In the beginning there was barter system. People used to exchange their commodities among themselves to fulfill their needs. Whatever they needed, they looked for it with someone and gave their excess commodity to him in exchange. There were no bargains. Each got what they required. Since this method had so many shortcomings it did not continue for long.

For example three people had three different things, but their requirement was different from what they all had. So a tender system got established. The tender was some authority. Different people gave to some authority that collect things and issued tender. Using the tender he could get what he requires from the different authorities who have collected items. This was also as REQUIRED. But still there were no means of measurement by weight or volume used.

On those days no body felt the necessity of storing the commodities for future, and there were no such facilities also for it. If the commodities were available, they were available to everybody, if not available, they were not available to anybody. What an excellent atmosphere of living.

Time passed by hundreds of years. Population increased. When scarcity came for a commodity the quantity for a tender had to be limited. For a tender it was weighed and given. The initial weighing could be using hands taking things in hands with mental weighing only. Apple can be weighed with mango, rice can be weighed with wheat, but everything cannot be weighed, for example a cloth piece cannot be weighed with mango. The production effort of cloth is much more comparing to plucking and bringing a few mangoes, otherwise also one cannot measure the cloth with mango. So value for the commodity and the nature of the commodity came into existence.

So a system of token with some value and giving things by weighing them came into existence. Even now many a time we make mental weighing. Many people can tell the weight of Gold, silver etc by just lifting it in hand palm. The weight is almost 80% correct weight. For measuring cloth the measure of one arm (from elbow to the tip of the middle finger) and hand (from the tip of the smallest finger to the tip of the thumb stretched) were in use before. But this system also had the problem because everybody could not guess the correct weight and every hand and arm had different measurement.

During the excavation of Harappan Mohenjedaro Indus Valley, some polished stones of same size but different weight are found from the excavation. The weight of the marble stone is lesser than granite stone. These were used for weighing the commodities earlier before using metallic weights.

All Commodities Cannot be Measured or Weighed Equally
Measuring and weighing are two different things -
Measuring is used for those things which could be spread and be measured by the help of yard, foot, or meter. These things are those which have a length and breadth, for example land, cloth, paper, buildings etc

Weighing should be used for measuring those items which can be filled in something, though they might be different in weight. That is why two types of weighing methods are in use - a can or a container which is used to measure 1 liter water, although the same can cannot be used to measure 1 liter oil, because the oil's weight will be different and that one can oil may not be 1 Kilo as we want to measure. Water is lighter while the oil is heavier. Its another way is to weighing by weights. Weighing by weights is different from weighing by a can or a container etc. Weighing by weights means using the scale or balance and making the two panes equal in weight weighing against some weight like Kilogram, Pound etc.

Weighing. Measuring and Counting
These are all different methods of accounting. For example, coconuts are counted by number; betel nuts can be counted but for convenience, are weighed or counted. Rice is either measured or weighed. Plantains may be counted by number as well may be weighed too. In 1966 when I was in madras rice was measured, not weighed. Shopkeepers kept a can, which they claimed that if that can was filled with rice, it weighed about 1 Kilo, but there was no way to know that all shopkeepers had the same size cans. Even other dry things like spices were also sold by measuring in the same way. Liquids, like milk etc are also measured by cans or containers. Similarly petroleum items like petrol or diesel or oils. But cooking gas is weighed. Many items which were measured earlier, are now weighed. Examples are rice, pulses, commodities etc.

Volume and Weight
One liter of water weighs 1 kg. But one liter of oil weighs only 850-900 grams and not 1 kg, since oil is heavier in weight than water. Lesser density - compact is less of molecules, is the reason of lightness. One liter of kerosene weighs only about 750 or 800 grams, still lesser weight than edible oils. This is the reason why the same volume of silver and gold weigh very much different. Silver has lesser density - compact is less for molecules. Thus different things have different density.

