Eighty-five years ago this month, the August 1939 issue of the British publication Practical Mechanics showed how to build this crystal set, which it dubbed a “stand-by arrangement for emergencies.”
The magazine noted that popularity of crystal receivers was on the upward swing. One possibility was that this was due to the splendid quality of reproduction. But it noted that it probably had something to do with the fact that they were not dependent on batteries or the mains supplies. They were ideal for “A.R.P. shelters.” The article didn’t say what “A.R.P.” stood for, but it apparently meant “air raid protection.”
With more and more high powered stations on the air, it was important that a crystal set be selective, and this one was designed with that parameter in mind.
Of course, the following month, Britain was at war, so building one of these for the ARP shelter was probably a prudent idea.