From very early times, civilizations used weighing machines in business transactions. Over the years, the sensitivity and accuracy of these machines were improved. Modern instruments approach the limits of weighing accuracy, for no instrument can be completely accurate; nearly all of them measure the force of the Earth’s gravity, which varies from place to place (see gravitation).
The simplest form of weighing machine is the balance, so called because it balances two loads from opposite ends of a horizontal bar. The Egyptians used instruments called equal-arm balances 4,000 to 5,000 years ago. They balanced a beam from a vertical support and suspended two pans from either end of the beam. One pan held a known weight, the other the object to be weighed.
The delicate precision balances used today in scientific laboratories are based on the same principles. Probably the first true precision balance was built in the early 1770s by British clockmaker John Harrison. His balance had an adjustable center of gravity and was the first to balance the beam on a triangular knife-edge—a wedged fulcrum that reduced friction between the beam and its support. Modern precision balances, known as macrobalances in scientific laboratories, are sensitive enough to measure loads up to 7.05 ounces (200 grams) to the nearest 3.5 millionths of an ounce (0.1 milligram). There are even more sensitive balances, called microbalances, that can weigh a maximum load of 0.00353 ounce (0.1 gram) to the nearest 3.53 hundred-millionths of an ounce (1 millionth of a gram). Most precision balances are kept in glassed-in cabinets so that they are unaffected by external temperature changes and drafts.
Equal-arm and precision balances require that the “known,” or standardized, weights be at least equal to the heaviest load to be weighed. For weighing heavier loads, there are scales based on the steelyard, first invented by the Romans. The steelyard consists of a suspended beam with two unequal arms or levers. It uses a small weight to balance a heavy load. The load is hung from the short arm, near the fulcrum, and a weight, or counterpoise, is moved along the longer arm until a balance is reached. Markings on the long arm indicate the weight.
Spring scales were invented around 1700. The load may be hung from a hook or placed in a pan below a spring or laid on a platform supported by a spring. The stretching or compressing of the spring moves a pointer to register the weight on a dial or may be converted into a digital readout. Most modern bathroom scales are spring scales.
The mechanical scale was invented by Thaddeus Fairbanks in 1830 (see Fairbanks, Thaddeus). It uses a system of two or more connected levers to support the platform. When a load is placed on the platform, the levers divide the load so that a small weight can be used to balance a load thousands of times heavier.
Electronic scales were introduced during the 20th century to measure loads faster than traditional scales or balances. Electronic measurements can also be easily displayed, stored, and transmitted. Many electronic scales can transmit weight readings directly to adding machines, calculators, and computers.
Electronic scales use one of three principal mechanisms: the strain-gauge load cell, the pressductor, and the force balance. In electronic scales that use a load cell, the applied weight compresses a column. Bonded to the surface of the column is a strain gauge, a fine wire that undergoes a change in electrical resistance when it is either stretched or compressed. A measurement of this change in resistance yields a measure of the applied weight.
The pressductor is a laminated magnetic block with four holes drilled through it. A wire carrying alternating current is threaded through two of these holes, and a pickup wire connected to a voltage meter is threaded through the other two. In the absence of a load, no voltage is detected by the pickup wire. However, an applied load induces a voltage that is proportional to the weight.
Both the load cell and the pressductor are used to measure large weights—from 2.2 pounds (1 kilogram) to many tons. The force balance is used to weigh light objects rapidly. A weight placed on a force balance deflects a moving coil—a conductor carrying a current in a steady magnetic field. This deflection is corrected by a balancing current proportional to the deflecting weight.
Industries use electronic scales to weigh trucks, railroad cars, and other heavy equipment. Supermarkets use these scales to weigh items such as meat and produce.
Many retail stores have scales that register the price of the article weighed: an indicator is set at the price per unit and moves exactly as does the weight indicator. Other scales count small articles, such as nails or bolts, by assuming a constant weight for a given number of the articles.