Fox dip and intensity apparatus, c 1840.

Detail of jewel bearings. Fox dip and intensity apparatus made by W George of Falmouth, Cornwall. A magnetic needle on a horizontal axis lays horizontally only at the magnetic equator where the attraction to the poles is evenly balanced. Elsewhere, the needle will dip, to an angle dependent upon its distance from the nearest magnetic pole. Dip circles were frequently taken on scientific expeditions from the 18th century onwards. R W Fox introduced a variation in design in 1834, to allow for use on a moving ship and the measurement of magnetic force as well as dip.