Futuristic gadgets of the 1930s

Nowadays voicemail services are taken for granted, and most people will remember the clunky tape-based answering machines of the 1980s and 90s. But back in the 1930s, answerphones were a marvel of science fiction, as the following quote from the May 1935 issue of Doc Savage magazine illustrates:

‘At exactly noon, the telephone buzzer whirred in Doc Savage's New York skyscraper headquarters. The buzzer whirred three times... Then an automatic answering device, an ingenious arrangement of dictaphone voice recorder and phonographic speaker – a creation of Doc Savage's scientific skill – was cut in automatically... "This is a mechanical robot speaking from Doc Savage's headquarters and advising you that Doc Savage is not present, but that any message you care to speak will be recorded on a dictaphone and will come to Doc Savage's attention later," spoke the mechanical contrivance. "You may proceed with whatever you wish to say, if anything".’

The caller leaves a message, after which ‘The mechanical device in Doc Savage's New York office ran on for some moments, and a stamp clock automatically recorded the exact time of the message on a paper roll; then the apparatus stopped and set itself for another call, should one come.’

Doc Savage’s answering machine isn’t the only high-tech wonder described in this story (called The Secret in the Sky, and written by Lester Dent under the pseudonym of Kenneth Robeson)... it also features a touch-sensitive intruder alarm that works by detecting changes in electrical capacitance, and a superfast air vehicle that is characterized by the sonic boom it creates (actually described in the story as a “crack” rather than a “boom”, by analogy with the crack of a bullet... real-world supersonic aircraft were still more than a decade in the future).