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When certain substances are cooled to extremely low temperatures, they become superconductors, conducting electrical current entirely without resistance. This applies to helium-4, the most common form of helium, but for a long time the superfluidity of helium-3 was in dispute. The different types of helium are described by different quantum mechanical rules and equations under which helium-4 has a whole-number spin while helium-3 has a half-number spin. After it was discovered that at extremely low temperatures helium-3 also becomes superconducting, Anthony Leggett formulated a theory that explained this.