gauss
Magnetic flux densityexact by definition
The gauss is the CGS unit of magnetic flux density, equal to exactly 1e-4 tesla. The conversion is exact by definition rather than measured, since it is a fixed relation between coherent unit systems. Although it is not an SI unit, it remains in daily use in magnet manufacturing, geophysics and medical imaging safety practice.
Watch out: The gauss is a flux density and pairs with the oersted, which is the CGS field strength. In free space a field of one oersted produces a flux density of one gauss, and the two are casually treated as interchangeable, but inside magnetic material they are not.
| 1 G | 0.0001 T |
Named for Carl Friedrich Gauss, who with Wilhelm Weber built the first systematic absolute measurements of the Earth's magnetic field in the 1830s and set up a magnetic observatory at Göttingen.
About the gauss
The gauss persists because it sizes ordinary magnetism conveniently. The Earth's field is about half a gauss, a fridge magnet is 50 to 100 gauss at the surface, a loudspeaker gap runs around 10000 gauss, and a 1.5 T MRI bore is 15000. The habit is deeply embedded in the magnet trade: catalogues quote surface gauss, gaussmeters read gauss, and residual induction \(B_r\) for a neodymium grade is usually printed in kilogauss even in otherwise metric documents.
The number that shows up most often outside the trade is the 5 gauss line, the perimeter around an MRI magnet inside which the field exceeds 0.5 mT. It is the conventional exclusion boundary for pacemakers, implanted devices and loose ferromagnetic objects, and it is marked on the floor of every scanner suite. Beyond that line the field falls off steeply, roughly as the inverse cube of distance for a dipole, which is why a step or two makes a large difference.