The Weights and Measures Act 1878 had laid down the Pound and the Yard measure as the fundamental units of length and of weight. The Imperial Standard Yard and Imperial Standard Weight provided the benchmark from which all other denominations of weight and measure were derived. The imperial reference derived from the Report of the Weights and Measures Commission 1821, which was given effect in the Weights and Measures Act, 1824.
The original Yard and the Troy pound standards were damaged in a fire which destroyed the Houses of Parliament in 1834. Scientific investigation and research were immediately required to re-establish the primary standards of weight and measure. The new standards were equalized by Standards legislation.
The Avoirdupois pound of 7,000 grains was constituted the imperial measure of weight. The Imperial Standard Avoirdupois pound is in a platinum form. The fundamental unit is the pound, and all other units are defined as fractions or multiples of it.
A new yard standard was constructed based on two previously existing standards known as A1 and A2, both of which had been made for the Ordnance Survey, and R.S. 46, the yard of the Royal Astronomical Society. All three had been compared to the Imperial standard before the fire. The new standard was made of Baily’s metal No. 4 consisting of 16 parts copper, 2 1⁄2 parts tin, and 1 part zinc. It was 38 inches long and 1 inch square.
The Weights and Measures Act of 1855 granted official recognition to the new standards. Between 1845 and 1855 forty yard standards were constructed, one of which was selected as the new Imperial standard.
A number of copies of the imperial standards were made and deposited in the standards department of the Palace of Westminster as well as in a number of other key places. They were to be compared every ten years under the legislation.
Formerly there were also local standards which were copies of the Board of Trade standards. They were provided by the local authorities for use by their inspectors of weights and measures.
Latterly, the imperial measures were fixed with reference to metric measures. In 1959, the USA, UK, Canada, Australia, New Zealand, and South Africa agreed to adopt the international yard of exactly 0.9144 meters. In the UK, the provisions of the treaty were ratified by the Weights and Measures Act of 1963. The Imperial Standard Yard of 1855 was renamed the United Kingdom Primary Standard Yard and retained its official status as the national prototype yard.[
The Metric System
EU Directives and national regulations provide for the technical aspects of the metric system. They prescribe the units of measurement which may be used in particular fields. The national standards for the meter and the kilogram are specified.
Persons may not offer for sale, rent or lease any weighing instruments for retail use, other than those indicating metric units. The regulations apply to measurements, dimensions or quantities expressed in units, for trade-economic, public safety or administrative purposes. Certain other fields are exempted. The regulations deem references in earlier legislation to imperial measurements as a reference as to metrical measurements, as of 1993.
Phasing out of Imperial Measures
EU Directives and regulations made in the 1990s phased out most remaining imperial measurements. Imperial units were continued for a certain period for road signs for distance and speed measurements and in respect of certain products. They provided for postponement and ultimately the phasing out and the conversion of road distances and speed measurements to the metric system.
Certain derogations were granted, postponing indefinitely the requirement to withdraw certain nonmetric units. This derogation preserved the pint measure for draft beer, cider and lager and the Troy ounce for precious metals permanently. They may be withdrawn voluntarily. Products may bear dual markings.
Modern Weights and Measures Legislation
The legislation in respect of weights and measures was modernised in 1996. The Metrology Act established the Legal Metrology Service and the Office of the Director of Legal Metrology. The body is based within the National Standards Authority of Ireland. It replaced the Weights and Measures service, based within the Department of Enterprise and Employment and local authorities who administered the Weights and Measures Acts.
The meter is defined by reference to an international standard reference to wavelengths of radiation. The kilogram is equal to the mass of the international prototype of the kilogram. The metric measurements of length, area, volume capacity mass or weight are prescribed.
The legal units of measurements derive from EU Directives and regulations. They may be varied from time to time by ministerial order. The “SI” system of units, is the international system of units. It is promoted and recommended internationally by the General Conference on Weights and Measures.
A person may not possess for a prescribed use, a linear or capacity standard which is not of an approved denomination. This does not apply to capacity measures over 10 litres or 1 gallon. Breach is an offence.
Facilitating the SI System
The Legal Metrology Service provides and maintains, facilities, equipment, and reference materials, which are necessary to facilitate the SI system unit of measurement. The Minister may authorise other organisations to be responsible bodies in relation to specialised fields of measurement.
The Service may determine the circumstances in which instruments may be adjusted in order to make them accurate. An inspector may give notice to the owner in relation to the inaccuracy of an instrument. The owner may elect to carry out the work or have it carried out by a third party. The owner may authorise the inspector to make the adjustments. The owner is liable for the cost.
Units of Measurement
The international system of units, the SI, is an internationally agreed basis for measurement in science, technology and human endeavour. The SI system is universally recognised. It is promulgated by the Bureau International des Poids et Mesures (BIPM).
There are two types of SI units; base units and derived units. The base units are measured scientifically in an objective manner with reference to legally defined physically based measurement. There are base units of:
- Length – the metre,
- Mass – kilograms
- Time – the second
- Electric current – the ampere
- Temperature – Kelvin
- Amount of a substance – mole
- Luminous intensity – candela
Derived, Multiple and Sub-Multiple Units
Derived units are multiples or products of the base units. For example, they include area and volume which are derivatives of length. Speed, velocity and acceleration are derived from time, length and, in the latter case mass.
Dimensionless qualities are the ratios of the two quantities of the same type. The unit of a dimensionless quantity is a ratio.
Some derived units have a special name. This includes, for example, radians for angles, Newton for force, Pascal for pressure, joules for energy, watt for power, coulomb for electric charge and volt for electric potential difference. Other examples are Celsius for temperature, a lumen for luminous flux and ohm for electric resistance.
There are common multiple and submultiple prefixes for use in the SI unit system. The prefixes depend on the factor, to the power of 10, of the particular quantity, e.g., hectogram (10 to the power of 2), kilogram, (10 to power of 3), megagram, (10 to the power of 6).
Equally, there are standard names for factors to the power of -1, -2, -3 et cetera. (centimetre, millimetre etc.) Kilograms are an exception as the prefix is included for historical reasons.
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