Compasses Point the Way from Geomancy to Navigation
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Compasses Point the Way from Geomancy to Navigation "How do we get there from here, and how do we get home?" are questions as old as humankind. Today the answer might be with a GPS (Global Positioning System) and a dedicated computer, but for hundreds if not thousands of years the answer was "with a compass." The unusual property of certain rocks to attract each other or pieces of iron was known very early on, at least by the time of Thales, ~600 BC. Such rocks contain the mineral magnetite, mainly FesOj, often with admixtures of Mg, Zn, Mn, or Ti. These rocks are often strongly magnetized, sometimes possess polarity, and are then known as lodestones, from Old English meaning "leading" or "guiding." We now know that magnetite is a ferrimagnet (one in which the spins of neighboring atoms are antiparallel but unequal resulting in a net magnetization) and mat its magnetization is lost at the 580°C Curie point. The magnetization arises from the rock's geologically slow cooling through its Curie point in the earth's magnetic field, hence the directionality, or possibly from the tremendous currents of lightning strikes. The north-south-orienting capability of lodestone was discovered during the Han dynasty in China sometime between 2nd century BC and 1st century AD. It was first used for divination or geomancy. For the Chinese this meant "the art of adapting the residences of the living and the tombs of the dead so as to cooperate and harmonize with the local currents of the cosmic breath," that is, adopting a north-south orientation. Chinese diviners made use of a board composed of two plates: the square lower plate represented the earth and was inscribed with the cardinal directions; the upper heaven-plate, round and free to rotate, bore a representation of the Big Dipper constellation, the outer rim of which always points to Polaris. Over the years the upper plate was transformed to a three-dimensional representation in the form of a spoon, fashioned from lodestone and pivoted on the polished bronze earth plate so that the spoon handle pointed south (see Needham's text for a figure). The spoon shape was not only a geometric analogue of the constellation, but the long handle acted as a pointer and the large length-diameter ratio counteracted the otherwise demagnetizing effect of poles too close to each other. The spoonshaped "south pointer" was referred to in Chinese literature nearly 1,400 years before the first European mention of the existence of the directional compass by Alexander of Neckam -1180 AD.
The Chinese plate-spoon contrivance could hardly have been a very convenient or inexpensive device, and the next improvement in the compass by the Chinese made use of the principle of magnetic induction. Instead of lodestone, small strips of iron were used, sometimes in the shape of a fish or tadpole. Interestingly, the Chinese term for tadpole, "tou," is also the radical for "ladle" or "spoon." Iron needles were also used. These could be magnetized in three ways: by stroking with a lodestone, by hammering a heated s
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