Magnetic Resonance of realistic materials is often confounded by magnetic susceptibility mismatch effects which dramatically increase the MR linewidth. In an ideal world one would wish to choose the maximum field strength (for more signal) consistent with a tolerable magnetic susceptibility mis match effect. For several years we have been employing a cryogen free magnet with an integrated magnet power supply which permits us to increase or decrease the static field as desired, controlling linewidth and susceptibility effects for relaxation and imaging studies. 
There is however another advantage of the variable field magnet approach to materials MR/MRI. If we are studying a system with a complicated apparatus (for example a battery or a system at high temperature and pressure) the simplest possible RF probe is desirable. The variable field magnet makes it possible to bring different nuclei into resonance while maintain the original resonance frequency. The RF probe in this case maintains a constant frequency and constant sensitivity even as we change the nuceli being interrogated. For example, at 0.79 T the 1H frequency is 33 MHz. If the field is increased to 2.99 T we measure 23Na at 33 MHz and if we increase the field to 3.1 Tesla we measure 13C at 33 MHz. 
In this presentation we will outline measurements where we adjust the field to control susceptibility mismatch effects, and we will describe measurements in systems where we adopt the field changing approach to undertake 19F, 13C, 23Na and 7Li studies in conjunction with 1H for battery studies and studies of fluids in porous media.