Calculator
From Tardis Wiki, the free Doctor Who reference
A calculator could be used to perform mathematical calculations.
The Second Doctor elected to use an abacus to calculate the stability of bubbles within a singularity rather than a calculator as the latter's accuracy could be affected by the singularity. (AUDIO: The Dead Star [+]Loading...["The Dead Star (audio story)"])
At Brendon School, Vislor Turlough had a digital watch which could also function as a calculator. (AUDIO: Power Game [+]Loading...["Power Game (audio story)"])
