Complex sentence: The "macroscopic" set of Maxwell's equations defines two new auxiliary fields that describe the large-scale behaviour of matter without having to consider atomic scale details, but their use requires experimentally determining parameters for a phenomenological description of the electromagnetic response of materials.
Split into simpler sentences: The "macroscopic" Maxwell equations define two new auxiliary fields that describe the large-scale behaviour of matter without having to consider atomic scale details. However, their use requires experimentally determining parameters for a phenomenological description of the electromagnetic response of materials.