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 freely-suspended magnetic needle lies horizontal only at the magnetic equator, where the attraction to the poles is equally balanced. Elsewhere, the needle dips to an angle depending on the distance from the nearest magnetic pole, and vertically at the poles. Scientists frequently took dip circles on field expeditions from the 18th century onwards. This example was made by Dollond of London.
© Science Museum / Science & Society Picture Library