The International Bureau of Weights and Measures 1875-1975 U.S. DEPARTMENT OF COMMERCE National Bureau of Standards ""EAU of NBS SPECIAL PUBLICATION 420 Aerial view of the Pavilion de Breteuil and the immediate environs. To the east, the Seine and the Pont de Sevres; to the northwest, the Pare de Saint-Cloud: between the Pavilion de Breteuil (circled) and the bridge: the Manufacture Nationale de Porcelaine de Sevres. The new laboratories (1964) are situated north of the circle and are scarcely visible; they were built in a way to preserve the countryside. (Document Institute (leographique National, Paris). Medal commeiiKiraUn-i the centennial (if the Convention cif tlie Metre and the International Bureau of Weights and Measures. (Desifined by R. Corbin. Monnaie de Paris) The International Bureau of Weights and Measures 1875-1975 Edited by Chester H. Page National Bureau of Standards, U.S.A. and Pan I Vigoiireiix National Physieal Laboratory, U.K. Translation of tlie BIPM Centennial Volume Piibli>lieH on the ocrasioii <>( the lOOth Aniiiver^ai y ol tlie Treaty of tlie Metre May 20, 1975 U.S. DEPARTMENT OF COMMERCE NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Direcior Issued May 1 975 National Bureau of Standards Special Publication 420 Nat. Bur. Stand. (U.S.), Spec. Publ. 420. 256 pages (May 1975) CODEN: XNBSAV U.S. GOVERNMENT PRINTING OFFICE WASHINGTON: 1975 For sale by the Superintendent of Documents U.S. Government Printing Office, Washington, D.C. 20402 Paper cover Price $3.00 Stock Number 003-003-01408 Catalog Number C13.10:420 FOREWORD The metric system was made legal by Congress in 1866, the United States of America signed the Treaty of the Metre in 1875, and we have been active in international coordination of measurements since that time. In science, in pharmacology, and increasingly in industry, the United States is making ever greater use of the International System of Units (SI). In view of this ongoing conversion to the metric system, the celebra- tion of both the Centennial of the Treaty and the first century of metrology at the International Bureau of Weights and Measures (BIPM) holds con- siderable interest for the American public. It therefore gives me great plea- sure to make available this translation of the BIPM's official centennial volume. Richard W. Roberts Director, National Bureau of Standards III THE INTERNATIONAL BUREAU OF WEIGHTS AND MEASURES AT THE TIME OF ITS CENTENNIAL We have all heard of the International Bureau of Weights and Mea- sures; it is even one of the memories which have been part of us since our school days. And it is still more true in countries other than France, without doubt because there is added to the prestige of the "Standard Metre" and the "Standard Kilogram," that of the metric system conceived by the Paris Academy of Sciences under the French Revolution. The International Bureau of Weights and Measures is situated on French ground, but it is independent of France: decisions concerning it rest with a Conference which assembles the delegates of 43 countries, furthermore, the 18 members of the Committee elected by the Conference, who constitute, with the Director of the Bureau, its operations directorate, must be scientific persons of entirely different nationalities. These 43 countries contribute, each its share, to the operating expenses of the BIPM, which was therefore since the beginning, and still is, truly interna- tional. That which characterizes the BIPM now, at the time of its centennial, is the blending of permanence, which comes from its mission, and of rapid renovation, as in the techniques used to accomplish this mission. Its per- manent mission is to achieve uniformity of physical measurements throughout the world, at the highest level of precision; this uniformity, recognized as necessary as early as 1875, has become even more so since the advent of the rapid scientific and industrial progress which dominates our civilization. As for the level of precision, it raises itself today, by virtue of the discoveries in atomic physics, to a degree which would have been in- conceivable 10 or 20 years earlier; the former standards of a mechanical nature tend to be replaced by standards of an atomic nature controlled by rigorous universal quantum laws. This is why the BIPM needs a staff of an ever higher scientific level: additionally, its technical equipment must be regularly updated. Thus, in the fields where it specializes narrowly, the BIPM keeps itself at the level of the best metrological laboratories in the world. The BIPM furnishes standards to countries needing them, compares the standards of countries which are sufficiently advanced to establish their own standards, and contributes to research looking forward to the still higher precision of the future; it serves as a neutral and impartial scientific arbiter when national sensitivities impede an international agreement. It is v also a center for discussion, for coordination, and for preparation of inter- national