= Purcell nuclear resonance absorption experiment
{c}
<Edward Mills Purcell>, <Henry Cutler Torrey>, and <Robert Vivian Pound> placed <paraffin wax> in a capacitively loaded coaxial cavity near 30 MHz between magnet poles. Coupling loops supplied and extracted power; oscillating and steady fields were perpendicular. Output balanced against a generator reference fed amplification, detection and a microammeter. A slow field sweep revealed absorption above fluctuations at room <temperature>. Weak drive avoided population equalization. Calibration was rough; field inhomogeneity could contribute to linewidth.<Resonance Absorption by Nuclear Magnetic Moments in a Solid by Edward Mills Purcell and others>[pp. 37–38, apparatus and results; weak drive, balanced cavity output, microammeter, field sweep, calibration and linewidth limitations]{ref}
\Image[/nuclear-magnetic-resonance-induction-versus-absorption.png]
{title=Two electrical routes to bulk <nuclear magnetic resonance>}
{description=Original functional diagrams of the distinct instruments described above. Arrows show connections and field directions, not scale or winding geometry.}
These are driven experiments with continuous radiofrequency excitation and changing static field, not demonstrations of modern pulsed <Fourier transform> <nuclear magnetic resonance>. Bloch's theoretical paper explicitly distinguishes proposed free precession after a pulse from the forced precession actually studied.<Nuclear Induction by Felix Bloch>[pp. 460–461, introduction: proposed pulse/free-precession experiment versus performed forced-precession experiment]{ref}
An experimental failure need not mean absent <nuclear magnetic moments>. In the model below, excessive drive reduces the population imbalance through <nuclear magnetic resonance saturation>; waiting too briefly can leave insufficient polarization. Separately, ordinary electrical losses can cause heating without identifying a nuclear resonance. The absorption experiment measured a balanced electrical <signal>, rather than using heating as its readout. Useful diagnostic checks include moving off resonance, changing the frequency, checking field uniformity and reducing drive. These are general checks; the historical operations actually documented are specified above.
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