simple pulsed-NMR experiment begins with the ap-
plication of a near-resonant excitation pulse applied to a
coil containing the sample of interest. This pulse tips the
nuclear magnetization from its equilibrium orientation,
along a static magnetic field, into the plane perpendic-
ular to the field. Following the pulse, precession of the
magnetization about the static field induces a voltage
in a receive coil. This signal is amplified, filtered, and
digitized for further processing and display. One attrac-
tive feature of NMR in the earth’s field is that if care
is taken in siting and choice of materials, the field over
the sample volume can be very uniform, giving NMR
linewidths significantly less than 1 Hz. Because the net
nuclear magnetization in thermal equilibrium at room
temperature in low field is very small, such NMR ex-
periments are often preceded by a polarization period,
during which a much larger, but potentially more inho-
mogeneous, magnetic field is applied in order to increase
the net polarization of the nuclear spins.