Chemical Shift
Again, consider Maxwell’s theorem that an electric current in a loop
generates a magnetic field.   Effectively, the electron distribution in the
chemical will cause distinct local magnetic fields that will either add to or
subtract from Bo 
HO-CH2-CH3
Aromaticity, electronegativity and similar factors will contribute
to chemical shift differences
Beff = Bo( 1 - s )
de-shielding                                      high shielding
Shielding – local field opposes Bo