Time sensor
From Tardis Wiki, the free Doctor Who reference
A time sensor was a device which, according to Jo Grant, "detect[ed] disturbances in the time field". It was most useful as a "TARDIS sniffer-outer", In particular, it was used by the Third Doctor to detect the presence of the Master's TARDIS. It was calibrated to provide distances to a disturbance in Venusian feet and miles. It was unknown whether this meant the time sensor was of Venusian origin, or if the Doctor invented the device and intentionally used Venusian units due to his fondness for that planet. (TV: The Time Monster)
