The potion of magnetization is brewed by adding a compass to an awkward potion in a brewing stand.
The effect is to accelerate nearby item entities so they are on a collision course with the player. If the player is stationary, they are accelerated straight towards him, if he is walking/sprinting/flying/swimming/etc, they are accelerated towards a location some (speed dependent) distance in front of him.
Item entities whose PickUpDelay is greater than zero are not effected. Also, effected item entities' ages are decreased slightly each time they are accelerated.
The base duration is five minutes, and the base range of effect is double the distance which the player can normally pick up items.
Adding redstone extends the duration as expected, and glowdown extends the distance from the player within which item entities are affected.
Adding a fermented spider's eye to a potion of magnetization produces a Potion of Tantalus, which accelerates nearby item entities in precisely the opposite direction.