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A rhondium sensor was an ad hoc device built at least twice by the Doctor. As they were built from whatever materials were available at the time, the versions created by the Third and Tenth Doctors varied greatly in design. They were used to detect delta (or rhondium) particles. These particles indicated the presence of some kinds of space-time fissures.
In the case of the Third Doctor, the device alerted the Doctor to the use of the Sontaran osmic projector, itself a device that created a quantum tunnel that connected two disparate points in time. (DW: The Time Warrior, MA: Lords of the Storm) For the Tenth, the device detected the presence of a growing wormhole created by the Swarm which linked central London to the planet San Helios. (DW: Planet of the Dead)