This disclosure is directed to improved equipment in detection of desired nuclei or electron response. In a magnetic field thought to be inhomogeneous, an improvement which incorporates means for summing repetitive samples is disclosed. Excitation is applied to the sample of interest, and the received signal is broken into samples over the life of the received signal and temporarily stored. The flux is varied by varying the magnet current, and several nuclear magnetic resonance signals are received. They are similarly sampled periodically and added to earlier responses. They are preferably stored in a buffer after conversion by an analog-to-digital convertor. After stepping through several magnetic levels, the testing is completed. The summation of the various signals is a signal enhancement technique which overcomes the lack of a uniform optimum field impressed on the sample of interest.