combining Rk with a value of the Planck constant, the latter obtained by realizing the watt in a special way. This realization of the SI watt is achieved by the moving-coil watt balance, which is a modern version of the absolute ampere experiment.

The NIST watt balance has been designed to measure the ratio of mechanical to electrical power, linking the artifact kilogram, the meter, and the second to the practical realizations of the ohm and the volt derived from the quantum Hall effect (QHE) and the Josephson effect, respectively. The first results from the NIST watt experiment, sometimes called an ampere experiment, were published in 1989 [8], giving a relative standard uncertainty for K, of 6.7 × 107. That experiment was a prototype for the next version in which the magnetic field was increased a factor of 50 using a superconducting magnet, resulting in similar increases in the force and voltage. During the next decade many improvements were made [9,10]. In 1998 the latest results were published [11] by E. R. Williams, R. L. Steiner, D. B. Newell, and P. T. Olsen. That work, which used a NIST calculable capacitor measurement of RK, reports that K, 483 597.892 GHz/V with a relative standard uncertainty of 4.4× 10-8. This is the most accurate measurement of the Josephson constant to date.

=

The experiment is automated and runs nightly and over holidays to reduce vibrations. Recent measurements recorded 989 values of the SI watt over a 4-month period. The total uncertainty is dominated by Type B uncertainty components, that is, components that have to be evaluated by means other than statistical analysis of repeated measurements. Of the possible Type B error sources that contribute to the uncertainty, the three largest components arise from the following: (1) the index of refraction of air; (2) the present alignment procedures; and (3) residual knife-edge hysteresis effects during force measurements. Using the data discussed above, Williams et al. [11] obtained a relative standard uncertainty of 0.087 μW/W.

By connecting the macroscopic unit of mass (the kilogram) to quantum standards based on the Josephson and quantum Hall effects, this result provides a significant improvement in the Josephson constant as well as many other constants. For example, recent measurements of the Plank constant h can be derived directly from this work with a relative standard uncertainty of 8.7 × 10-8.

The NIST watt experiment is being completely rebuilt to reduce the uncertainty by a factor of ten, with a goal of less than 10 nW/W relative standard uncertainty. At that level of measurement uncertainty, the watt-balance experiment becomes a very good means of monitoring the mass artifact that is used in the weighings. The present definition of the unit of mass in the SI is based

-8

on the International Prototype of the Kilogram, which is a cylinder of platinum-iridium housed at the BIPM in France. The Prototype and a set of duplicate standards of mass accumulate contaminants on their surfaces, and must be cleaned to achieve fractional changes over the long term of less than 10 per year. Since the kilogram is the last SI base unit defined in terms of a material artifact, a quantum standard of mass founded on electrical measurements would complete the modern trend of removing all artifacts from the definitions of SI units.

The largest uncertainties in the NIST watt experiment of the 1990s arose from operating in air, which required that the changing air buoyancy and refractive index be calculated from many readings of pressure, temperature, and humidity sensors. Almost every part of the balance assembly is being rebuilt to operate inside a specially constructed vacuum system consisting of two chambers, schematically represented in Fig. 1. The upper chamber houses the balance section, and a toroidshaped chamber houses the inductive coils, located 3 m below and centered about the liquid helium cryostat containing the superconducting magnet.

Bibliography

[1] Roger W. Curtis, Raymond L. Driscoll, and Charles L. Critchfield, An absolute determination of the ampere, using helical and spiral coils, J. Res. Natl. Bur. Stand. 28, 133-157 (1942).

[2] Harvey L. Curtis, Roger W. Curtis, and Charles L. Critchfield, An absolute determination of the ampere, using improved coils, J. Res. Natl. Bur. Stand. 22, 485-517 (1939).

[3] Harvey L. Curtis and R. W. Curtis, An absolute determination of the ampere, Bur. Stand. J. Res. 12, 665-734 (1934). [4] Harvey L. Curtis, Review of recent absolute determinations of the ohm and the ampere, J. Res. Natl. Bur. Stand. 33, 235-254 (1944).

[5] R. L. Driscoll, Measurement of current with a Pellat-type electrodynamometer, J. Res. Natl. Bur. Stand. 60, 287-296 (1958). [6] R. L. Driscoll and R. D. Cutkosky, Measurement of current with the National Bureau of Standards current balance, J. Res. Natl. Bur. Stand. 60, 297-305 (1958).

[7] Peter J. Mohr and Barry N. Taylor, CODATA recommended values of the fundamental physical constants: 1998, J. Phys.

Chem. Ref. Data 28, 1713-1852 (1999); Rev. Mod. Phys. 72, 351-495 (2000).

[8] P. Thomas Olsen, Randolph E. Elmquist, William D. Phillips, Edwin R. Williams, George R. Jones, Jr., and Vincent E. Bower, A measurement of the NBS electrical watt in SI units, IEEE Trans. Instrum. Meas. 38, 238-244 (1989).

