This quantity’s thermally-activated “creep” is modeled in Anderson–Kim theory. A “pinched” hysteresis loop is a signature of a device whose strength is the derivative of this quantity with respect to charge. The material derivative of this quantity vanishes for infinitely conducting fluids by Alfvén’s theorem. The voltage produced in the AC Josephson effect is equal to the frequency times the quantum of this quantity, which equals h over 2e. The (10[1])“pinning” of this quantity in superconductors (10[1])creates a force that allows them to levitate magnets. This quantity can be computed as the double integral of “B dot dA.” The EMF induced in a loop is proportional to the time-derivative of this quantity, symbolized phi, by Faraday’s law. For 10 points, name this quantity that measures the density of magnetic field lines passing through a surface. ■END■