To understand the principles involved is easy if the cobwebs and dust are shaken off the school books and memories of atomic particles. Remember that old friend, the hydrogen atom, first in the atomic table, with just one proton and one orbiting electron, as simple a thing as any alchemist could wish. The orbiting electron acts just like electric current in a coil of wire and sets up a magnetic field about the atom as seen in Fig. 1. The proton, the main mass of the atom, is also in motion, spinning about its centre, so that the whole atom looks like a magnetic gyroscope, whose magnetic poles are on its spin axis.

Gyroscopes have idiosyncrasies revealed to mechanical engineers, one of them being that they will precess if acted upon by an external field. In the hydrogen proton’s case, its spin axis will wobble or precess about the direction of the earth’s magnetic field if that is the only magnetic field acting on it. This is shown in Fig. 2.