I do have a couple of those circuits, they use instrumentation amplifiers on the front end, but do not have a scanner to post it here. Give me a few days perhaps I can ask someone to scan.
It supposedly can detect an automobile passing a block away; and it is not more than a zillion turns of hair-fine enameled wire on an iron bar. The instrumentation amplifier is fed by the coil ends and the output is a millivoltmeter with its pointer centered at midscale as zero.
Miguel