Contemporary International Metrological Work

G. D. Burdun

In recent years one may note an intensification of the activity of international metrological organizations in the field of scientific and applied metrology, and their adoption of important decisions concerning units of measurement of physical quantities and the standards that reproduce these units.

In October 1954 the Tenth General Conference on Weights and Measures was held. This conference is convened once every six years and reviews the work of the International Committee of Weights and Measures, its consultative committees, and the laboratories of the International Bureau of Weights and Measures. The conference discussed questions concerning new definitions of the metre and the second, adopted a decision on defining the thermodynamic temperature scale by means of a single fixed point, and established the basic units of the international system of units.

In October 1956, at a session of the International Committee of Weights and Measures, a new definition of the second was adopted and an international system of units was established. At the first international conference on legal metrology, held soon afterward, the foundation was laid for the existence of a new metrological organization dealing, on the international level, with questions of applied metrology.

Below are given data on certain of the most important international metrological works and decisions.

1. On the Question of a New Definition of the Metre

When the metric system of measures was established, one ten-millionth part of a quarter of the Paris meridian was adopted as the unit of length. A direct measurement, carried out by a commission of French scientists, of the section of the Paris meridian from Dunkirk to Barcelona made it possible to establish the length of this measure and to manufacture its standard in the form of a platinum end measure, which was deposited at the end of the eighteenth century in the French State Archives and received the name of the archival metre.

However, repeated measurements of the meridian showed that the length of the archival metre was somewhat shorter than that of the true “natural” metre. Since subsequent, more accurate measurements might have yielded different values for the basic measure of length, the First General Conference on Weights and Measures in 1889 decided to adopt, as the standard of the unit of length, a specially manufactured platinum-iridium line measure, called the international prototype metre and, within the limits of attainable accuracy, equal in length to the archival metre. Thus the metre lost its significance as a “natural” measure.

The development of physics at the end of the nineteenth century led to the establishment of the possibility of returning to a natural standard of the unit of length. On the basis

of the work carried out at the International Bureau of Weights and Measures by Michelson and Benoît, the Second General Conference on Weights and Measures in 1895 adopted the decision that “the natural witness of the prototype meter shall be the ratio of the meter to the lengths of light waves.” Subsequent work, carried out in a number of laboratories in various countries, led to the conclusion that it was possible to establish a natural standard of length in the form of the meter expressed in wavelengths of light (the light meter). It was proposed to use as the standard radiation first the yellow line of sodium, then the green line of mercury, and later the red line of cadmium.

In 1927 the Seventh General Conference on Weights and Measures legalized the numerical relation between the meter and the wavelength of light. The meter was adopted as equal to 1553164.13 wavelengths of the red line of cadmium, emitted under specified conditions.

Subsequently, in metrological institutions of a number of countries, new work was carried out on the creation of standard sources of radiation using isotopes that give finer, monochromatic lines. In Germany work was conducted with the isotopes of krypton \( \mathrm{Kr}^{84} \) and \( \mathrm{Kr}^{86} \), in the USA with the isotope of mercury \( \mathrm{Hg}^{198} \), and in the USSR with the isotope of cadmium \( \mathrm{Cd}^{114} \).

In 1948 the Ninth General Conference on Weights and Measures adopted the following resolution: “The General Conference on Weights and Measures, having acquainted itself with the new possibilities presented by the spectral lines of elements with a single isotope, combining to the highest degree the qualities required for the creation of standards of wavelength, welcomes the scientists whose work has culminated in the production of significant quantities of these elements; recognizes that in these lines lies the possibility of finding a natural basis of high precision for the unit of length; invites the large laboratories and the International Bureau to continue the investigation of these lines with the aim of establishing in the future a new definition of the meter, based on the wavelength of a selected line emitted under specific conditions.”

In 1952 the International Committee of Weights and Measures created a special Advisory Committee on the Definition of the Meter. This committee, having studied the work of major metrological laboratories in various countries on the light meter and having recognized at its 1953 session that the time had come to consider favorably a new definition of the meter based on the wavelength of light, nevertheless could not make its choice of any standard radiation possessing the best metrological properties, and deemed it necessary to continue the work on the investigation of monochromatic radiations.

The Tenth General Conference on Weights and Measures in 1954, having considered and discussed the report of the Advisory Committee on the Definition of the Meter, as well as reports of national metrological laboratories on this question, recognized that there was still no possibility of choosing any one spectral line, and resolved to retain the former definition of the meter until the Eleventh General Conference on Weights and Measures (1960).

