Magna-Clamps were metal devices found by Torchwood 1. When attached to an object it cancels the objects mass enabling it to be easily lifted. They are capable of lifting two Imperial tons even if only held in one hand. Magna-Clamps were not available for public use. They are activated by pressing a button on the handle.
Torchwood 1 found the Magna-Clamps buried in a crashed spaceship (possibly the Vinvocci spaceship that also held the Immortality Gate) at the foot of Mt. Snowdon. They were then removed and taken back to Torchwood Tower (Canary Wharf). Yvonne Hartman showed the Magna-Clamps to the Tenth Doctor and Jackie Tyler during their tour, and later on during the Battle of Canary Wharf the Doctor and Rose Tyler used the Magna-Clamps to hold onto a wall when the Void was open. This stopped them from falling into the Void until Rose let go and almost fell in. (DW: Army of Ghosts / Doomsday)