The high Q sample coil surrounds a 125 ml plastic bottle containing
a liquid rich in either hydrogen or fluorine nuclei. The sample is placed in a uniform
part of the Earth's magnetic field with the coil's axis aligned perpendicular
to this field. The sample coil is connected to either a dc current regulated
power supply or a high-gain, low-noise, tuned amplifier, with both
switching and tuning electronically controlled. The output of the tuned
amplifier is presented on a storage oscilloscope for observation and
At the beginning of an experiment, the sample coil is connected to
the dc power supply. This supplies a maximum of 3 amperes for a pre-determined
length of time. The electronic circuit then disconnects the
supply, quickly dissipates the stored energy, and connects the coil to the
The sample nuclei (usually protons), having been polarized in a large
magnetic field created by the power supply, find themselves oriented
with their net magnetization perpendicular to the Earth's field. With the
polarizing field now off, this magnetization precesses in the Earth's field,
producing a time varying magnetic flux through the sample coil. The
time varying flux creates an emf at the coil's terminals, which is magnified
by the tuned amplifier.
Although the fundamental ideas behind the instrument are simple,
the actual design of the apparatus has important subtleties.
As the photographs show, there are actually two coils around the sample.
The outer or "halo" coil is a bucking coil, designed to significantly
reduce random noise pick-up from the always present stray electromagnetic
fields. The bucking coil has the same turns-area as the inner
sample coil. The coils are connected in series but in opposition.
The output of the two coils is connected to the receiver. Since
the two coils are in opposition, local noise fields produce
equal and opposite emf in each coil and the net noise emf at
the receiver is theoretically zero. Because the precessing magnetization
is a dipole field, it couples primarily to the sample coil.
This common mod rejection is essential to the outstanding signal-to-noise
ratio of the apparatus. The students can study this system by
rearranging the bucking coil connections.