Scale, Weighing, is a device used to measure weight or mass. Scales are used in business and science to weigh loads ranging from extremely light substances, such as drugs, to railroad cars that weigh many tons. There are three main types of weighing scales. In order of their historical development, they are (1) balances, (2) mechanical scales, and (3) electronic scales.
Balances,
the oldest type of scale, were first used in ancient Egypt about 2500 B.C. The two main kinds of balances are the equal-arm balance and the steelyard.
The equal-arm balance
consists of a horizontal bar with a pan suspended from each end. The bar is supported at the center by a piece of metal or other hard substance, called a pivot. The load being weighed is placed in one pan. Weights of known quantity are placed in the other pan until the two pans balance. A pointer indicates balanced pans. Scientists use balances to weigh very light substances. See Balance .
The steelyard
was developed by the ancient Romans about 2,000 years ago. It uses a small weight to balance a large load. The horizontal bar on a steelyard has arms of unequal length. A pan or hook on the shorter arm holds the load. A small weight called a poise is moved along the longer arm until it balances the load. Markings on this arm indicate the load’s weight.
Mechanical scales
were first developed in the 1700’s. Nearly all mechanical scales have levers that reduce the force of a large load to a smaller force that is read by a sensitive weight indicator. Mechanical scales may be grouped according to the type of weight indicator used. There are three types of mechanical indicators: (1) a beam, (2) a spring, and (3) a pendulum.
Beam scales
use poise to indicate the weight of the load. The poise is moved along the beam until the load is balanced. Markings on the beam give the weight. Many physicians use beam scales to weigh patients.
Spring scales
use one or more springs to balance the load, which rests on a pan or platform. The weight of the load stretches or compresses each spring. This action causes the weight indicator to move and automatically give the weight. A bathroom scale is a common type of spring scale. Some spring scales, such as hanging produce scales, operate without levers.
Pendulum dial scales
use a dial containing a weighted pendulum to indicate the weight. The pendulum swings out to one side to balance the platform containing the load. A needle hooked to the pendulum rotates on the dial to give the weight. This mechanism is used in many types of scales throughout industry. Food stores use these scales to weigh meat and produce.
Electronic scales
were first introduced commercially in the 1950’s. They use various electronic methods to measure and indicate weight. Most electronic scales contain a device called a strain-gauge load cell. This device measures the force created by the weight of the load placed on the scale. It then converts the force measurement into an electrical signal and transmits the signal to an electronic weight indicator. The weight indicator converts the signal into a weight reading.
Electronic scales can weigh loads faster and less expensively than can other kinds of scales, and they usually provide more accurate readings. Most of them also contain microprocessors (tiny computer chips) that enable the scales to perform activities in addition to weight measurement. Such activities may include determining the number of items in the load, calculating the price of the load, and transmitting information to printers and computers.