[9] Aaron D. Gillespie, Ken-ichi Fujii, David B. Newell, Paul T. Olsen, A. Picard, Richard L. Steiner, Gerard N. Stenbakken, and Edwin R. Williams, Alignment uncertainties of the NIST watt experiment, IEEE Trans. Instrum. Meas. 46, 605-608 (1997).

[10] Richard L. Steiner, David B. Newell, and Edwin R. Williams, A result from the NIST watt balance and an analysis of uncertainties, IEEE Trans. Instrum. Meas. 48, 205-208 (1999).

[11] Edwin R. Williams, Richard L. Steiner, David B. Newell, and Paul. T. Olsen, Accurate measurement of the Planck constant, Phys. Rev. Lett. 81, 2404-2407 (1998).

[12] R. E. Elmquist, M. E. Cage, Y-H. Tang, A-M. Jeffery, J. R. Kinard, R. F. Dziuba, N. M. Oldham, and E. R. Williams, The Ampere and Electrical Units, J. Res. Natl. Inst. Stand. Technol., January-February (2001).

Residents of the United States born much after 1930 can have little appreciation for what it was like to mobilize for total war. In World War II, everyone and every facet of daily life was affected. All citizens had to learn to live with food and fuel rationing, and no new cars or other consumer products made from steel could be purchased. There were blackouts, air raid drills, scrap drives, school children buying War Bonds (a 10 cent stamp at a time), and, of course, able-bodied men and women taken either into military service or placed in critical jobs in industry and elsewhere. Institutions such as the National Bureau of Standards were likewise totally involved in the war effort. The Bureau found itself with a number of very important technical assignments and, for a change, the resources to carry them out In October 1939, after Albert Einstein and Leo Szilard urged the President to launch a major research program on the possibility of producing nuclear fission and

utilizing it in the likely war effort ahead, NBS Director Lyman Briggs was placed in charge of a new Advisory Committee on Uranium to look into this proposal. By 1941 some 90% of the NBS staff was doing war work.

The Bureau worked on a great diversity of war projects ranging from high technology to evaluating materials for blackout curtains and blackout masks. A major effort carried out with the Navy and the Radiation Laboratory at MIT was the development and fielding of the Bat, the first combat success with a fully automatic guided missile (really a bomb with wings and a tail looking rather like a modern Unmanned Aerial Vehicle or UAV). The story of the Bat has significant technology in common with the proximity fuze program; namely, the use of electromagnetic radiation sources on flying ordnance and the interpretation and use of the reflected waves to carry out the mission.

This is the atmosphere in which the Bureau undertook the work on proximity fuzes, work that had a profound impact on NBS for many years thereafter as well as providing the military with breakthrough applications of technology. Bureau staff at the end of World War II prepared the book titled Radio Proximity Fuzes for Fin-Stabilized Missiles [1]. It summarized all the work done from about 1940 through 1945. It is a monograph; that is, a stand-alone comprehensive presentation of the entire program. Allen Astin, then assistant chief of the Ordnance Development Division of the Bureau, edited the work. (Harry Diamond was chief; W. S. Hinman, Jr. was chief engineer.) The work was published under the auspices of the National Defense Research Committee (NDRC) and the Office of Scientific Research and Development (OSRD) and classified at the Secret level. It was declassified in 1960. The volume under discussion is one of three covering the work done by Division 4 of NDRC titled Ordnance Accessories. It should be pointed out that parts of the program were ably carried out by contracts with companies and universities, a mode of operation learned well in the war and likely very comfortable for the Bureau staff, given their history of collaborations with the private sector. It has been asserted that the radio proximity fuze effort consumed “about 25 % of the electronics manufacturing capacity and 75% of the plastics molding capability" of the Nation during the war [2]. Authorship of the various chapters is by Bureau staffers who spent the war in the Ordnance Development Division and its subsequent subdivisions. Authors included such well-known names as Robert D. Huntoon, Chester Page, and Jacob Rabinow; Rabinow's wife, Gladys, is listed as a contributor to one chapter.

The problem assigned to the Bureau was to conceive and realize a fuze system for non-rotating, fin-stabilized munitions (ordnance) such that detonation could be obtained at a specified distance from the target. Such performance is desirable for two reasons: first it is often difficult to produce a direct hit, and it is acceptable to achieve detonation close to the target. A good example is trying to hit an airplane or a rocket. A direct hit was well beyond the technology available in the 1940s and remains a challenge today for the anti-ballistic missile program. Secondly, more damage can often be obtained by detonation at some distance removed from the target. Air bursts over ground targets are effective over wider areas than explosions on the surface, where much of the energy goes to producing a crater. Air bursts are particularly effective against dug-in ground troops and against stored materiel. The ordnance. addressed at NBS were rockets, bombs, and mortar shells. Both the Army and the Navy provided performance specifications for such fuzes and the program

came to the Bureau for execution. The specifications varied for the several applications, and the program was directed to make as much as possible common to all.

