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.

This magnetometer was used at the Royal Observatory between 1864 and 1917. This instrument measures small changes in the vertical force of the Earth's magnetic field. There were effectively 4 properties to the Earth's magnetic force that that were measured at the Observatory - the direction and intensity of the vertical force, and the direction and intensity of the horizontal force and often a different instrument was used for each of these properties. There are four additional numbered parts.

Vertical force magnetometer. 18-inch bar magnet, tapering and pointed at both ends, held in metal clamp carrying 20-cm steel knife edge, two mirrors for recording magnet's motions and two brass counter balances on right angle screws. Either end of knife edge rests on two agate planes set in a heavy gun-metal mount, fitted with a lever for raising the magnet, clamp and knife edge. 4 securing screws for mounting.