In 1851, Michael Faraday secretly measured the muscle movements of Ouija board users who believed that the planchette was under ghostly control. According to Faraday, the users were unconsciously moving their muscles and but truly thought a spirit was pushing the planchette. A few decades later, physiologist William Carpenter dubbed this the "ideomotor effect." To this day, the ideomotor effect is a powerful phenomena and one that scammers have used to sell bogus "scientific" instruments. From the Wellcome Collection:
For example, in 2014, James McCormick, a British businessman, was convicted of selling fake bomb detectors to various international police forces. McCormick’s devices were marketed as using principles similar to dowsing, with extreme life-or-death stakes. The operator was supposed hold the device, called the ‘ADE 651’, like a wand, and allow its subtle movements to direct them towards dangerous substances.
The devices themselves have been determined to be entirely non-functional. But thanks in part to the ideomotor effect, they could easily feel functional, especially if the operator were confident in their legitimacy.
Since the late 1990s, non-functional detection devices with names such as ‘Sniffex’, ‘GT 200’ and ‘Alpha 6’ were sold by various scammers to governments throughout the world, including those of Iraq, Egypt, Syria, India, Thailand and Mexico. The World Peace Foundation of Tufts University, which tracks corruption related to international arms trading, estimates that fake bomb detectors generated more than $100 million in profit between 1999 and 2010.