Portion of a declination magnetometer, made up of four numbered parts: a bar magnet and its carrier, a mirror and its mount. Rectangular bar magnet mounted in a carrier originally fitted with two mirors (now missing). The whole fitting may be orientated on a graduated circle from which the magnet carrier is depended.The bar magnet was used at the Royal Observatory, Greenwich, from 1840. It was used to study the earth's magnetic field and was observed hourly until 1847, when photographic self-registration was introduced. It was replaced by a smaller instrument in 1914.

A magnetometer is a device used to measure the strength and/ or direction of the magnetic field near or around it. Magnetometers at the Royal Observatory were used to measure the Earth's magnetic field at Greenwich and how that varied from day to day. The reason for doing this was primarily to aid the use of then modern, very sensitive compasses used on board ships. That a better understanding of how the Earth's magnetic field changed from day to day and if that change related to any other observable phenomenon (such as an increase in the number of sunspots observed for example) might aid navigation.