Bibliography

[1] G. C. Paffenbarger, J. A. Tesk, and W. E. Brown, Dental Research

at the National Bureau of Standards: How It Changed the Practice of Dental Health Service, J. Am. Dent. Assoc. 111, 83-89 (1985).

[2] W. Souder and C. G. Peters, Investigation of Physical Properties of Dental Materials, Dent. Cosmos 62, 305-335 (1920).

[3] R. L. Coleman, Physical Properties of Dental Materials (Gold Alloys and Accessory Materials), Bur. Stand. J. Res. 1, 867-938 (1928).

[4] I. C. Schoonover, W. Souder, and J. R. Beall, Excessive Expansion of Dental Amalgam, J. Am. Dent. Assoc. 29, 1825-1932 (1942).

Standard Reference Materials (SRMs) are wellcharacterized materials produced in quantity and certified for one or more physical or chemical properties. They are issued under the NIST trademark and are characterized using state-of-the-art measurement methods. SRMs are designed to ensure the accuracy, traceability, and compatibility of measurement results in many diverse fields of science, industry, and technology both in the United States and throughout the world. SRM users recognize that reliable measurements can help avoid costly manufacturing mistakes and unnecessary over-design of products and systems. Good measurements can provide the basis for sound and economical environmental and safety regulations and can improve health care by enhancing the validity of clinical tests and procedures. Thus the use of SRMs for measurement reliability contributes to the strength of this nation's economy and the well being of its citizens.

Many users of SRMS are interested in the details of the procedures used at NIST to certify the SRMs. In 1985, the late John Keenan Taylor prepared SP 260-100, Handbook for SRM Users [1] to provide guidance for the use of SRMs and to explain the philosophy behind the SRM Program. The book is dedicated to the dissemination of information on the phases of preparation, measurement, certification, and use of SRMs. While written from the viewpoint of a chemist, the basic concepts described are applicable to most areas of metrology. Taylor arranged the Handbook by sections in a logical progression, starting with the concepts of precision and accuracy, followed by discussions of calibration procedures and quality assurance of the measurement process, the use of SRMS to evaluate various kinds of measurements, and the reporting of data with evaluated limits of uncertainty. The statistical considerations most frequently applicable for the evaluation and interpretation of measurement data are reviewed in the Appendices. Each section is written with some degree of independence so that it can be comprehended without frequent reference to the content of others.

The original Handbook was published in 1985 and required a second printing in 1987. In 1992, work was begun on a revision of Special Publication 260-100 [2] to upgrade dated information and to reflect significant changes that had occurred within NIST and the Standard Reference Materials Program since the document was

first issued in 1985. N. M. Trahey of the Standard Reference Materials Program performed an extensive editorial and technical review of the Handbook and its appendices. The text was determined to be consistent with current NIST guidance on measurements, but various sections and appendices (including Sections 8 and 9, Appendices A, B, and C, and the Guide for Requesting Development of Standard Reference Materials) required revision. There were extensive revisions in Appendix C. Statistical Tools, rewritten in accordance with current NIST policy by S. B. Schiller of the NIST Statistical Engineering Division. Two other sections and the remaining appendices were rewritten by N. M. Trahey, who also prepared new introductory material. The editing process had not yet begun when John Taylor passed away, and every effort was made to preserve those parts of the text that Taylor had prepared, essentially as he wrote them. Fifteen years after the original Handbook for SRM Users was published, the Standard Reference Materials Program continues to get approximately 800 requests for copies per year.

The National Bureau of Standards began to provide reference materials, originally known as standard samples, in 1906 in response to the needs of the metals industry. The SRM inventory has since become far more diverse and now contains over 1300 different SRMS and related samples. A large number of materials useful in physical metrology and engineering are included. Some technical areas are covered more completely than others for historical reasons, priorities for national issues, and to some extent the degree of industrial awareness of the quality assurance concept. Modern SRMS take into consideration the fact that a given substance for which an analysis is carried out may occur in different matrix environments. Thus the users must be made aware of the need to take the specific environment into account.

The Handbook is still making an impact on SRM users. It contains some of the course material used by John Taylor to teach classes at NBS/NIST and at national and international meetings of the American Chemical Society and the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy. At those meetings Taylor spent many hours in dialogues with reference materials users, analysts, and accreditors regarding reference materials and their importance in the metrology of chemical and physical measurements. Eight years after his death, the Handbook continues to

spark conversations with those who knew him and used his book. Former students are eager to tell how his teachings influenced their careers, and they cite the Handbook as one book they consider a collector's item that they will always keep in their library.

For 3 years at the Pittsburgh Conference, the Standard Reference Materials Program honored the author by sponsoring the J. K. Taylor Symposium on the Development and Use of Reference Materials. Taylor himself conceived the workshop, based on his feeling that U.S. industry needed to be both educated and informed of NIST's Standard Reference Materials role regarding measurement quality and data comparability.

