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Eavesdropping Machine Aids Police
January, 1936 — Popular Science
Secret conversations, picked up by concealed microphones and recorded on cellulose-acetate film by a new process, will aid police in securing evidence against criminals. The new sound-recording machine, invented by a Chicago scientist, employs a diamond needle to impress a record of speech or other sounds on a specially prepared sixteen-millimeter film, which is ready for reproduction without processing. To "play back" the sound record, it is necessary only to rerun the film through the machine. This feature is an improvement over former methods.
A new sound-recording machine which makes its record directly on film without processing. Connected to a hidden microphone, as above, it gathers evidence for police. (Can you see the 'bug' in the plant?)