The Tipping Device was a part of a weapon.
It was a single atom of the isotope Gold-197. The Device was stolen by the Crei Sergeant Kandula because he had grown tired of war. He slipped it into the Third Doctor's pocket. After Kandula died helping the Doctor stop the Crei, the Time Lord took his body and the Tipping Device to the safety of the void at the end of time. (PROSE: Midnight in the Café of the Black Madonna)