In October 1956, at a regular session of the International Committee of Weights and Measures, a report was heard by the Bureau’s vice-director, Prof. J. Terrien, on the work of the International Bureau on the investigation of monochromatic light sources for the reproduction of the unit of length by means of the wavelength of light. Light sources filled with the isotopes krypton 86 (presented by the Physico-Technical Institute of the GDR), mercury 198 (presented by the National Bureau of Standards of the USA), and cadmium 114 (presented by the D. I. Mendeleev Institute of Metrology of the USSR) were investigated. These investigations showed that the contours of the spectral lines

...of the isotopes in question, namely—the red line of cadmium 114, the yellow-green and orange lines of krypton 86, and the green line of mercury 198—are simple and symmetrical. However, a detailed study of mercury 198 and its green line showed that even at a source temperature below 20°C self-reversal is observed. Broadening of the line under the influence of external conditions also takes place. Studies of the lamp with cadmium 114 showed the symmetry of the contour of the red line and self-reversal in the green line. A certain dependence was observed between the temperature of the source and the wavelength of the red line of cadmium, apparently connected with a change in the pressure in the lamp. The lines of krypton were investigated; their width upon cooling to the temperature of the triple point of nitrogen proved to be the smallest.

In the case of all three isotopes (krypton, mercury, cadmium), in practice the width of the spectral lines proved to be 35% greater than the theoretical value; this has not yet been explained by the influence of the discharge itself, and further investigations are necessary.

Investigations in the laboratory of the International Bureau of Weights and Measures were carried out by photoelectric methods. As a result of the work performed, it was shown that the orange line of krypton 86 has the smallest width (13 mK) and, thus, is for the present the best candidate for its selection as the principal one in a future definition of the unit of length.

The International Committee for Weights and Measures recognized it as necessary to continue work in this field, in particular to carry out an exchange of light sources among the various laboratories, in order to investigate as fully as possible all the metrological properties of the light sources, including the use for this purpose of the modern technique of atomic beams.

2. NEW DEFINITION OF THE SECOND

As a natural standard of time, the rotation of the Earth about its axis was used, and until recently the second was defined as 1/86400 part of the mean solar day.

However, observations over a long period of time have shown that the rotation of the Earth is subject to irregular and unpredictable fluctuations, which do not permit it to be regarded as a natural standard of time and which deprive the concept of the mean solar day of metrological significance. From 1872 to 1903 the mean duration of the day increased by 7 thousandths of a second, and from 1903 to 1934 it decreased by 5 thousandths of a second, after which it again increases. Thus, the mean day is defined only with an accuracy of \(10^{-7}\); this accuracy is quite insufficient in the present state of frequency technology (modern molecular generators and cesium frequency standards can give an accuracy up to \(10^{-10}\)).

In 1952 the International Astronomical Union adopted a decision to appeal to the International Committee for Weights and Measures concerning a new definition of the second, and in 1955 the Ninth Assembly of the International Astronomical Union indicated that the second should be tied to the tropical year.

The Tenth General Conference on Weights and Measures of 1954, recognizing that the need had arisen to give a new definition of the second, instructed the International Committee for Weights and Measures to adopt a decision on this question, taking into account the decision of the 1955 session of the International Astronomical Union.

Taking all the foregoing into account, the International Committee for Weights and Measures at its session in October 1956 adopted the following resolution on the new definition of the second:

“By virtue of the authority received from the Tenth General Conference on Weights and Measures in its Resolution No. 5, the International Committee of Weights and Measures, taking into account: 1) that the Ninth General Assembly of the International Astronomical Union (Dublin, 1955) expressed a favorable opinion concerning the linking of the second to the tropical year; 2) that, according to the decisions of the Eighth Assembly of the International Astronomical Union (Rome, 1952), the second of ephemeris time (ET) is the fraction

\[ \frac{12\,960\,276\,813}{408\,986\,496} \cdot 10^{-9} \]

of the tropical year for 1900 January 0 at 12 hours ET, decides:

The second is the fraction \(1/31556925{,}9747\) of the tropical year for 1900 January 0 at 12 hours ephemeris time” *).

The adopted new definition of the second does not introduce a new unit of time. The duration of the second, linked to the tropical year for 1900, is, insofar as it was possible to achieve this, equal to the mean duration of the terrestrial second over the last three centuries. The natural standard to which it refers is defined with considerably greater accuracy than the former one. Consequently, the introduction of the new definition of the second may be compared with putting new graduations on a line standard, when broad and irregular marks would be replaced by fine marks with rectangular edges, placed on the mean axis of the old marks.