decisions, whenever these decisions concern standards, for exam- ple, those of length and time (change of the definition of the metre in 1960 and of the second in 1967), or the International System of Units, developed from the Metric System. The importance of the mission of the BIPM is well illustrated by the progressive adoption, in all countries, of the International System of Units; despite the force of habit which retards change, this system is spreading inevitably on its own merits, and is supplanting the multiplicity of disparate units which we have too long endured. In the present volume, the chapter on the measurement units of the International System of Units is due to Professor Jan de Boer (University of Amsterdam), Secretary of the International Committee on Weights and Measures. The history is by H. Moreau, metrological editor of the BIPM. The following chapters discuss the principal laboratory activities of the BIPM; they have been composed by its scientific staff. An editing commit- tee, consisting of J. Bonhoure, P. Carre, G. Leclerc, A. Rytz. and P. Giacomo, deputy director, has seen to making them homogeneous in presentation and so easily readable that a large number of readers can ab- sorb the most important aspects of the basic metrology, often ignored, sometimes even by scientists. J. Terrien Director, BIPM VI CONTENTS Page Foreword HI The International Bureau of Weights and Measures at the time of its centennial, by J. Terrien V From the earlier systems of measures to the International System of Units, by J, de Boer 1 I. HISTORICAL INTRODUCTION 21 The Metre Convention and its organs 21 A century of metrology 29 II. MASS 43 The Prototype Kilograms and periodic verifications 43 Determination of the mass of Kilograms outside the periodic verifications 48 Balances and methods used for the comparison of Kilograms, auxiliary measurements 50 Recent balances and current studies 54 III. LENGTH 65 Measurements of length based on the International Metre 66 Toward the change of the definition of the metre 72 Putting the 1 960 definition of the metre into practice 77 Complementary studies of the standard radiation 88 Lasers 92 IV. GRAVIMETRY 103 Metrological importance of the i<nowledge of the acceleration of gravity 1 03 Reversible pendulums 103 Free fall of a graduated scale 104 Method of two stations 106 V. MANOBAROMETRY 117 VI. THERMOMETRY 125 The gas thermometer and the mercury thermometer 125 The International Practical Temperature Scale 1 26 Pyrometric study of the freezing-point temperature of gold 131 VII. ELECTRICITY 135 Electric units 1 36 Standards 137 Installations of the BIPM 140 Periodic international comparisons 1 45 Maintenance of the ohm and the volt by the BIPM 151 Progress of metrology in the field of electricity 1 54 VIII. PHOTOMETRY 163 Apparatus and measurement methods at the BIPM 164 International comparisons of intensity and of luminous flux 170 Maintenance of the units and stability of the standard lamps 173 Spectrophotometric measurements 173 International comparison of distribution temperature scales 1 75 Prospects offered by radiometry 1 75 VII IX MEASUREMENT OF RADIOACTIVITY 179 Generalities on the methods of measurement 1 79 The method of counting by 4 tt/? (PC)- 7 coincidences 180 The BIPM laboratory for radionuclide measurements 181 Radioactivity standards 183 International comparisons of radionuclides 1 84 Statistics of counting 185 Energy measurement: alpha-particle spectrometry 190 X. X AND GAMMA RAYS 197 Generalities 1 97 Measurement of X Radiation 200 Measurement of Gamma Radiation 203 XL NEUTRON MEASUREMENTS 213 Generalities 213 Measurement of the emission rate of a neutron source by the manganous sulfate bath method 215 Comparative measurements of neutron sources 218 Measurement of the flux density of a source of fast monoenergetic neutrons 221 Appendixes 1 1 . Convention of the Metre (May 20, 875) 225 2 Member States of the Metre Convention 231 3- Members and officers of the CIPM, and Directors of the BIPM from 1 875 to 1 975 233 4 The base units of the'SI and their origins 238 5 Worldwide expansion of the Metric System 243 6. Publications of the Metre Convention Organs 245 7, Abbreviations used in this volume 247 VIII FROM THE EARLIER SYSTEMS OF MEASURES TO THE INTERNATIONAL SYSTEM OF UNITS J. de Boer Numbers and Measures On the occasion of the celebration of the centenary of the Convention of the Metre* it seems appropriate to review the origins and historical developments of unit systems and in particular of the International System of Units, one of the most important results of the broad international col- laboration that has been realized in the framework of the Convention of the Metre. The development of such an internationally accepted unit system is closely related with the history of science itself. The purely metrological aspects of the development of weights and measures and of measuring standards in general are of course directly influenced by science and technology.