Ancient Roman balance.
© Science Museum / Science & Society Picture Library
Replica. 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 allows the previously unknown mas to be determined as it must obviously be the same as the sum of the known weights.