86 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1953 



an alternating magnetic flux across the xz plane. Now I describe an 

 actual experiment, the first of its type. 



The sample is water (or something else) in a spherical container. 

 Around the container are wrapped two coils at right angles to one 

 another. The coil of which the axis is parallel to the x-axis produces the 

 alternating field. The coil of which the axis is parallel to the y-axis is 

 connected with a rectifier and a detector. At resonance there is an 

 alternating magnetic flux through the latter coil, and by the operation 

 of the rectifier this is converted into a signal on the scope. The signal 

 locates the resonance-frequency as accurately as does the peak in the 

 absorption-method. This is the phenomenon called ''nuclear induction." 



I terminate this section by mentioning a paradox resulting from 

 precession. Everyone has seen a compass-needle turning to point to 

 the north : it is natural to infer that when a magnetic field is applied to 

 a piece of matter, the elementary magnetic particles of which the 

 nuclei (and also the electrons) are examples will automatically turn to 

 point along the field. Yet the analogy fails and the inference is false: 

 the nuclei do not turn to point along the field, but each of them maintains 

 a constant angle with the field while it precesses. It seems to follow that 

 matter cannot be magnetized by a magnetic field, but again the inference 

 is false. Animistically speaking, the field makes the nuclei want to turn 

 into its direction, but they cannot fulfill their desire without assistance 

 from something other than the field. This something-other is not absent, 

 and in the section on "relaxation" we shall meet with it. 



THE MOLECULAR-BEAM EXPERIMENT 



There are three methods for detecting and locating nuclear magnetic 

 resonance, and we have now considered two of them. In one of these, 

 the resonating nucleus makes itself manifest by absorbing energy; in 

 the other, that of nuclear induction, by radiating energy; in the one 

 which is to come, by simply failing to turn up at the scene of the measure- 

 ment. This singular attribute is that of the molecular-beam experiment, 

 which (I repeat) was done before the others and so receives the credit 

 of revealing nuclear magnetic resonance. Molecular-beam experiments 

 are so remarkable that it is hard to speak of them without yielding to 

 temptation to say more than is essential to the purpose, but here the 

 temptation must be withstood. 



Conceive a narrow stream of hydrogen-containing molecules coming 

 along the (horizontal) axis of y, and cutting across a big magnetic field 

 parallel to the (vertical) axis of z. This big field differs from that of 



