Seems like a lot of work for free clean clothes, though. About 20 years ago, someone came up with a better way to make sure people were sticking real quarters in vending machines. Drop a coin into one of these babies and it rolls down a chute and over a rocker arm designed to open and kick the coin out if it is either below a preset minimum weight or over a preset maximum. If it passes this test, it flashes by a set of LED sensors which do a quick size verification. If the coins still looks good to the machine at this point, it is dropped through midair, passing between the poles of a magnet.
Eddy currents created by the magnet slow the coin down.
The change of speed is a function of size, weight and metal content. If slows down too much or too little, it lands in one of the slots that leads to the reject chute. Otherwise, the coin is considered valid, and you get your soda. Some mechanical coin validators use other tests to check for authenticity, including a quick zap of electric current, which serves as another test for metal content. The latest and greatest thing in coin authentication technology is the electronic coin validator.