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Over the past twenty years a surpri­ singly strong link has developed bet­ ween the applied science of metrology and atomic, molecular and solid state physics. The techniques of frequency multiplication, and the precise measure­ ment of frequency at infrared and visible wavelengths, have reached such a level of development that the metre has been redefined in terms of the distance travell­ ed by light in a given time 1, 2). The direct linking of atomic lattice spacings to opti­ cal wavelengths has produced a signifi­ cant improvement in the determination of the Avogadro constant 3, 4). Impres­ sive progress has been made in the pre­ cision of measurement of the electron anomalous magnetic moment 5) as well as the numerical evaluation of the asso­ ciated sixthand eighth-order Feynman diagrams 6). The most striking metrolo­ gical advance occurred when K. von Klitzing in 1980 observed the quantiza­ tion of electrical conductance 7) and achieved not only a direct macroscopic measurement of the fine-structure con­ stant, but the 1985 Nobel Prize in Phy­ sics as well 8). The CODATA Task Group on Funda­ mental Constants has recently comple­ ted a new evaluation of the fundamental physical constants 9) taking into ac­ count the truly extraordinary amount of experimental and theoretical work that has become available since the previous adjustment in 1973 10). Because of past problems associated with the statistical treatment of such a diverse set of experimental and theoreti­ cal data, increased attention was direc­ ted in the 1986 analysis to questions of statistical validity. The least-squares ap­ proach to the analysis of such data has been described in detail in previous re­ views 11, 12, 13, but in brief, each experi­ mental result with its estimated uncer­ tainty represents a constraint on the values of a set of physical quantities, ex­ pressed as an algebraic relationship in­ volving the auxiliary constants, the unknown stochastic data and their un­ known experimental and theoretical errors. The least-squares solution de­ termines the "best" values of these unknown quantities by finding the Table 1. 1986 Adjustment of the fundamental constants.

Fundamental Physical Constants 1986 Adjustments