John Keenan Taylor joined the National Bureau of Standards in 1929 at the age of 16; while working at the Bureau, he received his B.S. in Chemistry from George Washington University and later his PhD in Physical Chemistry from the University of Maryland. Over the course of a professional career spanning 57 years, Taylor performed research and directed activities in the Microanalysis, Gas and Particulate Science, and Standard Samples (a forerunner of the Standard Reference Materials Program) Sections of the NBS Analytical Chemistry Division. Even after his retirement. from NBS in 1986, he devoted much of his time and attention to measurement quality control and assurance in the field of analytical chemistry. Taylor served as coordinator for Quality Assurance of the NIST Chemical Science and Technology Laboratory and continued to write articles, design seminars, and teach

classes on the subject of quality assurance as applied to chemical measurements until shortly before his death on March 26, 1992.

Taylor was honored by the Department of Commerce with its Silver and Gold Medals. He was a member of the American Chemical Society, the Alpha Chi Sigma Chemical Fraternity, the American Institute of Chemists (AIC), and several ASTM technical committees, including Committee D22 on Sampling and Analysis of Atmospheres which he chaired from 1984 to 1990. He received an Award of Merit from ASTM, the Fitch Memorial Chemistry Award from George Washington University, the DC Educational Society Award, the AIC Honor Award, and the Chemical Society of Washington Achievement Award. He authored three books and some 200 scientific journal publications, served as editor of four books, and held two patents.

Prepared by Thomas E. Gills.

Bibliography

[1] John K. Taylor, Handbook for SRM Users, NBS Special Publication 260-100, National Bureau of Standards, Gaithersburg, MD (1985).

[2] John K. Taylor (edited by Nancy M. Trahey), Handbook for SRM Users, 1993 Edition, NIST Special Publication 260-100, National Institute of Standards and Technology, Gaithersburg, MD (1993).

This paper [1] is considered the seminal, definitive paper describing the revolutionary one-volt Josephsonjunction array standard. NIST changed forever highaccuracy voltage measurements with this development, which built on earlier work at NIST and a microwave feed design from the then West German standards laboratory, Physikalisch-Technische Bundesanstalt

(PTB). The basic element of the array is the Josephson junction, in the form of a superconductor-insulatorsuperconductor sandwich. When irradiated with microwave energy, such a junction exhibits a dc potential uniquely determined by the frequency of the radiation, the electronic charge, and Planck's constant, with a single junction providing a few millivolts. In other words, a Josephson junction can act as a superb frequency-to-dc voltage converter. A properly designed and fabricated array of junctions can be excited to produce a series of very accurate quantized voltages, or steps.

As developed and demonstrated by the NIST team [1-8], a Josephson-junction-based voltage standard system consists of microwave source and feed, cryostat, probe, chip, and readout and control system. Microwave energy is fed into the chip mounted in the probe's chip carrier and cooled by liquid helium. The array standard microchip is fabricated by techniques analogous to those used to fabricate silicon integrated circuits, although with very different material systems.

For almost 80 years, starting in 1901, the U.S. Legal Volt was maintained by several groups of standard cells. There was a large effort in the late nineteenth century and the early twentieth century to establish a standard for electromotive force (emf) based on electrochemical reactions within chemical cells. The first legal unit of voltage for the United States was based on the Clark cell, developed by Latimer Clark in 1872, with its output assigned a value of 1.434 international volts by the 1893 International Electric Congress. Public Law 105, passed by the U.S. Congress in 1894, made this the legal standard of voltage in the U.S. During the years between 1893 and 1905, the standard cell devised by Edward Weston was found to have many advantages over the Clark cell [9]. The Weston cell consists of a cadmium amalgam anode and a mercury-mercurous sulfate cathode with a saturated cadmium sulfate solution as the electrolyte. In 1908, at the London International Conference on Electrical Units and Standards, the

Weston cell was officially adopted for maintaining the volt. After 1908, only Weston cells were used for maintaining the national standard in the United States.

The Weston standard cell can be disturbed by transport or if it is subjected to a change in temperature or a small electrical current. When at times it was necessary to eliminate cells due to changes in emf of a cell relative to the mean of the group new cells could be added. In 1965 the National Reference Group of standard cells [10] included 11 cells made in 1906, seven cells made in 1932, and 26 cells made in 1948. Long-term stability of the volt reference was also maintained by comparisons of neutral and acid cells, preparing and characterizing new cells, and through international comparisons and absolute ampere and ohm experiments. According to Driscoll and Olsen [11], the results of the absolute current-balance measurements could be regarded "as assigning a value to the emf of the standard cell used to control the strength of the current" and as a check on the emf of the NIST standard cell bank. The use of the Weston cell as the national standard of voltage was supported by a considerable amount of research in electrochemistry and related fields at NBS.

Before the Josephson effect was discovered, it was difficult to provide incontrovertible evidence regarding the long-term stability of the U.S. Legal Volt. However, considerable evidence indicated that the unit of emf preserved with standard cells was unlikely to have changed by any significant amount, relative to the best measurements of the time, from the early 1900s to the 1960s.

