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,
Flux loop integrate the signal over a period of time to get the magnetic field at that instant (not just the change in magnetic fields). This is normally done by adding a integrator circuit which will passively integrate the electrical signal. The flux loop is common on tokamaks, magnetic mirrors, Field-reversed configurations, the Levitated dipole experiment and the polywell. In tokamaks a set of loops is used, and these are spaced slightly off-center and non-concentrically. Placing the loops intentionally asymmetrically allows the users to find the magnetic field density at different points inside the tokamak. The distance between the loops was determined