Weighing
For weighing anything, we usually use a standard weight to compare the commodity's weight. For this we keep the weight on one side and the commodity on other side. When the commodity is equal to the weight, we say that the commodity weighs "so much". To weight many kinds of balances are used.

Weighing Scale or Weighing Instrument?
A weighing scale (usually called just "scale" in UK and Australian English, "weighing machine" in south Asian English or "scale" in US English) is a measuring instrument for determining the weight or mass of an object.

Spring Balance
The spring balance is handy. Here a needle is attached to a spring move over a pre marked scale . When spring is more tensioned or pulled by weight, the needle moves to show the weight of the commodity. Post offices were using this kind of spring balance to weigh light material like envelopes and small parcels, since it is easy for weighing and it is not good to weigh heavy materials. A spring scale measures weight by the distance a spring deflects under its load.

Arm Balance
An armed balance is used to weigh heave things. In this type of machines, there is one stick, and a iron hook or a rope is tied in its middle. When this hook lifts the stick, its both ends should be in the same line. Now two panes are tied to these both ends and they are used to put weight and commodity on either side. If any pane is lighter, it will rise up than the other. An arm balance compares the torque on the arm due to the sample weight to the torque on the arm due to a standard reference weight using a horizontal lever.

Electronic Machines
Recently electronic machines have been installed to weigh postal articles for charging.
In banks, coins are not counted but weighed. Every coin is of certain weight, so it is easy to weigh them and find out how many coins are they. They keep a chart for the weight of the coins and its value.

Weight and Mass
Just to understand weight is different from the mass. Weight is attraction of an object towards the Earth . A man can fly in Moon's atmosphere, because Moon's pulling force or compression force or gravity is very less compared to the Earth's pulling force. For the same reason a commodity on the Moon weighs only 1/6th of its weight on the Earth.

Balances are different from scales, in that a balance measures mass (or more specifically gravitational mass), where as a scale measures weight (or more specifically, either the tension or compression force of constraint provided by the scale).

How much we can weigh using balance/ scale
Weighing scales are used in many industrial and commercial applications, and products from feathers to loaded tractor-trailors are sold by weight.

Specialized medical scales and bathroom scales are used to measure the body weight of human beings.

History of Balance
The balance scale is such a simple device that its usage likely far predates the evidence. What has allowed archaeologists to link artifacts to weighing scales are the stones for determining absolute weight. The balance scale itself was probably used to determine relative weight long before absolute weight. Relative weight means simply which is weighing more in comparison, its an easy example is - one bottle of kerosene oil will weigh less than water in an identical bottle which explains relative weight.

The oldest evidence for the existence of weighing scales dates to c. 2400-1800 BCE in the Indus River Valley. Uniform, polished stone cubes discovered in early settlements were probably used as weight-setting stones in balance scales. Although the cubes bear no markings, their weights are multiples of a common denominator. The cubes are made of many different kinds of stones with varying densities. Clearly their weight, not their size or other characteristics, was a factor in sculpting these cubes.

In Egypt, scales can be traced to around 1878 BCE, but their usage probably extends much earlier. Carved stones bearing marks denoting weight and the Egyptian hieroglyphic symbol for gold have been discovered, which suggests that Egyptian merchants had been using an established system of weight measurement to catalogue gold shipments and/or gold mine yields.

Although no actual scales from this era have survived, many sets of weighing stones as well as murals depicting the use of balance scales suggest widespread usage.

Variations on the balance scale, including devices like the cheap and inaccurate bismar began to see common usage by c. 400 BCE by many small merchants and their customers. A plethora of scale varieties each boasting advantages and improvements over one another appear throughout recorded history, with such great inventors as Leonardo Da Vinci lending a personal hand in their development.

Even with all the advances in weighing scale design and development, all scales until the 17th century CE were variations on the Balance Scale. Although records dating to the 1600s refer to spring scales for measuring weight, the earliest design for such a device dates to 1770 and credits Richard Salter, an early scale-maker.

