Dictionary:Nuclear magnetic resonance (NMR)
Nuclei with odd numbers of electrons behave as magnets because of their spin. Their magnetic moments tend to orient when located in a static magnetic field. An orthogonal oscillating magnetic field excites them (polarizes them) when it is at the Larmor frequency (their natural frequency) and causes them to precess about the static field; the polarized precessing signal can be measured in a coil orthogonal to the exciting field. Polarization does not occur instantaneously but grows with time. When the exciting field is removed, the spins gradually return to their original state. The rate of return (relaxation time) depends on interactions with neighboring atoms and hence on the molecule of which the nucleus is a part. Relaxation time is the time for the magnetization to reach 63% of its final value. T1 is the longitudinal or spin-lattice relaxation time and T2 is the transverse or spin-spin relaxation time.