Today, if you’re asked to think of a tractor, most of us probably imagine farm equipment. But in the late 18th century, a physician named Elisha Perkins made and sold a different kind of tractor – a device consisting of small metal rods that could cure what ails you simply through touch. And for several years, people were mad for the Perkins Patent Metallic Instruments, or Perkins Tractors as they became popularly known -- even though it all turned out to be what we now know as the placebo effect.