The first job was to select the basic concept. Although some thought was given to acoustic, photoelectric, and passive radio systems, an active radio scheme was adopted early on. As a result of the Bureau's great successes with radio earlier and with radiosondes more recently, the choice of the radio scheme led directly to a major technical strength of NBS. Management could and did assemble a powerful team, largely from within but augmented by cooperation with the Navy and, at times, with our Allies in England. Radio waves could be employed by using time-of-flight measurements on the path from and reflected back to the ordnance in flight or by using the Doppler shift of the reflected radiation. The reflected waves are shifted to a somewhat higher frequency as the missile flies toward the target. Using the transmitting antenna as the receiving antenna, this reflected wave sets up a beat frequency in the oscillator circuit. Transmitting at 75 MHz to 110 MHz produced a beat frequency of a few hundred hertz, the exact value depending on the relative velocity of missile and target and the transmitting frequency. These details were considerably different for rockets, bombs, and mortar shells. Nonetheless the concept worked in all cases.

It turns out that this situation is equivalent to a timevarying impedance at the antenna terminals. The amplitude of this signal increases with the strength of the reflection; i.e., with decreasing distance to the target. By using the impedance, it is shown that the sensitivity to reflection is independent of power level over a wide range, thereby enabling application to many different systems. A variety of interferences are treated in detail in Chapter 2 of the book, including different antenna designs, ground effects, target geometry, and the like.

The differential signal from the antenna circuit was fed to an amplifier and, depending on the design, through a rectifying diode and thence to the grid of a thyratron. When this signal reached a set magnitude, the thyratron discharged into a detonator circuit and the fuze mission was complete. Details of the control circuit are in Chapter 3. The concept was first demonstrated in February 1941. What lay ahead was a long, painstaking series of engineering projects to put together a series of fuzes that not only detonated the munitions successfully, but also met the armed services requirements for safety, reliability, ease of manufacture, and shelf life. In addition, for most of the fuzes there was an additional requirement that they had to fit into existing fuze wells on existing ordnance. These practical considerations

were non-trivial and required considerable ingenuity. Few of the available electronic components were suitable. Industry was pressed into service to design and prove out new triodes, diodes, pentodes, thyratrons, and a variety of power supplies-an assignment carried out in a timely fashion. The mechanical systems such as safety designs and arming schemes, as well as alternate detonation systems for situations where the proximity fuze did not work for whatever reason, were described in Chapter 4.

Chapter 3 also addresses the question of providing electrical power to the circuits, both by batteries and by mechanical generators. The batteries were either ordinary dry cells or "reserve" types that were activated by the forces of launch wherein an ampoule containing the electrolyte was broken and the electrodes and electrolyte brought together. The "reserve" concept was demonstrated but not fielded; the idea would come to be used often years later when the technique had been perfected. Dry batteries were used in the early fuzes but suffered shelf life problems. Much effort was directed to the notion of a wind-driven turbine connected to a generator. This had safety advantages-no possibility of electric currents prior to flight-was fairly simple in design and manufacture, and proved to be the method of choice.

The remainder of the volume contains a catalog of fuze types, details of laboratory testing, details of field testing, a resume of actual performance at the latter stages of the war, and a formal analysis thereof. It is interesting to note the thoroughness of field testing as exemplified by the number of units tested: for bombs15,000 dropped at Aberdeen Proving Ground in Maryland; for rockets fired from the ground-nearly 24,000 fired at Ft. Fisher in North Carolina and Blossom Point, Maryland (plus a few at Aberdeen); and some 3000 mortar shells fired at Blossom Point and the field station of the University of Iowa at Clinton, Iowa. Chapter 9 contains an analysis of performance. At the end of this chapter is a summary of conclusions by the armed services concerning results in combat.

Curiously, the driving force behind the work, Harry Diamond, seems not to have written any of the report. Nonetheless, he was the dominant figure throughout the war and established an atmosphere that made the great success possible. Diamond was a dynamo himself but evidently a very remarkable man to work for. He believed in delegation and a minimum of formality. Here is a quote from an appreciation written after his death [2]:

Vannevar Bush is said to have considered the radio proximity fuze the preeminent scientific and technical advance of the war. Considering the Manhattan Project and radar, this is a startling statement. Another historian ranked the fuze as follows "Considering the magnitude and complexity of the effort [it ranks] among the three or four most extraordinary scientific achievements of the war." General George Patton said, after the fuze had performed notably at the Battle of the Bulge, "The new shell with the funny fuze is devastating... I think that when all armies get this shell we will have to devise some new method of warfare." [2].

Allen Astin, in his closing comment, stated that, as of 1946, the Army Ordnance Department had already formulated a further program and that the Ordnance Development Division at NBS was working for the Army on new fuze challenges.

It turned out that the NBS effort on fuzes continued in a very strong manner in the post war years and that military work came to dominate the work of the Bureau. The basic concept of using the Doppler shift in frequency of the reflected radiation and beating it against the transmitter's oscillator frequency continued