the compass course. When uncorrecting, the deviation is applied in the opposite direction to that when correcting.Flinderssda (talk) 21:36, 5 April 2012 (UTC)