At the same time, taking into account the advances in the creation of microwave standards of frequency and time based on the use of oscillations of molecules and atoms, and with the aim of coordinating, on an international scale, investigations into the substantiation of the unit of time through the observation of physical phenomena, the International Committee of Weights and Measures decided to establish a Consultative Committee for the Definition of the Second, whose task would consist of consultations on decisions to be taken for the improvement of the standard of time. The first meeting of the Consultative Committee on the Second took place in June 1957. Questions concerning atomic standards of frequency and time were considered there.

3. INTERNATIONAL TEMPERATURE SCALE AND THE THERMODYNAMIC TEMPERATURE SCALE WITH ONE FIXED POINT

Fundamental decisions in the field of temperature measurements were adopted by the Ninth General Conference on Weights and Measures in 1948, which established the modern International Temperature Scale, and by the Tenth General Conference in 1954, which defined the thermodynamic temperature scale by means of one fixed point—the triple point of water.

The International Temperature Scale of 1948, adopted by the Ninth General Conference on Weights and Measures, is based on the following provisions.

The Kelvin scale, in which temperatures are denoted by °K, corresponding to the symbol \(T\), is recognized as the fundamental thermodynamic scale, to which every temperature measurement can ultimately be referred. On this scale the interval lying between the ice point \(T_0\) and the boiling point of water \(T_{100}\) is equal to 100 degrees. The Ninth Ge—

*) The date 1900 January 0 at 12 hours is expressed in the ordinal reckoning of time used by astronomers and corresponds to noon on December 31, 1899. Ephemeris time, to which the ephemeris coordinates of celestial bodies are assigned, is established from the results of astronomical observations of the Moon and is considered invariant, independent of fluctuations in the speed of rotation of the Earth.

The General Conference on Weights and Measures adopted in principle the Kelvin scale, as well as the thermodynamic Celsius scale, in which the temperature is equal to \(T - T_0\). Any temperature interval expressed on one of these scales has the same numerical value as when it is expressed on the other scale.

The experimental difficulties inherent in measuring temperature on the thermodynamic scale led in 1927 to the adoption, by the Seventh General Conference on Weights and Measures, of a practical scale, which was called the International Temperature Scale. This scale was to be brought into agreement with the thermodynamic Celsius scale as closely as the state of knowledge at that time permitted. It was established in such a way as to be conveniently and accurately reproducible and so that it would make it possible to define any temperature according to the International Scale in considerably narrower intervals than according to the thermodynamic scale.

The refinement of the International Temperature Scale in 1948 pursued the aim of achieving the closest possible agreement between it and the thermodynamic Celsius scale; for this purpose refinements were introduced into the numerical values of individual fixed points, and a number of other changes were also made.

In the International Temperature Scale of 1948, temperature is denoted by the symbol \(t\) and is expressed in degrees Celsius, denoted \(^{\circ}\mathrm{C}\) or °C (Int 1948). The scale is based on a certain number of constant and reproducible equilibrium temperatures (fixed points), to which numerical values have been assigned, and also on certain formulas establishing the relationship between temperature and the readings of instruments standardized by means of these fixed points.

The fixed points and the numerical value assigned to each of them are given in the following table:

Basic fixed points and primary fixed points at a normal pressure of \(1013250\,\dfrac{\text{dyn}}{\text{cm}^2}\)

By interpolation the scale is divided into four regions (from the boiling point of oxygen to \(0^\circ\mathrm{C}\), from \(0^\circ\mathrm{C}\) to the solidification point of antimony, from the solidification point of antimony to the solidification point of gold, and above the gold point), for each of which the corresponding interpolation formulas are given.

In the recommendations of the Ninth General Conference on Weights and Measures, appended to the scale, a description was given of the instruments, methods, and operational procedures for realizing the International Temperature Scale.

As early as the second half of the last century, Kelvin and Joule and, independently of them, D. I. Mendeleev proposed basing the temperature scale on a single fixed point. Such a scale has significant advantages and makes it possible to determine absolute temperature more accurately than a scale with two fixed points.

The reproducibility of the boiling point of water \((0.002—0.01^\circ C)\) is inferior to the reproducibility of the melting point of ice \((0.0002—0.001^\circ C)\). The triple point of water, which is the equilibrium point of water in its solid, liquid, and gaseous phases, can be reproduced with the greatest accuracy (up to \(0.0001^\circ C\)).

Taking all this into account, the Consultative Committee on Thermometry of the International Committee of Weights and Measures in 1954 adopted a resolution to proceed to the definition of the thermodynamic scale by means of one fundamental fixed point—the triple point of water.

