Fox dip and intensity apparatus, c 1840.
© Science Museum / Science & Society Picture Library
Detail of jewel bearings. Dip circles are used to measure the angle between the direction of the earth's magnetic field and the horizontal at any point on the earth's surface. A magnetic needle on a horizontal axis lays horizontally only at the magnetic equator, where the attraction to the poles is equally balanced. Elsewhere, the needle will dip to an angle which depends on the distance from the nearest magnetic pole. At the north and south magnetic poles the needle dips vertically. Dip circles were frequently taken on scientific expeditions from the 18th century onwards. This apparatus demonstrates the variation in design introduced by R W Fox in 1834, to allow for use on a moving ship and the measurement of both magnetic force and dip. Made by W George.