<p>Three-stage amplifying set device for detecting airplanes, Acoustic Laboratory, Frankford Arsenal, open.
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Operation [top]: 1. Connect condenser detector to input.; 2. Connect telephone to output.; 3. Decrease grid leaks to minimum resistance.; 4. Set filament controls at OFF position.; 5. Turn on A Battery by pulling out snap switch.; 6. Increase filament current of last tube to almost full value.; 7. Set filament control knobs of other tubes to index marks. (Never turn past these marks.); 8. Increase grid leak resistance until maximum amplification is obtained.; 9. No further attention is required.
National Institute of Standards and Technology
National Institute of Standards and Technology