File:Precession (expand the description below for a detailed explanation).webm

English: As most children learn from playing with magnets, two magnets placed on a table with their north poles facing one another will tend to rotate into alignment. The magnetic dipole of a proton is no different. Such is the case with this proton that has been placed in a magnetic field, as shown by the green arrow. Its dipole, the orange arrow, will experience a force pulling it into alignment with the overall magnetic field.

This may at first seem simple, but don't forget that, like a gyroscope, this proton is spinning. As gravity makes a gyroscope precess as it pulls it down, this magnetic field will make the proton precess at it pulls it up. The result is the characteristic motion seen here. The proton does indeed rotate to align with the magnetic field, but precesses as it does so. This seems trivial, but this concept is fundamental: if protons did not precess, there would be no such thing as MRI or NMR. This is because multiple protons precessing together produces magnetic fields oscillations that are measurable. Later graphics will better demonstrate this, but it is important for now to accept two important principles: 1. Protons precess when in a magnetic field. The frequency at which the precess is called the Larmor frequency.

2. This causes an oscillating magnetic field that can be measured.