Beryllium chip
From Tardis Wiki, the free Doctor Who reference
A beryllium chip was a component of some types of atomic clocks.
The clock designed by Professor Wagg used a beryllium chip. The Eighth Doctor stole the chip to repair a malfunction in the TARDIS which had been caused by the Master. (DW: Doctor Who) Daniel Joyce gave Wagg a new beryllium chip. (EDA: Unnatural History)
The timecode on the holo-prints of Lexy's body was made with a beryllium chip. (BFG: Square One)
