Harry -- this is a Pocket Sneakoscope. If there's someone untrustworthy around, it's supposed to light up and spin. Bill says it's rubbish sold for wizard tourists and isn't reliable, because it kept lighting up at dinner last night. But he didn't realize Fred and George had put beetles in his soup.
-- Letter from Ron Weasley on Harry's 13th Birthday
Edgar Strougler was the inventor of the Sneakoscope, a type of Dark Detector that warns when someone untrustworthy is nearby (FW).
Other canon notes and references
Ron believed that Sneakoscopes were not very useful because his older brother Bill told him they were "rubbish." That was reinforced when the pocket sneakoscope that Ron gave Harry for his birthday seemed to be whirling like a top and making a loud noise constantly as if it was broken (PA1, PA5, PA11). What Ron and Harry did not know was that the device was warning them that Scabbers the rat was actually Peter Pettigrew in Animagus form, which is why the Sneakoscope was sounding off even as Ron was attaching to Errol the Owl at the Burrow before sending it to Harry (PA1).