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

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