I have seen a device used called a Spanish dip needle. It was hung from a thread and I believe It was Magnetised. I would equate it to a vertical compess needle suspended from a thread. It was used on a map. Did it work? I suspect that magnetic pull was the reason that it moved. Since I wasn't involved in the actual hunt, I have no idea!
A few years ago at our metal detector club meeting a guy brought in an antique Fisher metal detector. It had vacuum tubes and a wooden coil. The battery was this massive thing that was about 5 inches long. It was produced by Fisher in the 1930's. It resembled some of the machines I've seen from the 1960's. I don't remember if it had a meter. I wonder if the article may be referring to one of the first metal detectors?