Atomic clock
From Tardis Wiki, the free Doctor Who reference
A beryllium atomic clock inside the Institute for Technological Advancement and Research. (TV: Doctor Who)
An atomic clock was a clock which used atomic vibrations to measure time.
Professor Wagg designed an atomic clock for the countdown to the year 2000 and the new millennium. However, the Eighth Doctor had to steal the clock's beryllium chip, rendering it inoperable. (TV: Doctor Who)
