Balance scales work by placing items of equal mass onto two hanging pans, according to Math Is Fun. In order to determine how much a single object weighs, a standard weight is placed in one pan and the object of unknown mass in the other.
Balance scales were the predecessors to modern weight scales, according to Wikipedia. There are several variations to traditional balance scales, including spring scales, which measure weight by balancing force exerted by gravity against the force exerted by a given object. Both traditional balance scales and spring scales are calibrated to read units of force. The traditional balance scales accomplish this by weighing items against a standard weight, while spring scales weigh items against gravity.
Because balance scales use a balance beam to hold eac;h pan and compare masses, they read correctly regardless of gravitational field strength, according to Wikipedia. This versatility in function has led to the continued use of balance scales in spite of the fact that more modern adaptations are available. Pendulum balance scales are another type of balance scales that uses pendulums rather than pans to operate. These scales also give accurate readings regardless of gravitational field strength, and they are still produced commercially.