Task: Text Categorization
Input (English): Birefringence results from these entities in the Voigt effect, while light is polarized with a rotation linearly dependent one of their components in the Faraday effect. The force exerted by them is proportional to the cross product of these entities with the velocity of a nearby charged particle, and they are expulsed by superconductors in a phenomenon discovered by Meissner. One differential equation stating that they have a divergence equal to zero, named after Gauss, is the second of Maxwell's equations. Their strength is measured in teslas and they are denoted by the letter B. For 10 points, name these entities that often surround a material that has north and south poles.
Output (English): Science