A general utility equal-arm balance
Make a base about 22 cm square from wood about 2 cm thick. Next make two uprights from wood15 cm long by 6 cm wide by 2 cm thick and join these near the centre of the base about 2.5 cm apart. They may be attached either with screws or by slotting the base and screwing the uprights to it. The top of every upright should be cut deeply sufficient with a thin saw to permit a razor blade to extend about 4 mm above the wood. The razor blades are wedged tightly in the slots.
The beam of the balance is made from a metre stick or same length of wood with a thin finishing nail through its exact centre of balance. The nail rides on the razor blades.