Having carefully considered all the numerical results obtained in various laboratories, the Consultative Committee on Thermometry recognized that the best value for the temperature of the triple point of water is \(273.16^\circ K\) on the thermodynamic scale.

The Tenth General Conference on Weights and Measures of 1954, having discussed the report of the Consultative Committee on Thermometry, adopted the following resolution:

“The Tenth General Conference on Weights and Measures decides to define the thermodynamic temperature scale by means of the triple point of water, as the fundamental fixed point, and to assign to it the temperature \(273.16\) degrees Kelvin, exactly.”

4. INTERNATIONAL SYSTEM OF UNITS

The concept of a system of units was first introduced by Gauss in 1832, who showed that if certain arbitrary units independent of one another are taken as a basis, for example a unit of length, a unit of mass, and a unit of time, then other units can be expressed as derived units by means of these fundamental ones.

The International Electrical Congress in 1881 established the CGS system of units, adopting as the fundamental units: for length—the centimeter, for mass—the gram, and for time—the second.

Since the CGS system proved inconvenient for practical measurements of electrical quantities, the congress at the same time established, for the convenience of practice, an “absolute practical” system of electrical units, obtained from the corresponding units of the CGS system by multiplying them by 10 to the appropriate power, and established the units of this system: the ampere as \(10^{-1}\) CGS units of current, the volt as \(10^8\) CGS units of electromotive force, and the ohm as \(10^9\) CGS units of resistance.

From the second half of the last century, the MKGSS system of units (meter—kilogram-force—second) found wide application in engineering for purposes of measurement and calculation; its fundamental units are: length—the meter, force—the kilogram-force, and time—the second.

In the 1920s, the MTS system was introduced in France, whose fundamental units were the meter, the tonne as the unit of mass, and the second. This system was recommended in the 1930s by Soviet standards as a system of units of measurement. However, both in France and in the USSR this system did not become widespread.

The use of several systems of units created great inconveniences both for practical purposes and for teaching. The need had arisen to establish a single international system of units.

In 1948, the Ninth General Conference on Weights and Measures considered a statement by the International Union of Pure and Applied Physics requesting the adoption, for international relations, of a practical international system of units, with a recommendation of the MKS system and one electrical unit from the absolute practical system. The Conference adopted a decision according to which the International Committee for Weights and Measures was instructed to conduct a survey of all countries and, on the basis of this survey, to establish an international practical system of units that could be adopted by all countries that had signed the Metric Convention.

In 1954, the Tenth General Conference on Weights and Measures adopted, as the basic units of the international practical system to be established, the following six units: of length—the meter, of mass—the kilogram, of time—the second, of electric current—the ampere, of thermodynamic temperature—the degree Kelvin, of luminous intensity—the candle.

In the same year, the International Committee for Weights and Measures selected from among its members a commission on the system of units. The commission included: Burdun—chairman (USSR), Barrell (England), Bur (Holland), Volle (International Bureau of Weights and Measures), Isnardi (Argentina), Cassinis (Italy), Stula-Getz (Austria), and Fivet (Federal Republic of Germany). The commission studied the replies of the countries to the survey conducted and presented to the International Committee for Weights and Measures a draft International System of Units.

At its meeting on October 6, 1956, the International Committee for Weights and Measures adopted the following resolution:

The International Committee for Weights and Measures, taking into account the task received from the Ninth General Conference on Weights and Measures in its Resolution No. 6 concerning the establishment of a practical system of units of measurement that could be adopted by all countries that had signed the Metric Convention; taking into account all the documents received from the 21 countries that replied to the survey proposed by the Ninth General Conference on Weights and Measures; taking into account Resolution No. 6 of the Tenth General Conference on Weights and Measures establishing the choice of the basic units of the future system, recommends:

1) that the system based on the basic units adopted by the Tenth General Conference and listed below be called the “International System of Units”:

*) The abbreviated designations of units are given in Russian transcription in accordance with the new GOST for units of measurement and in the transcription adopted by the International Committee for Weights and Measures.

