(1) In terms of a prototype object:

(i) Mass: The kilogram is the mass of a platin cylinder preserved at the International Bureau of Weights and Me Sèvres, France. Prototype No. 20 is kept at NBS; equivalent prot kept by other countries.

(2) In terms of natural phenomena:

(i) Length: The meter is the length of exactly 1,6 wavelengths of radiation in vacuum corresponding to the un transition between the levels 2p10 and 5d5 of the atom of krypton orange-red line.

(ii) Time interval: The second was long defined as 1 the time required for an average complete rotation of the earth on with respect to the sun. This, with daily corrections from zenith of a star, is the basis for a universal time scale (UT). With correction for polar motion, it becomes UT1, and with further co for annual seasonal variations, UT2. Also, the earth's averag rotation rate has been decreasing, thereby increasing the length year by about 6 ms over the length of the preceding year. Because and other larger random fluctuations, the universal second thus def not a constant. Consequently, the 11th Conference (1960) rati definition of a second based on ephemeris time (ET): "the 1/31,556,925.9747 of the tropical year for January 0, 1900 at 12 ephmeris time." The 12th Conference (1964) authorized the designat cesium atom transition as a standard of frequency to be used tem for the physical measurement of time. The 13th Conference (1967) a the 1960 action and decided that: The unit of time of the Inter System of Units is the second, defined in the following terms: "Th is the duration of 9,192,631,770 periods of the radiation correspon the transition between the two hyperfine levels of the fundamental the atom of cesium 133."

(iii) Temperature: The kelvin, the unit of therm temperature, is the fraction 1/273.16 of the thermodynamic tempera the triple point of water. It was decided by the 13th Conference same name, kelvin, and the symbol K should be used for ex temperature intervals, dropping the the former convention which exp temperature interval in degrees Kelvin or as abbreviated, deg K.

the old designations are acceptable temporarily as alternatives to ones. One may also express temperature intervals in degrees Celsiu

(iv) Electric current: The ampere is that constant which if maintained in two straight parallel conductors of infinite of negligible circular sections, and placed 1 meter apart in a would produce between these conductors a force equal to 2 x 10-7 ne meter of length.

(v) Luminous intensity: The candela is the luminous in in the direction of the normal, of a blackbody surface 1/600,000 meter in area, at the temperature of solidification of platinum pressure of 101,325 newtons per square meter.

(c) Although the six base units, and others derived from th exactly defined, their practical use requires a realization thr development of accurate measurement standards. Measurement standa

reference instruments. Extensive theoretical studies and laboratory periments are involved in their selection, design, construction, and oper

ation.

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(d) It will be noted that a kilogram mass standard can be calibrated only through a series of comparisons, starting from the International Prototype. The units for the other five base quantities, and all quantities derived from them, in principle are independently realizable many laboratories. In practice, however, inevitable minor differences among instruments, environments, and operators are bound to introduce small discrepancies. Periodic comparison of Periodic comparison of standards and the resolution of these discrepancies is required for compatibility among domestic standards laboratories, as well as internationally.

(e) Within the United States, NBS consults with the major industrial and governmental standards laboratories and cooperates with the Department of Defense and the National Conference of Standards Laboratories in conducting measurement agreement comparisons. Periodic intercomparisons of NBS standards with those of other countries are made through the International Bureau of Weights and Measures, through international scientific organizations, or by direct arrangement. The operations of the International Bureau are supervised by the General Conference of Weights and Measures to which U.S. delegates are appointed by the Department of

State.

(f) Frequency and time comparisons within the United States are made by the National Bureau of Standards, the United States Naval Observatory, and other organizations which have an interest in precise time and frequency standards. The data from world-wide astronomical observations and from standards laboratories in many countries are coordinated by the International Bureau of the Hour, which announces recommended approximations for the rate difference between atomic and universal time, as well as for epoch adjustments.

(a) NBS has developed instrumentation and techniques for realizing standards for the six base units of the International System of Units as agreed on by the General Conference of Weights and Measures. Reference standards have been established not only for these six base units but also for many derived quantities and their multiples and submultiples. Such reference standards, or equivalent working standards, are used to calibrate laboratory and plant standards for other organizations. Accuracy is maintained by stability checks, by comparison with the standards of other national and international laboratories, and by the exploration of alternative techniques as a means of reducing possible systematic error.

(b) Calibrations for many types of instruments and ranges of physical quantities are described in the itemized service schedules of MP 250. On those services for which fees have been established, the fees also are listed in MP 250. In cases where fees have not been fixed, services will be billed on the basis of actual costs incurred. (See Section 200.116 below for details relating to the description of service items and listing of fees.) Changes in services and fees are announced in supplements to 250 and in the monthly NBS Technical News Bulletin.*

MP

* For sale by the Superintendent of Documents, U. S. Government Printing Office, Washington, DC 20402. Annual subscription, $1.50.

made. These might might involve unusual physical quantities, upper or extremes of range, higher levels of accuracy, fast response speeds, durations, broader ranges of associated parameters, or special env mental conditions. Such inquiries should describe clearly the measur desired. Indication of the scientific or economic basis for requirement to be satisfied will be helpful in determining future programs. Fees for work accepted will be based on actual costs incurr (d) The principal emphasis of the National Bureau of Standards those calibrations and other tests requiring such accuracy as ca obtained only by direct comparison with its standards. However, in to maintain efficient utilization of specialized equipment and sk personnel, when workload permits NBS may calibrate devices requiring 1 accuracy but suitable for working standards in plant or laboratory.

