A want conduit was a small silver sphere that functioned like a control circuit. It was telepathic in nature, being controlled by the thoughts of the person using it.
When police were trying to uncover Mamps, Cosmo and Leeb Slitheen from their stone prisons, they used a want conduit to control the explosive to free them. The Tenth Doctor, not knowing what they were doing, stopped them and turned off the want conduit, making so it could only be turned on by thinking about sunflowers and ball bearings. (PROSE: The Slitheen Excursion)