Differential magnetometer
A differential magnetometer was a device invented by Miles O'Brien in 2369. 

This was accomplished aboard the runabout  as a means of detecting the magnetic resonance pattern of the missing  within a vast star system. O'Brien intended to launch probes from the runabout throughout the system equipped with differential magnetometers to detect the pattern that otherwise would have been impossible for the probes to detect. 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. ()
Sensor technology