The rules of quantum mechanics are highly successful in describing microscopic objects, atoms and elementary particles. But "macroscopic systems," like springs and capacitors, are accurately described by classical theories like classical mechanics and classical electrodynamics. If quantum mechanics were to be applicable to macroscopic objects, there must be some limit in which quantum mechanics reduces to classical mechanics. "Bohr's correspondence principle demands that classical physics and quantum physics give the same answer when the systems become large". Arnold Sommerfeld referred to the principle as "Bohrs Zauberstab" (Bohr's magic wand) in 1921.