80 years ago, this was the latest in metal detectors. This unit, shown in the August 1934 issue of Radio Craft magazine, is similar in concept, but larger in size, to inexpensive metal detectors available today. It consisted of a one-tube oscillator along with a five-tube receiver. When the oscillator’s coil came in proximity to metallic objects, the frequency shifted, and the change in frequency could be determined from the receiver. The oscillator is mounted in one cabinet, with the receiver in the other.

The complete plans were in the magazine, which also reported that the unit could be purchased commercially. It was reported as being sufficiently sensitive to detect a piece of metal 16 inches square at a depth of 15 feet.