The overall magnitude of a magnetic field is measured in units called webers (Wb). A smaller unit, the maxwell (Mx), is sometimes used if a magnetic field is weak. One weber is equivalent to 100,000,000 (108) maxwells. Conversely, 1 Mx = 0.00000001 Wb = 10−8 Wb.
The Tesla and the Gauss
If you have access to a permanent magnet or electromagnet, you might see its strength expressed in terms of webers or maxwells. But usually you’ll hear units called teslas (T) or gauss (G). These units are expressions of the concentration, or intensity, of the magnetic field within a certain cross section. The flux density, or number of lines per square meter or per square centimeter, is a more useful expression for magnetic effects than the overall quantity of magnetism. A flux density of 1 tesla (1 T) is equal to 1 weber per square meter (1 Wb/m2). A flux density of 1 gauss (1 G) is equal to 1 maxwell per square centimeter (1 Mx/cm2). It turns out that the gauss is equal to 0.0001 tesla (10−4 T). Conversely, the tesla is equivalent to 10,000 gauss (104 G).
The Ampere-Turn and the Gilbert
With electromagnets, another unit is employed: the ampere-turn (At). This is technically a unit of magnetomotive force, which is the magnetic counterpart of electromotive force. A wire, bent into a circle and carrying 1 A of current, produces 1 At of magnetomotive force. If the wire is bent into a loop having 50 turns, and the current stays the same, the resulting magnetomotive force is 50 At. If the current is then reduced to 1/50 A or 20 mA, the magnetomotive force will go back down to 1 At. The gilbert (Gb) is also used to express magnetomotive force, but it is less common than the ampere-turn. One gilbert (1 Gb) is equal to 0.796 At. Conversely, 1 At = 1.26 Gb.