Spring scales came into common usage in 1840 when R. W. Winfield developed the candlestick scale for use in measuring letters and packages.

By the 1940s various electronic devices were being attached to these designs to make readings more accurate. These were not true digital scales as the actual measuring of weight still relied on the quality of springs and balances.

Load cells, small nodes that convert pressure to a digital signal, have their beginnings as early as the late 19th century, but it was not until the late 2oth century that they became accurate enough for widespread usage.

Did Spring Balance or Beam Balance Come First into Use?
The balance (beam balance and laboratory balance) was the first mass measuring instrument invented. In its traditional form, it consists of a pivoted horizontal lever of equal length arms, called the beam, with a weighing pan, also called scale suspended from each arm. The unknown mass is placed in one pan, and standard masses are added to the other pan until the beam is as close to equilibrium as possible.

Technically the Arm Balance Measures Mass or Weight?
Although a balance technically compares weights, not mass, but the weight of an object is proportional to its mass, and the standard weights used with balances are usually labeled in mass units. Balances are used for precision mass measurement, because unlike spring scales their accuracy is not affected by differences in the local gravity, which can vary by almost 0.5% at different locations on Earth. A change in the strength of the gravitational field caused by moving the balance will not change the measured mass, because the moments of force on either side of the balance beam are affected equally. In fact, a balance will measure the correct mass even on other planets or Moons, or any other location that experiences a constant gravity or acceleration.

Precise Measurements
Very precise measurements are achieved by ensuring that the balance's fulcrum is essentially friction-free (a knife edge is the traditional solution), by attaching a pointer to the beam which amplifies any deviation from a balance position; and finally by using the lever principle, which allows fractional masses to be applied by movement of a small mass along the measuring arm of the beam, as described above. For greatest accuracy, there needs to be an allowance for the buoyancy in air, whose effect depends on the densities of the masses involved.

Original Balance
The original form of a balance consisted of a beam with a fulcrum at its centre. To determine the mass of the object, a combination of reference masses was hung on one end of the beam while the object of unknown mass was hung on the other end.

Off Center Balance
To reduce the need for large reference masses, an off-centre beam can be used. A balance with an off-centre beam can be almost as accurate as a scale with a centre beam, but the off-centre beam requires special reference masses for calibration. Such measuring equipment is a conical rod with increasing weight. The item to be measured is suspended at smaller end and strong movable holder is moved towards the expanding conical side, where the rod is horizontal, the pre-marked weight is read as the weight of the item. This was more used to weigh very light items such as to weigh gold, silver and medicines.

Analytical Balance
An analytical balance is used to measure mass to a very high degree of precision and accuracy. The measuring pan(s) of a high precision (0.1 mg or better) analytical balance are inside a transparent enclosure with doors so that dust does not collect and so any air currents in the room do not affect the balance's operation. Analytical precision is achieved by maintaining a constant load on the balance beam, by subtracting mass on the same side of the beam to which the sample is added.

Spring Scale
In a typical spring scale, the spring stretches in proportion to how hard the Earth pulls down on the object. It is therefore affected by the local gravity. By Hooke's law, every spring has a proportionality constant that relates how hard it is pulled to how far it stretches. Some weighing scales such as a Jolly balance (named after Philipp von Jolly who invented the balance in about 1874) uses a spring with a known spring constant (see Hooke's law) and measures the displacement of the spring by any variety of mechanisms to produce an estimate of the gravitational force applied by the object, which can be simply hung from the spring or set on a pivot and bearing platform. Spring scales measure weight, or more precisely, the tension force of constraint acting on an object, opposing the force of gravity.

Rack and Pinion Mechanisms are often used to convert the linear spring motion to a dial reading.

The Hooke's law in summary states, the spring constant is depend on the quality of spring material. Hence if something overweight is weighed the spring may expand more and will not contract back, or the scale may not show 0 point.