Graphic representation of interactions between various groups in the techniques network.

materials in environments of plasma temperatures for examplewhere even the usual definition of temperature does not apply—is a great deal more difficult than doing so at more mundane temperature

ranges.

In this country an extensive network of institutions and organizations has grown up, aimed in one way or another at proper utilization of the national measurement system for the making of meaningful measurements. This techniques network has not yet been as well examined or understood as the instrumentation and data networks. At a minimum, it includes professional journals and other publications; meetings of professional societies; organizations and institutions that provide training in measurement techniques; standardizing bodies such as the USA Standards Institute, the American Society for Testing and Materials, and the International Standards Organization; standards of practice which include agreed-upon procedures for making measurements; and the educational institutions that provide the trained manpower to operate the national measurement system.

As an institution which has developed the capability for leading the national measurement system, NBS has a responsibility for making available the information and know-how it has acquired in developing this capability. To fulfill this responsibility, the Bureau renders consultative and advisory services to standards laboratories, publishes data and information on measurement techniques, sponsors symposia and training courses on measurement topics, and cooperates extensively with standardizing bodies, particularly through participation of Bureau staff members in the committee work of these organizations.

INSTITUTE FOR BASIC STANDARDS

The Institute for Basic Standards (IBS), one of three institutes which comprise the National Bureau of Standards, has as its first responsibility the provision of "the central national basis for a complete, consistent system of physical measurement properly coordinated with those of other nations." As a second responsibility IBS develops and maintains standards for physical quantities and for the measurement of physical properties. In concert with the Bureau's Institute for Materials Research, IBS shares the responsibility for providing physical data on the properties of matter and materials.

Implicit in the assignment of the first responsibility is the recognition that there does exist a national system of measurement and that this system is a centralized one, with a central laboratory which develops and maintains the national standards for physical measurement and provides the starting point for a chain of measurement leading from those standards to the ultimate users of the system. This chain must provide for measurements of all necessary magnitudes, from the properties of atoms to those of the universe.

From the point of view of the ultimate user who faces a measurement problem, such as finding the diameter of a ball bearing or the melting point of a metal, the measurement chain can operate in two different ways: (i) It can provide the user with a proven measurement technique or with a calibrated instrument, traceable back to the national standards, with which he can measure the diameter or the melting temperature. (ii) In the case of the melting temperature or other similar properties, it can provide him with an immediately available answer in the form of critically evaluated data which previous investigators have obtained in measurements based on the national standards.

As the nation's central measurement laboratory, NBS exercises leadership in both these measurement areas. In the Bureau's laboratories the acquisition of standard reference data by precise measurement goes on side by side with research to develop and improve the national standards and associated measurement methods.

PHYSICAL QUANTITIES

The strength and utility of the national measurement system depend fundamentally upon the existence of a complete, consistent system of units and standards around which the system can develop. The In

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ternational System of Units (SI), defined by the 11th and 12th General Conferences on Weights and Measures, is the base for the international system of measurement and for most national systems. Six of these SI units-the kilogram, meter, second, degree Kelvin, ampere, and candela-are the arbitrarily chosen values of six quantities of the physical world-mass, length, time, temperature, electric current, and luminous intensity. Consistent units for other quantities may be derived from these, with appropriate values fixed by the units selected for the basic six. The English system-pound, inch, second, degree Fahrenheit, etc.-and other systems of units are related to the SI units by definite conversion factors.

The research at NBS on physical quantities is concerned with the establishment of these units by international agreement, the realization of the standards which represent them, and the development of a chain of measurement from these standards to the multiples and submultiples needed by our technologically based society. These activities offer an exciting field of technical endeavor which reaches to the frontiers of science and technology. Indeed, the state of sohistication of the U.S. national measurement system is an important gage of the scope and utility of our science and technology. Current work in this area is described below.

INTERNATIONAL BASE UNITS

Length

International Comparison of Laser Wavelengths.—In order to make accurate use of the gas laser in dimensional metrology it is necessary to know the wavelength of the laser light being used. NBS metrologists recently determined the wavelength of the "6328" line of a helium-neon gas laser by direct comparison with the krypton wavelength standard of length. The same laser was then sent to the National Laboratories of Great Britain and the Federal Republic of Germany for similar measurements. None of the results differ from the average of the three-632.991418 nm in vacuum-by more than 3 parts in 10o. This round-robin helped clarify a mystery that appeared in 1965. At that time each laboratory measured a different laser, with much poorer agreement. It is now clear that lasers, even though of identical manufacture, may have wavelengths differing slightly from one another. Because of this, NBS has established a program that will lead to a calibration service for laser wavelengths.

