Help identify the principle of operation of my grandfather's apparatus, with a lead disk.

[zhasmin]

As a child, my grandfather showed me his metal detector, which only found gold. (don't take this as a joke).
I remember that the apparatus consisted of a heavy, several kilograms, lead disc, 30 centimeters in diameter.
He repeatedly demonstrated the work of the apparatus on gold jewelry, the apparatus squealed when gold was brought to the disc by 30-50 cm. The device did not react to iron and copper coins. The device has not survived.
What could be the principle of its action?
I have not found a single device where the lead disk is based.

[Carl-NC]

Never heard of such a thing, and I seriously doubt it was real. Did your grandfather find a lot of gold?

[GeoMax]

Hi
it maybe sounds like the story about the legendary NAZI gold detector device. But whether this device ever exists, was never proven. Came your grandfather from Germany? Or was he during and/or after the WWII based in Germany?

[Qiaozhi]

This is how many so-called "gold only" detector stories start.

[waltr]

If he was like my grandfather he rigged a way to make the device beep manually just to 'pull your leg".

[kt315]

yup. your grandfather was big prankster.