An anti-robot bomb, fixed to a surface, and primed to detonate. (TV: The Robots of Death)
Anti-robot bombs were manufactured by Lish Toos and Kiy Uvanov from z-9 electron packs. The Fourth Doctor's instructions were to pass "a positive charge through the metal plate" so that "you'd be able to magnetise them and have anti-robot bombs". Uvanov fixed one against a door, which detonated, destroying V5. (TV: The Robots of Death)
