Pennies contain enough of copper for the Cu+2 ions to carry a charge that the gastropods do not like. It gives them a shock that repels them. You can line the walls of a raised bed garden like this:
A ball made like the one above can make the garden bed pretty, but remember, a repellent can drive away a food source for something else, like a frog. If your slugs are a problem, perhaps this can be utilised to reduce numbers and create a balance.
I remember hearing Geoff Lawton, of the Permaculture Institute of Australia, once saying, "You don't have a grasshopper problem, you have a turkey deficiency" !!!