Admiralty steering compas, mid 19th century.
© Science Museum / Science & Society Picture Library
A compas is a device for determining a horizontal geographical direction or bearing; invented in China in 1117, and in Europe in 1190. The magnetic compas depends on a magnet, free to rotate in a horizontal plane, which locates itself in line with the Earth's magnetic field. It is subject to the irregularity and to the short and long period variation of the Earth's field. This example was constructed for the British Admiralty, for use in smaller vesels. The instrument is fitted with a dry card of 6 1/2 inches ( 165 mm) diameter, and is suspended, in gimbals, in an octagonal wooden case. It has two laminated magnets, and the upper part of the bowl is fitted with a copper ring. These features were intended to increase sensitivity, and to promote eddy-current damping.