Magnetic dipole magnetic dipole as the distance from the source increases. In classical physics, the magnetic field of a dipole is calculated as the limit of either a current loop or a pair of charges as the source shrinks to a point while keeping the magnetic moment constant. For the current loop, this limit is most easily derived for the vector potential. Outside of the source region, this potential is (in SI units) with 4"π r" being the surface of a sphere of radius "r"; and the magnetic flux density (strength of the B-field) in teslas is In spherical coordinates with the
Flux tube density of the tube increase with the same proportionality. If a flux tube with a configuration of a magnetic field of B and a plasma density of ρ confined to the tube is compressed by a scalar value defined as λ, the new magnetic field and density are given by:formula_10formula_11If λ < 1, known as traverse compression, B and ρ increase and are scaled the same while transverse expansion decreases B and ρ by the same value and proportion where B/ρ = constant value. Extending the length of the flux tube by λ* gives a new length of L =