The Doomsday Magnet is a magnet-like cannon that is a weapon of Apokoliptian origin and it is featured in the second part of the episode "Little Girl Lost" of Superman:The Animated Series.
Darkseid ordered the Female Furies and the Intergang to construct and place the magnet in Metropolis. It would be used to change the path of Fleischer's Comet to Earth; after the disaster Darkseid could invade Earth.
Supergirl managed to destroy the magnet, and later the comet.
After Darkseid's first invasion of Earth, Highfather placed the planet under his protection. Uninterested in reigniting a bloody war with his rival, Darkseid came up with a plan to take over Earth through complete manipulation. He sent Granny Goodness to Metropolis where she used her influence to recruit teenagers to form a mock Intergang. Armed with Apokoliptian weapon gauntlets, a sudden crime wave swarmed the city. The common element was theft of electronics. Based in the Hobs Bay Rec Center, Granny secretly used the electronics to build a device she named the Doomsday Magnet. Since the weapon was made with Earth technology, Darkseid was not directly responsible for anything that happened. When completed, it would attract the passing Fleischer's Comet, and force it to impact onto Earth. In the face of utter devastation, Darkseid would offer to rebuild the planet and slowly take over.
Superman got his first lead when a theft happened at the Metropolis Expo Center. Kara Kent became interested in fighting crime and despite her 'cousin' Clark Kent and his wishes, she secretly investigated the crime wave with Jimmy Olsen. They tracked a game token to the Rec Center where Granny's Intergang attacked. However, Kent revealed herself as Supergirl. The battle took a turn for the worse when the Female Furies arrived and kidnapped Superman. Supergirl boomed to Apokolips and rescued him. Meanwhile, Olsen and one of the teens, Amy, fully activated the Magnet. Impulsive and inexperienced, Supergirl destroyed the Doomsday Magnet before Superman could shut it off. They both took off to stop it but Supergirl was injured during the struggle.