The copper on the pennies reacts with the mucus that the snails & slugs
secrete. The reaction creates a slight electrical current, strong enough
to discourage them from crossing the copper.

Forgive me...but this sounds like folklore. I checked around google, and it
seems to be common knowledge! How strange! Don't you need two types of metal
to create a current? Weird. You learn something every day. Anyway, I am off
to collect slugs. I plan to power my pump with them and some pennies.

Copper works to repel snails (1) but I've always doubted the mechanism is electric current.
I suspect copper sulfate and other copper compounds on the penny, copper is highly
toxic to invertebrates.

(1) Organic Gardening magazine once did a comparison of all the snail repellants
on the market and copper foil was the clear winner, might have been the only
one 100% effective, I can't recall...