In the late 1950s, research in solid-state physics stimulated the growth of the semiconductor industry. A new type of voltage standard based on a solid-state device, the Zener diode, appeared in the early 1960s. W. G. Eicke at NBS first reported the possibility of using Zener diodes as transport standards [12]. In the following years, after several manufacturers started making commercial Zener voltage standards, these references began to replace standard cells in commercial use. Although Zener voltage standards exhibit higher noise characteristics than standard cells and are affected by environmental conditions of temperature, atmospheric pressure, and relative humidity, they are now widely used in many metrology laboratories because of their robust transportability.

In 1962, Brian Josephson, a graduate student at Trinity College, Cambridge, England, predicted that electrons can tunnel in pairs (Cooper pairs) between two superconductors separated by a thin insulating barrier (a weak link or Josephson junction). An applied dc voltage V across the barrier would generate an ac current at the frequency f=2eV/h, where e is the elementary charge and h is Planck's constant. Conversely, an applied ac current of frequency ƒ would generate a dc voltage V, at the quantized values

where n is an integer and the value of 2e/h is approximately 483.6 MHz/μV.

One of the issues was whether this relationship was materials independent. In 1968 Parker, Langenberg, Denenstein, and Taylor [13] compared, via a potentiometer, the Josephson voltages of junctions consisting of five different superconducting materials and various combinations of thin-film tunnel junctions or point contacts with 1.018 V Weston saturated standard cells [10] calibrated by NBS. They obtained a value of 2e/h with a one-standard-deviation fractional uncertainty of 3.6×10-6.

It was argued on fundamental grounds that the above must be exact. The use of superconducting-quantuminterference device (SQUID) null detectors in the early 1970s allowed this to be tested to a few parts in 109, and thus the Josephson effect had obvious potential for use as a voltage standard [14]. By the early 1970s, NIST staff had set up a potentiometric measurement system in Gaithersburg that compared 2 mV to 10 mV dc Josephson junction voltages with 1.018 V standard cells to a few parts in 108 [15,16]. International comparisons in 1971-72 among national metrology institutes (NMIs), including NBS, the National Physical Laboratory (NPL) in the U.K., the National Research Council (NRC) in Canada, the National Standards Laboratory (NSL) in Australia, and the PhysikalischTechnische Bundesanstalt (PTB) in Germany, as well as the International Bureau of Weights and Measures (BIPM), found that the measured values of 2e/h agreed with each other to within 2×107 [17].

These results from the NMIs suggested the course of adopting a value of 2e/h for use in maintaining units of voltage. The United States was the first nation to do this, and the value of 2e/h to be used at NBS was chosen to prevent a discontinuity when NIST converted from standard cells to the Josephson effect [18]. NBS began maintaining and disseminating the U.S. volt. based on the Josephson effect in July 1972, using a 10 mV measurement system with relative uncertainty of 2×10. Soon after, the Consultative Committee on

=

Electricity (CCE) of the CIPM recommended the value KJ-72 483 594 GHz/V, which was adopted by all countries except the United States, France, and the Soviet Union.

In many applications, Josephson junctions were undoubtedly better references than standard cells, which are sensitive to environmental conditions, can shift values on transport, and can drift by a few parts in 108 per year. The typical 5 mV to 10 mV reference output from early Josephson devices made from a few junctions required both very low-level voltage balances and scaling by a factor of 100, both of which seriously limited the accuracy of measuring 1.018 V standard cells.

Then in 1977, M.T. Levinson and colleagues showed that unbiased Josephson junctions would spontaneously develop quantized dc voltages when irradiated with microwaves, opening the path to successful Josephson junction arrays. C. A. Hamilton, R. L. Kautz, F. L. Lloyd, and others of the NBS Electromagnetic Technology Division at Boulder began developing and improving Josephson standards based on series arrays of junctions operated near zero de voltage bias [3,19].

Stable 1 V zero-crossing arrays were operating at NBS [1] and PTB [20] by 1985, using about 1500 junctions and rf fields of 70 GHz to 90 GHz. Arrays with output voltages at the level of 1 V soon were used in NMIs throughout the world [21]. By 1989, NIST had made a 19 000 junction, 12 V array [2]. The widespread use of Josephson junction arrays in national standards laboratories, and better SI determinations of 2e/h, led the CCE to recommend a new exact conventional value for the Josephson constant:

which is fractionally larger by 8×10-6 than the 1972 conventional value. The new value was adopted worldwide on January 1, 1990, and thereby became the new basis for the U.S. Legal Volt. This definition of KJ-90 is the present volt representation, based on an ideal Josephson voltage standard. The conventional value was assumed by the CCE to have a relative standard uncertainty of 0.4 μV/V. By convention, this uncertainty is not included in the uncertainties of the representation of the volt, since any offset from the SI volt will be consistent among different laboratories using the Josephson effect standard.

The term "intrinsic standard" is sometimes used to describe a type of standard, such as a Josephson Voltage Standard (JVS), quantum Hall resistance standard, triple point cell, deadweight pressure gauge, etc., based on physical laws rather than on the stability of physical