In our day-to-day life as well as in the practice of pharmaceutical profession, it is always necessary to have some unit to measure a quantity.

The science, which deals with weights and measures, is called Metrology. As each and every pharmaceutical operation implies the fundamental knowledge of metrology, it appears appropriate to acquaint us with various aspects of metrology.

The need for a system of weights and measures might have been felt as a basis for comparison in day-to-day dealings and therefore mechanical devices like balances or scales were devised. Historically, the ancient standards referred to various parts of human body e.g., fathom, cubit, span, foot and nail etc.

Later, the objects around human beings like grains or wheat were preferred for comparison. As the facilities for trade and transport widened and the geographical barriers crossed, a need was realized to bring about uniformity in the system of weights and measures which might have started initially on a regional basis and developed through provincial to a national and international basis.

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Such an attempt to adopt an uniform, scientific and methodical system naturally culminated in most of the civilized countries of the world having adopted the metric system of weights and measures which is certainly the simplest and the best.

However most of the elder physicians still don’t follow the metric system in prescription writing and hence it is desirable that a pharmacist should be acquainted with other systems of weights and measures normally encountered in prescription writing.

The Indian system of weights and measures having the units tola, masha and ratti etc., is no more official since April 1, 1956 when the Government of India recognized the metric system as the only official one.

In India the Standards of Weights and Measures Act controls the weights and measures and the Pharmacopoeia of India has also recognized the metric system.

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Mass is a constant based on inertia while, weight changes with altitude, latitude, temperature and pressure. The measure of the gravitational force acting on a body is directly proportional to its mass and is known as weight.

The unit of weight is Gram, which is equal to 1/1000 of the mass of the International Prototype Kilogram. The extent of volume of a body is called measure. The units commonly employed are litre and metre for capacity and length, respectively.

The Standards of Weights and Measures Act, 1976 provides for the reference standard, secondary standard and working standard for calibration of weights and measures.

The Act also provides for the preparation of national prototypes of the kilogram and metre by the Central Government. The Standards of Weights and Measures Act, 1976 provides the calibration standard of weights and measures. Under the Act, ‘International Bureau of Weights and Measures’ means the Bureau International des Poids at Sevres in France.

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‘International prototype of the kilogram’ is defined as the prototype sanctioned by the First General Conference on weights and measures held in Paris in 1889, and deposited at the International Bureau of Weights and Measures. Kilogram is recognized as the base unit of mass and is equal to the mass of the International prototype of kilogram. Metre is recognized as the base unit of length and is equal to 1650763.73 wavelengths in vacuum of the radiation corresponding to the transition between the levels 2p10 and 5d5 of the krypton-86 atom.

The Act also provides for the preparation of national prototypes of the kilogram and metre by the Central Government.

English System:

The earlier units were based on the weight of 32 grains of wheat, which equaled a silver penny (Sterling), 20 penny weights (Pence) made an ounce, twelve ounces pound and eight pounds made a gallon of wine, etc. The word ‘haberdupois’ was used first in English laws in 1303. The ‘troy’ weight has still earlier origin. Troves is a French city in which great fairs were held during 8th and 9th centuries. However these systems caused great inconvenience to buyers and sellers of medicines. In 1790 Washington recommended the establishment of uniformity in currency, weights and measures; and in 1836 the troy pound (5760 grains), the avoirdupois pound (7000 grains) along with yard were introduced.

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Metric System:

In 1783, James Watt first proposed the use of decimal system and the commensurability of weight, length and volume. Metric weights were officially recognized in Great Britain in 1864, in United States in 1866 and in India in 1956. The principal attributes of this system are its simplicity, brevity and adaptability to everyday needs. It is the system of decimal progression meaning thereby that every unit is multiplied or divided by the same number (i.e. 10) to obtain various denominations. Latin prefixes like milli, centi, deci, etc., are used to indicate subdivisions while Greek prefixes like deca, hecto, myria, are used to denote multiples of the principal units.