Hannah Fry in The New Yorker:
Dr. John Haygarth knew that there was something suspicious about Perkins’s Metallic Tractors. He’d heard all the theories about the newly patented medical device—about the way flesh reacted to metal, about noxious electrical fluids being expelled from the body. He’d heard that people plagued by rheumatism, pleurisy, and toothache swore the instrument offered them miraculous relief. Even George Washington was said to own a set. But Haygarth, a physician who had pioneered a method of preventing smallpox, sensed a sham. He set out to find the evidence. The year was 1799, and the Perkins tractors were already an international phenomenon. The device consisted of a pair of metallic rods—rounded on one end and tapering, at the other, to a point. Its inventor, Elisha Perkins, insisted that gently stroking each tractor over the affected area in alternation would draw off the electricity and provide relief. Thousands of sets were sold, for twenty-five dollars each. People were even said to have auctioned off their horses just to get hold of a pair. And, in an era when your alternatives might be bloodletting, leeches, and purging, you could see the appeal.
Haygarth had a pair of dummy tractors created, carved carefully from wood and painted to resemble the originals. They were to be used on five unsuspecting patients at Bath General Hospital, in England, each suffering from chronic rheumatism. Using the lightest of touches, the fakes were drawn over the affected areas, with remarkable results. Four of the five patients declared that their pain was relieved. One reported a tingling sensation that lasted for two hours. Another regained the ability to walk.