Time and Frequency

A New Location for WWV.-At 0000 Greenwich Mean Time, December 1, 1966, WWV transmissions were transferred to Fort Collins, Colo. The obsolescent Greenbelt, Md., facility was replaced in its en

tirety at the Fort Collins site. The new facility located on a site near the present WWVB/WWVL Fort Collins, Colo., not only provides more reliable service throughout the country, but the station is near the NBS Radio Standards Laboratory, which is responsible for its technical and administrative supervision. The transmission building occupies a floor space of 6880 square feet and is located in the side of a hill. It was designed in this manner to prevent deleterious effects to the omnidirectional radiation characteristics of the antennas which are located in an arc of a circle on the ridge of a hill overlooking the building. The half-wave center-fed vertical dipoles are connected to their respective transmitters with rigid coaxial cables and are matched to 50 ohms. An average power of 10 kW is radiated on 5, 10 and 15 MHz, and 2.5 kW is radiated on 2.5, 20 and 25 MHz.

Frequency control equipment consists of three complete and independent frequency generating systems. Each is controlled by a cesium. 133 frequency standard that is phase-referenced to the NBS Frequency Standard located at Boulder, Colo. This station maintains a transmitted accuracy exceeding 1 part in 1011.

Frequency-Time Dissemination Research.-Time and frequency have the unique characteristic that enables NBS to distribute them to users by means of standard radio broadcasts. However, the accuracy of the current methods of distributing them has not kept pace with the needs, and therefore, research has been underway on new and improved methods of dissemination. One potential time distribution system being investigated by NBS is the use of satellites. Using the VHF transponder aboard the AFS-1 satellite, an atomic clock at the NASA Mojave site in California was compared to one at NBS, Boulder, Colo., with an indicated precision of about 10 microseconds.

Synchronization of Atomic Clocks.-An important problem in synchronizing widely separated clocks using VLF radio signals is the determination of propagation delays from the transmitter to the receiver. As in electric circuits, the propagation medium at VLF introduces phase and group delays, which need to be known before widely separated clocks may be accurately set to agree with a master clock at the transmitter and therefore with each other. Recent phase velocity measurements have been made with an accuracy of a few parts in 10,000. They were accomplished by using special VLF receivers and an atomic reference standard carried in a mobile laboratory which traversed a path from Boulder, Colo., to Austin, Tex. It was found that the VLF waves travel with a speed of around 99 percent of that of light in a vacuum. Results of these types of measurements are part of a continuing effort at NBS to obtain the basic information needed to

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disseminate standard time at VLF using a multiple carrier system devised by NBS several years ago. Such a system, with worldwide coverage, would be invaluable to the nation's space effort by permitting rapid synchronization of the clocks at all the NASA satellite and deep space tracking stations, as well as those of other agencies of the Government, such as the Department of Defense.

Time and Frequency Bulletin Available.-During the past year, at approximately one-month intervals, NBS disseminated to persons requesting it, a Time and Frequency Bulletin giving corrections and announcements for WWV transmissions.

Temperature

Nuclear Resonance Thermometry.-The temperature dependence of the chlorine 35 nuclear quadrupole resonance frequency in potassium chlorate (KCIO,) has been measured between 12 °K and 300 °K. It has been shown that it is practical to use this resonance frequency as a thermometer with a precision of one thousandth of a degree in the temperature range 50-300 °K. The sensitivity deteriorates at lower temperatures.

Electric Current

Ampere Determination. The NBS Pellat-type dynamometer has been modified for use in a new ampere determination. The balance arms were stiffened and a new rotatable coil was wound on a fused silica form. Preliminary results indicate a slightly smaller difference between the NBS and Absolute Ampere than resulted from the 1958 determination. The new determination is, therefore, in somewhat better agreement with the 1958 value published for the NBS Current Balance.

Surveillance of the NBS Ampere.-Continuing measurements of proton precession frequency in the field of a stable solenoid excited by a current defined in terms of the NBS Volt and Ohm, demonstrate that the ratio of these units (as maintained) has not changed by as much as 1 ppm over a six-year interval. In view of other evidence that the NBS Ohm has been stable within considerably closer limits, it may be presumed that the NBS Volt has not changed by as much as 1 ppm. Data now available include the year that has elapsed since the standard cells used to maintain the NBS Volt were moved from Washington to their present location in the Gaithersburg, Md., laboratory.