In the photo have used an ordinary top pan balance but when I do this as a class practical we will use small battery powered balances (Ebay). Using these cheap balances I can afford to supply a balance to each pair of students.The magnets used are two 2cm radius Neodymium magnets (Ebay) held in a piece of steel bent into a U shape. Without these strong magnets the force will not be big enough to measure on the balance...