The basis of NMR is founded on and works with Nuclear Spin States. A Spin State results from an uneven number of protons to create a difference of magnetic charge and thus a magnetic field called a magnetic moment arises. In the presence of a magnetic field there are two spin states possible a +1/2 or -1/2. The +1/2 state is aligned with the magnetic field and the -1/2 state is higher in energy and spins in an opposing direction to the external field.
The difference in energy between the two spin states is directly proportional to the external magnetic field applied to it. The magnetic field that arises from it is always much smaller than the magnetic field applied to it. The magnetic field is generated by a powerful magnet and the magnetic field strength is measured in Tesla Units often between 1-20. 1 being relatively weak. The magnetic field of the earth is not constant, however, it is generally about 10-4 Tesla. As stated, the difference in Energy (ΔE) between the two spin states is extremely small and is measured as a frequency in MHz units. This electromagnetic radiation falls within the radio and television broadcast spectrum.
The difference between the two spin states at a given magnetic field strength will be proportional to their magnetic moments. This field strength is the number within a range that shows up on the bottom of the spectra.
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