History of Scales
Before strain gage-based load cells became the method of choice for industrial weighing applications, mechanical lever scales were widely used. Around 7000BC equal armed balance scale used by Sumerians for weighing precious metals and gems. The balance consists of a beam oscillating around a fulcrum and carrying a pan at each end. Weighing involves comparing standard weights of known mass with an object of unknown mass There are records from around 3000 BC showing that balance scales were used by Egyptians for general trade purposes.
Around 55BC the Romans brought steelyard scales to Britain. A steelyard differs from the balance, which has equal arms, because it has a beam with unequal arms. The steelyard is suspended from a hook, whose position represents the fulcrum. The shorter arm is fitted with a hook to hold the object being weighed. A counterweight, called a counterpoise, can be shifted along the length of the longer arm. The counterweight is moved until the longer arm reaches horizontal equilibrium. We read the weight from the position of the counterpoise on the scale. Graduated scales were marked on two or three sides of the beam giving the steelyard two or three capacity ranges.
In 1830 Thaddeus Fairbanks build the first platform scale by digging a pit for the lever arms of the scale, placing the platform level with the ground. This modification ended the task of having to hoist the entire load.
In 1843, English physicist Sir Charles Wheatstone devised a bridge circuit that could measure electrical resistances. The Wheatstone bridge circuit is ideal for measuring the resistance changes that occur in strain gages. The first bonded resistance wire strain gage was developed in the 1940s, roughly 97 years after Sir Wheatstone invented his circuit, when modern electronics caught up that the new technology feasible.
In 1973, the first fully electronic precision balance was introduced by Mettler Instrumente AG.
Today, except for certain laboratories where precision mechanical balances are still used, strain gage load cells dominate the weighing industry. Even in environments where a mechanical scale might do as well, a load cell is often used because of its compatibility with digital, computer-based instrumentation.
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