A nuclear magnetic resonance (NMR) apparatus includes a pair of permanent magnets spaced apart from each other and defining a sample space, the permanent magnets providing a magnetic field in the sample space, an array of magnetic field sensors arranged relative to the sample space to provide, during operation, information about a homogeneity of the magnetic field, an array of magnetic coils arranged relative to the sample space so that each magnetic coil, during operation, generates a magnetic field that changes the magnetic field in the sample space, and an electronic controller programmed to receive information about the magnetic field from the array of magnetic field sensors and to variably energize the magnetic coils in the array of magnetic coils so that the magnetic fields generated by the array of magnetic coils reduces inhomogeneities of the magnetic field.