Theory Introduction Atoms consist of a nucleus and at least one electron, both of which have an electric charge.
Some nuclei also have an associated spin .
When these nuclei are placed in a strong magnetic field they all line up in one of two directions, either with the field or against the field. It requires more energy for these to oppose the magnetic field than to be in line with it, as can be experienced by holding two magnets in opposite and similar directions. This leads to two possible energy levels that the nuclei can occupy. This nucleus will then drop from the higher energy to lower energy by emitting exactly the same amount of energy as heat. When a nucleus undergoes this cycle of excitation and relaxation it is said to be 'in resonance', hence nuclear magnetic resonance .