it’s a skin galvanometer. It measures sweating directly based on the fact that sweat is electrically conductive, then interprets more sweat as more stress. this is a fallacy as the fact that stressed people tend to sweat does NOT imply that sweaty people tend to be stressed. this works to the advantage of scientologists because genuinely stressed people will measure high, but so will a lot of unstressed people or people who are only stressed by the fact that they suddenly find themselves in an experiment. False positives and true positives are much more common than true or false negatives, and also much more profitable for scientologists. When you successfully beat the test, the person administering it insisted you go again because it’s kinda rigged and they assumed that a second reading would come back with the needle pointing strongly toward “give us a bunch of money”.
this is the central fallacy behind lie detectors as well, as they measure skin galvanic response, heart rate, and other things that are correlated with stress but can have myriad other causes, then they assume that people are stressed when they lie, then they take a flying leap to the conclusion that anyone displaying symptoms of stress must be lying.