The signal strength is proportional to the stimulating magnetic field and the number of nuclei of that isotope in the sample.
Thus, in the 21 tesla magnetic field that may be found in high-resolution laboratory NMR spectrometers, protons resonate at 900 MHz.
However, in the Earth's magnetic field the same nuclei resonate at audio frequencies of around 2 kHz and generate feeble signals.
EFNMR signals can be affected by magnetically noisy laboratory environments and natural variations in the Earth's field, which originally compromised its usefulness.
are necessarily sophisticated and are too costly for most hobbyists, internet search engines find data and designs for basic proton precession magnetometers which claim to be within the capability of reasonably competent electronic hobbyists or undergraduate students to build from readily available components costing no more than a few tens of US dollars.
EFNMR equipment typically incorporates several coils, for stimulating the samples and for sensing the resulting NMR signals.
Examples of molecules containing hydrogen nuclei useful in proton EFNMR are water, hydrocarbons such as natural gas and petroleum, and carbohydrates such as occur in plants and animals.