A differential magnetometer was a device invented by Miles O'Brien in 2369. He did so aboard the runabout USS Rio Grande while searching for the USS Yangtzee Kiang and its complement, which included Kai Opaka and Benjamin Sisko. O'Brien needed a means to detect the magnetic resonance pattern of the runabout within a vast star system. He intended to launch probes from the runabout throughout the system equipped with differential magentometers to detect the pattern. Jadzia Dax was unfamiliar with the concept of such a device, and needed O'Brien to explain it to her after he finished inventing it. (DS9: "Battle Lines")