90 as detected by conductor or probe 92 a signal source 94 provides the selected signal pulses to probe 92 and the return signal is received and interpreted by measuring device 96 the output of which is then sent to computer 98 the chosen applied magnetic field direction can be fixed or variable the intensity and the gradient of the applied magnetic field can also be fixed or variable depending on the particular application of the invention in this manner the inventive method and apparatus can be used to monitor the progress of a chemical reaction or physical change in the process as an example without limitation the invention can be used to monitor the physical changes that occur in the hardening of a resin containing a substance such as a paramagnetic material the polarization response of which to an applied magnetic field changes as the resin changes from a liquid state to a solid state similarly the invention can be used to monitor the progress of a chemical reaction if the reaction either produces or consumes a substance measurably responsive to the applied magnetic field as an example if a chemical reaction either produces or consumes a paramagnetic ion then the change in the concentration of that paramagnetic ion as the reaction proceeds will cause a change in the total magnetic susceptibility of the sample and thereby cause a change in the performance characteristic of the probe as measured by the measuring device even if the reaction under direct study does not produce or consume a paramagnetic ion the sample can be u201c spiked u201d either with a paramagnetic ion that is consumed by one of the reaction products or with a.