Semiconductors are used in certain applications because they make extremely powerful magnetic fields utilizing relatively small amounts of energy in relatively small equipment. Probably the most common use of superconductors is in nuclear magnetic imaging, particularly with medical applications like MRI. Chemists utilize magnetic imaging with NMR, or nuclear magnetic resonance spectroscopy. NMR measures the energy required to change the spin of an atomic nucleus in a chemical sample. Since the amounts of energy and the energy differences being measured here are extremely tiny (measured in parts per million), very powerful magnetic fields are required to measure energy at this level. Superconducting magnets in NMR's cooled by liquid nitrogen and helium provide this level of power.