= Maser discovery experiment
Ammonia gas entered a vacuum chamber through fine channels; the selected beam entered a cavity with input and output waveguides.<The Maser—New Type of Microwave Amplifier, Frequency Standard, and Spectrometer by James Power Gordon and others>[pp. 1264–1265, Fig. 1, source and vacuum]{ref} With external <microwaves> swept across a molecular <resonance>, the 1954 experiment read an increase in transmitted <power>; with sufficient beam emission, it sustained <oscillation>.<Molecular Microwave Oscillator and New Hyperfine Structure in the Microwave Spectrum of NH3 by James Power Gordon and others>[p. 283, operation as spectrometer, amplifier and oscillator]{ref}
The 1955 readout mixed two maser signals with a local <oscillator> in a crystal detector, amplified them at an intermediate <frequency>, and combined them in a diode detector. An <oscilloscope> showed their <beat> <frequency>. Changing cavity tuning, source pressure or focuser field tested sensitivity; excessive detuning stopped <oscillation>.<The Maser—New Type of Microwave Amplifier, Frequency Standard, and Spectrometer by James Power Gordon and others>[pp. 1266–1267, Experimental Results, heterodyne and controls]{ref} The <beat> tests relative <frequency> stability; it does not by itself establish an absolute <frequency> or an intrinsic linewidth independent of detector noise. Detuning also changes the field–molecule interaction, so the threshold model's constant $G$ cannot describe arbitrary tuning scans.
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