The Difference Engine: More pennies, please

The Economist:

EVER since 1982, when the American penny (one-cent piece) ceased being minted from brass and started being made instead from zinc with a thin coating of copper, eighth-graders at some of the country’s more inspired schools have been given a nifty little experiment in electrochemistry to do for homework. Your correspondent’s 13-year-old came home recently with goggles and instructions to find the amounts of copper and zinc in a modern penny. While in class, each kid had first carefully weighed three such coins on a scientific balance. After that, the rest was up to them (and their dads).
The experiment is designed to test the pupils’ knowledge of the galvanic series, and the science that explains how corrosion occurs. The series lists metals according to their resistance to electrochemical reaction–with the “noblest” (eg, palladium, platinum and gold) at the top of the rankings, and the most reactive or “basest” (eg, beryllium, zinc and magnesium) at the bottom. Copper comes 11 places above zinc in the table. Thus, when the two metals share an electrolyte, the zinc (being much the more reactive) will dissolve into the solution long before the copper. In a similar way, zinc anodes attached to the hulls of ships protect the vessels’ steel plates from rusting away by being sacrificed instead.