Comment by mannykannot

I had a vague recollection that rectifiers for battery chargers were once made out of stacked layers of oxidized copper disks, and then, in the article danbruc linked to, I saw this:

Note that other semi-conducting materials, such as a rusty nail or an oxidised piece of metal, also generate harmonic frequencies and may there­fore cause an NLJD to generate a false positive.

https://www.cryptomuseum.com/df/tscm.htm#nljd

It turns out that the rectifiers in question were copper - cuprous oxide - lead sandwiches:

https://hackaday.com/2022/04/20/copper-rectifying-ac-a-centu...