I recently got a popular mechanics from may 1940 that has a 5 page article on metal detectors. The writer refers to them as "doodlebugs" and goes on to make some rather incredible claims as to what they can do. Those things were immense and consisted of a framework that you stood inside of with a pole on either side and a suitcase sized box on each end. One was horizontal , the other vertical. There are no technical drawings or explanations on how they worked but at the end of the article he states someone with one may have located the treasure of Maxmillian, the selfproclaimed emperor of mexico. The hoard was supposedly smuggled out of Mexico before his execution and hidden in California. The problem is the detector has pinpointed it beneath the parking lot of the Hollywood bowl. lol