The idea of perpetual motion, which has been around for centuries, is to make a device that will produce more work output than the energy input—in short, to get something for nothing. Robert Fludd in 1618 was one of the first to discover it is impossible to get something for nothing. He designed a pump to drive some of the output from a water wheel to recirculate water upstream, which would then run over the wheel again. The remaining portion of the output could then be used to operate a flour mill. The only problem with this device is that it took more energy to pump the water than the entire energy output of the water wheel. Friction will always cause such a water wheel to "grind" to a halt even in the absence of doing useful work.

Would-be inventors frequently employ magnetic or electrostatic interactions...