Electromagnet

An electromagnet was a type of magnet related to electromagnetism.

The Third Doctor's sonic screwdriver could not undo bolts, unless the polarity of the power source was reversed, making it into an electromagnet. (TV: Frontier in Space)

To get out of the magnetic clamps which Kasgi used to bind him to the TARDIS console, the Seventh Doctor "reversed the polarity" of the electromagnet in the main control column. (COMIC: Who's That Girl!)

Before the Seventh Doctor could close a Dalek time corridor in Michael Faraday's house, two more fully active Daleks, one of whom was the Dalek Prime, came through it. After activating Faraday's electromagnet, which disrupted the two Daleks' functioning, the Doctor was able to escape through the dumbwaiter. (AUDIO: The Four Doctors)