Scale, weighing, instrument for measuring weight. Balance scales, which date to about 2500 B.C. and are still in use, measure an unknown quantity by balancing it against established weights. Mechanical scales, which date from the 18th century, use beams, springs, or pendulums to convert the measurement of weight into a precise reading on a graduated scale. For example, spring scales, including the ordinary bathroom scale, measure weight according to the tension created when the weight stretches or compresses a spring. Electronic scales, which have been in commercial use since the 1950s, convert the force exerted by the weight into an electronic signal.