L9504-002

Steel bar declination magnetograph, detail, circa 1840 Long Caption: Portion of a declination magnetometer, made up of four numbered parts: a bar magnet and its carrier, a mirror and its mount. 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. Credit line: © National Maritime Museum, Greenwich, London Object: AST0705 Artist: unknown Date: circa 1840 Medium: steel Size: 76 mm x 432 mm x 127 mm Click here to buy a bespoke print of this image.