2) that the units of this system, listed in the following table, be applied without predetermining other units that may subsequently be added:

5. ESTABLISHMENT OF THE INTERNATIONAL ORGANIZATION OF LEGAL METROLOGY

The development of international relations in the field of industry and trade, as well as progress in measurement technology, created the need for a further growth of international cooperation on measures and measuring instruments. It became necessary to supplement the already sufficiently firm foundations of international agreement concerning units of measurement and standards with an agreement on the conditions for the manufacture and use of measuring instruments covered by legal rules in force in industry and trade. Beginning in 1920, a number of countries put forward the proposal that all questions relating to legal metrology be studied jointly on an international scale. In 1933 the Eighth General Conference on Weights and Measures adopted the following resolution: “Having considered the proposal submitted by the delegation of the USSR concerning the creation of an advisory committee on applied metrology under the International Committee of Weights and Measures, the Conference instructs the latter to undertake the study of this question.” Since the International Committee of Weights and Measures could not take on extensive work that would broaden the scope of its activity into the field of measuring instruments used in industry and trade, in 1937 an International Conference on Practical Metrology was convened in Paris. Representatives of 37 countries took part in this conference.

The Conference adopted a decision to establish a Provisional Committee, which was entrusted with working out the procedure for creating an International Organization of Legal Metrology, intended to solve, on the international level, technical and administrative problems arising in connection with the use of measuring instruments covered by official rules.

The Committee consisted of eighteen members—representatives of various countries. The secretariat of the Committee was provided by the French Service of Measures and Measuring Instruments. In connection with the events of the Second World War, the Committee was able to reconstitute itself and resume its work only in 1950. By 1955 the Provisional Committee had drafted a convention establishing the International Organization of Legal Metrology, which by October 1956 had been signed by 25 countries.

In accordance with the convention, the International Organization of Legal Metrology has as its principal tasks:

1) to create a center for documentation and information on national services for the control of measuring instruments subject, or liable, to rules established by law, as well as on measuring instruments, from the point of view of their design, manufacture, and application;

2) to translate and publish the texts of legal rules on measuring instruments and their use;

3) to determine the general principles of legal metrology;

4) to study, with the aim of unifying methods and rules, those problems of a legislative and administrative nature in legal metrology whose solution is of international interest;

5) to compile a model draft law and regulations on measuring instruments and their application;

6) to develop a draft of the material organization of a model service for the verification and control of measuring instruments;

7) to establish the necessary and sufficient characteristics and qualities that measuring instruments must meet in order that they may be approved by member states and that their use may be recommended on the international level;

8) to promote relations among services of weights and measures or other services engaged in legal metrology.

The organs of the organization established by the Convention are:

a) the International Conference on Legal Metrology, composed of official delegates of the member states and convened once every six years. The Conference adopts decisions on all questions that are the purpose of the organization and elects its governing bodies;

b) the International Committee of Legal Metrology, which is the working body, consists of not more than twenty members, and holds its meetings once every two years.

c) the International Bureau of Legal Metrology, which constitutes a documentation center on measuring instruments and on verification and inspection services and performs work relating to the duties of the organization.

The Convention provides for the participation of the national services of weights and measuring instruments of the member states in carrying out special surveys, experimental investigations, and laboratory work in the field of weights and measuring instruments.

On October 10–12, 1956, the first International Conference on Legal Metrology was held in Paris. Representatives of all the countries that had signed the Convention took part in the Conference. Representatives of a number of international organizations attended the Conference as observers: the United Nations Educational, Scientific and Cultural Organization (UNESCO), the International Committee of Weights and Measures, the International Organization for Standardization, the International Electrotechnical Commission, and the International Commission on Illumination.

The Conference considered organizational questions. Taking into account that more than 16 countries, as required under the Convention for the establishment of the organization, had already signed the Convention, the Conference established the beginning of the functioning of the International Organization of Legal Metrology as of October 1, 1956.

The Conference elected the International Committee of Legal Metrology, consisting of representatives of the following 13 countries that had previously been members of the Provisional Committee of Legal Metrology and had signed the Convention by October 1, 1956: Austria, Belgium, Germany, Holland, Denmark, India, Poland, the Soviet Union, France, Czechoslovakia, Sweden, Switzerland, and Yugoslavia.

In considering questions concerning the organization’s plan of work for the immediate future, the Conference decided that this work would proceed along two channels:

a) work performed by the International Bureau in a centralized manner—the creation of a documentation and information center, the translation and publication of the texts of legal regulations on measuring instruments, and assistance in developing links among the services of weights and measures of the member states;

b) work performed by the national metrological services—determining the characteristics and necessary qualities that measuring instruments must meet in order to be approved by the member states and in order that their use be recommended internationally. The national metrological services may participate in resolving these questions either as rapporteurs or as experts.

The Conference discussed and adopted the subjects of work and the list of weights and measuring instruments whose characteristics and qualities, as well as methods of inspection in manufacture and use, are to become the subject of study by the International Organization of Legal Metrology.