"- - -"[Shshsh!] MAGNETIC RESONANCE

The frequency of that radiation gives away what it is that is resonating: a piano with only a few strings could be tested this way to ascertain which strings are there, and the natural frequency of a swing bearing a 3-year old will be different from (higher than) that of the same swing when the Hon. Tip O'Neill sits in it.

Placing a material in a strong magnetic field, tickling it with a high-frequency electric field, and observing the resonance pattern of the nuclear radiation is known as Nuclear Magnetic Resonance (NMR); it will identify the types of radiating nuclei (the chemical element) and more.

Funnily enough, the "nuclear" in NMR is not only semantically indispensable ("magnetic resonance" is something else), but as in the case of nuclear power, it denotes a method that is both superior and safer than what we have been hitherto using for the same purpose (X-rays).

For reasons that would take us too far, not every type of atom will undergo NMR, but one that will do so is the hydrogen atom, which is present in every organic compound, in particular, throughout the human body. The radiation (radio-frequency, and therefore non-ionizing, i.e. not of the radioactive type) depends on several factors, of which two are particularly important: the strength of the applied magnetic field, and the concentration of the resonating atoms (for the evoked radiation is extremely weak and lots of sources must be present to make it observable).

A given nucleus (say, of hydrogen) in a given magnetic field will always resonate at the same NMR frequency; but its chemical environment, i.e. the type of molecule in which the atom is bound, will modify the magnetic field locally and slightly change the NMR frequency, so that the hydrogen nuclei of the atoms in water will resonate at a different frequency than those in gasoline.

Thus, a chemical compound has an NMR "signature" by which it will identify itself. The NMR phenomenon was predicted in 1936 by Dutch physicist Gorter, and demonstrated in 1946 by Felix Bloch of Stanford and Edward Purcell of Harvard (bringing the latter two the 1952 Physics Nobel Prize). For the next two decades it was used, as might be expected from the description above, as a tool of chemical analysis. But in the early 70s, a new direction began to be explored.