A beam balance is the simplest instrument to measure mass (weight). In a beam balance, the mass of an object is measured by comparing it with standard masses called standard weights. A simple beam balance consists of a straight beam of metal (generally iron), supported at its centre with the help of an iron loop. A pointer is fixed at the centre of the iron loop. Two identical pans are are suspended at equal distances from the centre, at the two ends of the beam. Working: The object whose mass has to be measured is placed on one of the pans (generally the right pan). Standard weights are placed on the other pan until the metallic beam becomes horizontal and the pointer becomes vertical. The sum total of all weights used gives the mass of the object.