The need for measurement and measuring devices dates back to antiquity. When the humans became civilised, started cultivating and living in communities they realised that one cannot do everything and they need to be interdependent. This paved the way for trade and then probably a need of a measure was felt.
Various ways of measurements were adopted. The system of measurement has evolved a lot since then.
The recorded history shows ample evidence that the different parts of the human body were used as a point of reference while making measurements. Some of these were, digit: the width of a single finger; foot: the length of a foot; cubit: length of an arm; hand span: the distance between the tip of the thumb and the tip of the little finger when the hand is fully stretched out. Similarly, fathom meant the distance between the ends of the hands of a Anglo-Saxon farmer when his arms were fully out stretched.
Certain historical units were based on the things around us. For example, Romans used a unit called pace which was equal to the stride of their army contingent and they called the distance travelled by it in 1000 paces to be equal to a mile. Similarly, the grain was used as the unit of mass in sixteenth century and was equal to the weight of a wheat grain.
Need for Standard Unit
The units based on parts of human body are arbitrary and inaccurate. The results of the measurements vary from person to person because size of the unit is different for different person. For example, the units like a cubit or a foot would depend on the person making measurement. This created problems in trade between different countries and obviously in the day to day transactions. In order to overcome the limitations of body parts as units, and to bring about uniformity in the measurement system, the need for exact measurement was felt. For this, a standard of measurements had to be developed which is acceptable to everybody.
The problem of measuring lengths accurately was first solved by the Egyptians as far back as in 3000 B.C. It was done by defining the standard cubit. It was defined to be equal to the distance between the elbow and tip of the middle finger of the Pharaoh ruling Egypt at that time. Measuring sticks of length exactly equal to that of standard cubit were made. In this way they made sure that the cubit was the same length all over Egypt.
Similar efforts were made by other rulers also. For example, the British King Henry-I (1100-1137) decreed that a yard would exactly be equal to the distance from the top of his nose to the end of his thumb on outstretched arm. Queen Elizabeth-I declared a mile to be exactly equal to eight furlongs. A furlong (furrow long) was the distance a pair of oxen could plough in a field without stopping to rest. It was found to be 220 yards.
These standards proved to be useful but were short lived, as once a given ruler went out of power or died, the system was not followed and a newer system came into being. Further, since different countries and the different provinces in a given country were governed by different rulers; they followed different systems of units. As a consequence, by the eighteenth century a large number of units for mass, length, area and volume came to be in widespread use.
Indian Measurement System
The measurement system in India also has evolved a great deal from the ancient times.
Indian measurement system in the ancient period
In ancient periods in India, the lengths of the shadows of trees or other objects were used to know the approximate time of the day. Long time duration were expressed in terms of the lunar cycles, which even now is the basis of some calendars. Excellent examples of measurement practices in different historic periods are available. For example, about 5000 years ago in the Mohenjodaro era, the size of bricks all over the region was same. The length, breadth and width of bricks were always in the ratio of 4:2:1 and taken as a standard.
Similarly around 2400 years ago during the Chandragupta Maurya period there was a well-defined system of weights and measures. The government at that time ensured that everybody used the same weights and measures. According to this system, the smallest unit of length was 1 Parmanu. Small lengths were measured in anguls. For long distances Yojan was used. One Yojan is roughly equal to 10 kilometres.
The Indian medicine system, Ayurveda, also had well-defined units for the measurement of the mass and volume. The measurement system was strongly followed to ensure the proper quantity of medicine for particular disease.
Indian measurement system in the medieval period
In the medieval period also the measurement system was in practice. As described in Ain-i-Akbari by Abul Fazl-i-Allami, during the period of Moghul Emperor Akbar, the gaz was used as the unit of measuring length. Each gaz was divided into 24 equal parts and each part was called Tassuj. This system was extensively used to measure land pieces, for construction of buildings, houses, wells, gardens and roads.
The gaz was widely used as a unit of length till the metric system was introduced in 1956. Even today in many parts of our country, particularly in the rural areas, gaz is being used as a unit of length.
Indian measurement system during British period
In order to bring about uniformity in the system of measurement and the weights used, a number of efforts were made during the British period. The British rulers wanted to connect Indian weights and measures to those being used in Great Britain at that time. During this period the inch, foot, and yard were used to measure length whereas grain, ounce, pounds, etc. were used to measure mass. These units and weights were used in India till the time of Independence in 1947.
The essential units of mass used in India included Ratti, Masha, Tola, Chhatank, Seer and Maund. Raatti is a red seed whose mass is approximately 120 mg. It was widely used by goldsmiths and by practitioners of traditional medicine system in India.
Modern Measurement Systems
Immediately after the French Revolution (1790) the French scientists took lead in establishing a new system of weights and measures. They advocated the establishment of national standards for the purpose and the use of decimal arithmetic system. This led to the birth of metric system which like Hindu-Arabic counting system is based on the multiples and subdivisions of ten.
After detailed deliberations the basic unit of length and mass were defined and their working standards were prepared. The working standard for meter was prepared by marking two lines a metre apart, on a platinumiridium bar. Similarly, a platinum - iridium cylinder was constructed, equal to the mass of 1 cubic decimetre of water, as the working standard for mass. These two standards have been preserved at the International Beurau of Weights and Measures at Serves near Paris. The copies of these were prepared and sent to different countries. As regards the time, the concept of hour, minute and second based on the rotation of earth was retained.
An international treaty, called Metre Convention was signed in 1875 to follow metric system through out the world for trade and commerce.
In the course of development of units a number of systems were adopted. Two systems which were extensively used were the cgs and mks systems. The cgs system was based on centimetre, gram and second as the units for length, mass and time while mks system used metre, kilogram and second for the same. In 1958 it was realised that the units defined as standard needed to be redefined.
Since 1983, it is defined as the length of the path travelled by light in vacuum in 1/299,792,458 of a second.The new exercise of redefining the system of units led to the birth of SI system of units which is currently the system in use.