When a dilute solution of copper acetate is placed on a bright silver
plate and a strip of zinc is touched to the silver beneath the copper, a
series of rings of copper are formed by electrolysis around the zinc.
These are Nobili’s rings.

If for the copper acetate a solution of lead oxide in potassium hydrate
solution is substituted, and if the polished plate which may be German
silver is connected to the positive electrode of a battery, and a
platinum wire connected to the negative pole is immersed in the liquid,
it determines the formation of beautiful iridescent rings of lead
binoxide. The platinum wire is sometimes sealed in glass so that only
its point projects.

The colors are due to interference of light, the layers of lead oxide
being extremely thin.

The lead binoxide is formed by secondary reaction. Metallic lead is
first deposited on the negative pole. The oxygen which goes to the
positive pole formed by the polished plate produces lead binoxide which
is deposited there in rings. The reaction is comparable to that of a
storage battery.

Synonyms–Metallochromes–Electric Rings.