Brass scales are mandatory items for all first year Hogwarts students. They are used to weigh potion ingredients, and are thus an important tool for Potions.
History
A set of brass scales were owned by a Gryffindor student while at Hogwarts School of Witchcraft and Wizardry. They left them behind after a Charms class with Professor Flitwick, and put up a notice on the Gryffindor pasteboard. Harry Potter, Ronald Weasley and Hermione Granger later found them and returned them, earning ten house points.
They are sold at Wiseacre's Wizarding Equipment for 3 galleons.