to suck up ghosts
The Ecto-Magnetizer was part of the original Ghostbusters equipment which they carried with them in their Ghost Pack.
It was similar to a vacuum cleanera and was attached to a separate power source by way of a cable. Therefore, at least two people had to work simultaniously to use the Ecto-Magnetizer. One to switch on and man the power source while the other held up the magnetizer itself to suck in ghostsly adversaries.
Eddie Spenser Jr. used a different looking Ghost Vacuum cleaner in the episode Ghostnappers. Unlike the Ecto-Magnetizer, this version appeared to be sentient, not unlike Ghost Buggy and items located at Ghost Command.
The Ecto-Magnetizer was only used in the episode Father Knows Beast.