Persian equal-arm balance, c 1800-1810.
© Science Museum / Science & Society Picture Library
These jeweller's scales have an ornamental polished steel beam and gallows suspension, with two bras pans which are supported by woven silvered cords. Such scales would have been used for measuring gem stones and precious metals. Equal-arm balances of this type are the simplest and earliest method of weighing. It consists of a bar which has a pan suspended at either end. To use the balance, objects of unknown mas are put into one pan, and weights of known mas are put into the other pan until the two are balanced and the bar is level. This demonstrates the weight of the previously unknown mas as it must obviously be the same as the sum of the known weights.