Flux surface surrounding a charge "q" is "q"/ε. In free space the electric displacement is given by the constitutive relation D = ε E, so for any bounding surface the D-field flux equals the charge "Q" within it. Here the expression "flux of" indicates a mathematical operation and, as can be seen, the result is not necessarily a "flow", since nothing actually flows along electric field lines. The magnetic flux density (magnetic field) having the unit Wb/m (Tesla) is denoted by B, and magnetic flux is defined analogously: with the same notation above. The quantity arises in Faraday's law of induction,
Magnetic flux quantum holes. The magnetic flux quantum may be measured with great precision by exploiting the Josephson effect. When coupled with the measurement of the von Klitzing constant , this provides the most precise values of Planck's constant obtained to date. This is remarkable since is generally associated with the behavior of microscopically small systems, whereas the quantization of magnetic flux in a superconductor and the quantum Hall effect are both collective phenomena associated with thermodynamically large numbers of particles. Magnetic flux quantum The magnetic flux, represented by the symbol , threading some contour or loop is defined as the magnetic field