This image was acquired from
wikimedia. It was marked as Public Domain or CC0 and is free to use. To verify, go to the source and check the information there.
Keywords from Image Description:
Airplane Detection Appt. Threestage amplifying set device for detecting airplanes Acoustic Laboratory Frankford Arsenal closed Operation top Connect condenser detector to input Connect telephone to output Decrease grid leaks to minimum resistance Set filament controls at OFF position Turn on Battery by pulling out snap switch