text: Power lost due to a variation in this quantity is proportional to its peak value squared times frequency squared, all over resistivity, density, and a constant that is 1 for sheets and 2 for wires.
full_question: Power lost due to a variation in this quantity is proportional to its peak value squared times frequency squared, all over resistivity, density, and a constant that is 1 for sheets and 2 for wires. This quantity causes a combination of circular motion and guiding center drift, and can be measured at low values via a device with parallel Josephson junctions, known as SQUIDs. Type I and II superconductors differ in the number of critical values for this quantity, which is crossed with the velocity in the Lorentz force and is given by a line integral in the Biot-Savart Law. This quantity, which has zero divergence and is the curl of the vector potential, is expelled from superconductors in the Meissner effect. For 10 points, name this quantity measured in teslas, whose lines run from north to south poles.
first_sentence: Power lost due to a variation in this quantity is proportional to its peak value squared times frequency squared, all over resistivity, density, and a constant that is 1 for sheets and 2 for wires.
raw_answer: magnetic field [accept B field or H field]