Usually the strength of the magnetic field. The stronger that magnetic the field, the higher the radio frequency at which the spins will oscillate, and the further apart the resonances will be. The strength of the magnet and the resolution of the instrument are actually usually described in terms of the frequency of the tetramethylsilane (TMS) resonance - a low resolution machine might be 60 or 90 MHz, while high resolution machines are over 600 MHz now.