Spatial geometer

The spatial geometer was a component of the Doctor's TARDIS that was crucial for its movement in space. It was described as "four perspex cylinders of varying size and technical complexity". Without this piece of technology the TARDIS could still move about in time, but was bound to the same location. With portions of the geometer in place, though, the TARDIS could move a very limited distance, but no more than a few thousand miles.

History
In 2009 the Doctor was performing some maintenance on the TARDIS and had disassembled a number of its major components across the parlour and dining room of his countryside home, Nest Cottage. While part were strewn about, his housekeeper Fenella Wibbsey inadvertently scooped up the spatial geometer and sold it at the local Church "Bring and Buy" sale, in exchange for a bag of various crafted items. (AG: The Relics of Time)

The Doctor led Mrs Wibbsey on temporal jaunts to Sussex in the Roman era and 1890s, Eastern Europe in the Middle Ages, and New York City in 1976 to recover various portions of the geometer. (AG: Demon Quest)