## The rotating coordinates or rotating frame

It is convenient to figure only the magnetic momentum of the excess of the nuclei in their fundamental states (Fig.3)

M is the macroscopic magnetization as a result of the individual nuclear moment
. The xy plane does not contain any magnetization component. In addition to the coordinate system K(x,y,z), another system K'(x,y,z) rotates around the z axis with the angular speed
thus
. In this figure,
is a virtual ﬁeld Bf opposite to B_{0}, which is generated by the only relative rotation from one versus the other of the two coordinate systems K and K'. This means that the
vector is static in the system K' which is called rotating frame, it happens when
has the same sign and the same value than
.^{[1]}