The compass is a key instrument for guidance at sea and in air.
The principle on which the compass is based is the interaction between the magnetic needle and earth's magnetic field: a magnet free to move, whose poles are not placed along the line joining the two opposing magnetic poles of earth, undergoes the action of a pair of equal forces, that make it finally align with earth's magnetic field lines.
The magnetic needle should have been used in China since 2634 a. C. Ancient Chinese records refer, in fact, of a cart on which some dummy humans had the arm pointing south, the most important cardinal point for the Chinese. A complete description of the cart is available only during the reign of Emperor Hian-tsoung (806-820 AD), while the first description of magnetic properties appear in a Chinese dictionary of the second century AD. Marco Polo (XIII century) does not speak of the compass.
Early historical records of magnetic properties in Europe date from the late twelfth century or beginning of thirteenth and attribute to the Republic of Amalfi the knowledge on the properties of the magnet. The news that they had invented the compass, given by Panormita and repeated by Flavio Biondo, gave rise to the legend of a nonexistent Flavio Gioia of Amalfi, who was said to have built the first compass in 1302. After that the instrument spreaded in the Mediterranean and was then imported to the Nordic countries.
The primitive compass consisted of a small bar set on a wooden flat floating in a bowl of water. Italians improved the instrument with the addition first of a container, so that the float was not disturbed by wind, and then, at mid XIV century, put a magnet suspended above a pin and finally completed the compass with a windrose, usually very ornate.
The gimbal appeared in the mid XVI century. A detailed description is in Breve compendio de la esfera y de la arte de navegar by Martin Cortes (1551), where clear guidelines for the construction of the compass are given. To the refinements to the instrument are largely attributable the great discoveries of new lands, by mid XV century onwards.
With the supremacy of Flemish, English and French in the sixteenth - seventeenth centuries, the Italians were overcome in the art to build compasses. At the beginning of the nineteenth century appeared the first dry compasses made with scientific criteria. In 1838 a Commission met in England, to define the so-called standard compass of the British Admiralty (Admiralty Standard Compass). It used four needles and was built according to strict principles of mechanics and magnetism. It was used on iron ships until, in 1877, Sir W. Thomson built a compass based on the principle of a very light windrose with eight small needles, that rapidly had large acceptance.
The DRY "THOMSON" COMPASS was in use until the early decades of the twentieth century, despite the arrival of more modern but still imperfect liquid compasses. The windrose, drawn on paper and supported by a very thin aluminum ring is connected with silken threads to the cap and to the sensor, consisting of eight needles in steel wires. To further reduce friction, the tip of the suspension is made of iridium and the cap takes a sapphire stone. The total weight of the rose is less than 20 grams.
Only in the late nineteenth century were introduced liquid compasses, almost simultaneously in various countries including Italy.
The magnetic compass shows the direction of the horizontal component of terrestrial magnetic field. Essentially consists of a magnetic needle placed in its center of gravity on a pin and free to rotate horizontally. To the needle is fastened a ring graduated from 0° to 360° (the windrose). The magnetic needle, under the influence of terrestrial magnetism, constantly directs one end to the magnetic North Pole. The North in the windrose will match this end of the needle. Everything is contained in a cylindrical metal box - the mortar - protected by glass.
The mortar is suspended by means of concentric rings moving at right angles to one another (gimbal) to keep the compass horizontal, despite the rolling or pitching of the ship. On the internal periphery of the mortar, usually painted in white, is marked a vertical black stripe, the reference line, lying in the longitudinal plane of the vessel or in a plane parallel to it.
The helmsman, handling the wheel, keeps the reference line to match the graduation of the rose corresponding to the route to be followed by the ship. In modern compasses there are more needles in parallel to increase the magnetic moment of the rose, windrose being the word commonly used to indicate the assembly of needles and graduated circle.
Dry compasses are equipped with smaller and lighter needles, joined together with thin strands of silk, on which there is a graduated circle of paper. The rose weighs very little, to minimize friction on the pin. Liquid compasses, however, are equipped with powerful and heavy magnets, contained within a float, carrying the graduated ring. The whole is immersed in a liquid which is usually a mixture of water and alcohol, in order to lower the freezing point of the liquid.
This gives large magnetic moments of the windroses, considerable damping, due to the liquid, and with a well studied float, very little friction on the pin or tip of the suspension. Liquid compasses are required on ships subject to sudden movements or vibrations produced by the thrusters.
Graduation on a metal ring is based on the float with four bundles of three needles each, arranged symmetrically in pairs in opposite positions.
The high number of needles was required because of the limited magnetic properties of materials at the time, while the float reduce the weight of the rose to a few grams in the alcohol mixture, ensuring the necessary sensitivity. The instrument, whose weight is 4.7 kg, bears the engravings "154" on the mortar, and "Hydrographic Office 390" on the rose.
(In 1898 the Hydrographic Institute published The liquid compass of the Royal Navy built at the Hydrographic Office after the drawings of G.B. Magnaghi. In 1904 and again a few years later they published The amended Magnaghi liquid compass: description and compensation).
The mortar containing the rose rests on the gimbal, on a sturdy ring attached to a wooden column that is called the binnacle, this one being firmly secured to the deck. Ring's position may be adjusted around a vertical axis passing through its center, so that the reference line is exactly in line with the fore-aft direction of the ship.
The binnacle contains the compensating permanent magnets and on one side, in cross-ship direction, are applied spheres or cylinders of soft iron, to compensate the compass. To protect the compass binnacle on top of it there is a cap with openings (bonnet).