A magnetic field was a form of technology that attracted metals.

The bodies of Magnedons were held together by an internal magnetic field and the First Doctor believed that they could attract other metallic creatures to prey on them. (TV: "The Dead Planet")

A Dalek duplicate of Ralph Hellcombe emitted a magnetic field when the Eleventh Doctor used ball bearings to learn from where the field came. (COMIC: The Dalek Project)

According to the Thirteenth Doctor, particle accelerators stored positrons very carefully inside electric and magnetic fields. (TV: The Tsuranga Conundrum)

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