(e) Other services which may be obtainable include:

(1) Tests of measuring instruments to determine compliance specifications or claims when the evaluation is critical in nat scientific or technical operations and when suitable facilities are available elsewhere.

(2) Referee tests in important cases when clients are unabl agree upon the method of measurement, the results of tests, o interpretation of these results, but have agreed in advance in writin accept and abide by the findings of NBS.

(f) NBS reserves the right to decline any request for service! the work would interfere with other activities deemed by the Director of greater importance. In general, measurement services are not pro when widely available from commercial laboratories or for organization individuals in foreign countries.

(g) Suggestions will be offered on measurement techniques an other sources of assistance on calibration or measurement problems whe Bureau's own equipment and personnel are unable to undertake the work. National Conference of Standards Laboratories issues a Director Standards Laboratories in the United States which perform calibration (obtainable from NCSL Secretariat, c/o National Bureau of Stand Washington, DC 20234). Those which perform testing are listed in the Directory of Testing Laboratories, Commercial and Instituti (Directory available from the American Society for Testing and Mater 1916 Race Street, Philadelphia, Pennsylvania 19103.) Similar lis appear in buyer's guides for commercial products and in technical concerned with physical measurement.

200.104 Consulting and advisory services.

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(a) In areas of its special competence, the National Burea Standards offers consulting and advisory services on various pro related to measurement, e.g. details of design design and construction, ational aspects, unusual or extreme conditions, methods of statis control of the measurement process, automated acquisition of labor data, and data reduction and analysis by computer. Brief consultatio be obtained at no charge; the fee for extended effort will be base actual costs incurred.

(b) To enhance the competence of standards laboratory personnel at irregular intervals conducts several group seminars on the prec

nity of laboratory observation and information discussion. A summer urse in electromagnetic measurements and standards is conducted periodilly by the NBS Radio Standards Laboratory at Boulder, Colorado.

200.105 Standard reference materials.

Often the performance of a device or structure can be evaluated at the er's laboratory by comparing its response to unknown materials with its sponse to a standardized specimen substance of certified compotion, properties, or purity. Types of such specimens are listed in veral of the detailed fee schedules in MP 260.* For information garding carefully characterized materials, see Subchapter B of Chapter , 15 CFR. The Office of Standard Reference Materials in the NBS stitute for Materials Research administers a program to provide many

pes of well-characterized materials that are needed to calibrate a asurement system or to produce scientific data that can be readily ferred to a common base.

200.106 Critically evaluated data.

of well-defined

Data on the physical properties of the thousands bstances which are commercially available need to be compiled and aluated to be useful for reference in engineering design. The Office of andard Reference Data in the NBS Institute for Basic Standards provides o-way communication with a number of governmental and non-governmental ta centers throughout the country. Its present compilation and dissemition activities cover seven technical areas--nuclear data, atomic and lecular data, solid state data, thermodynamic and transport data, emical kinetics, colloid and surface properties, and mechanical operties. Accounts of progress appear in the National Standard Reference ta System News (available upon request from the Office of Standard ference Data) and in the monthly NBS Technical News Bulletin.*

200.107 Publications.

MP

Changes in services and fees, as published in the supplements to 0, appear also in the monthly NBS Technical News Bulletin.* The Bulletin describes recent results of Bureau staff work, lists currently issued blications, and carries sections giving up-to-date detailed information standards and calibrations, standard reference materials, seminar anuncements, conference summaries, and the National Standard Reference Data stem. An index to publications by the Bureau authors will be found in pplements to Miscellaneous Publication 240, Publications of the National reau of Standards.* This index covers the publication series nographs and Technical Notes* and includes all papers appearing either the NBS Journal of Research or in outside technical journals.

of

U. S. Government Research and Development Reports, a semi-monthly nouncement journal covering over 1000 new documents per issue, is sold by e NBS Clearinghouse for Federal Scientific and Technical Information, ringfield, Virginia 22151. Other CFSTI series available on subscription clude the USGRDR Index and a Fast Announcement Service which highlights 57 categories those USGRDR items of particular technological and instrial significance. Any individual item announced is available as otostat or microfiche.

or sale by the Superintendent of Documents, U. S. Government Printing fice, Washington, DC 20402.

N

(a) The NBS Radio Standards Laboratory broadcasts various standard frequency and time signals as a service from three radic WWV, WWVH, and WWVB. A fourth station, WWVL, is engaged in an ex program to evaluate precise time synchronization techniques. laneous Publication 236, "NBS Standard Frequency and Time S contains information concerning the broadcasts of such signals. four stations. WWVH is located at Maui, Hawaii. WWV, WWVL, and located at Fort Collins, Colorado. Notices of precisions, accur adjustments in NBS clocks and radio emissions are published in th Register. Such notices are published also in the Time and Service Bulletin which is available on request from the NBS Radio Laboratory, Boulder, Colorado 80302.

2.5

(b) Broadcasts from WWV are at nominal frequencies of 15, 20, and 25 MHz and from WWVH at 2.5, 5, 10, and 15 MHz. Ti superimposed on these carrier frequencies occasionally are adi phase and also as to emission rate (by offsetting the carrier from nominal) in accordance with with the Universal Time Coordinat system, as recommended by the Bureau International de l'Heure (B carrier offset currently is minus 300 parts in 1010, giving pul occur at intervals slightly longer than one second. With phase

**

a

result approximates UT2 within about 100 ms and therefore is furnishing information about the earth's position on its axis as determine the observer's longitude. WWV and WWVH also provide musical pitch at 440 hertz, ionospheric propagation condition f and geophysical alerts. WWV broadcasts Greenwich Mean Time minutes, hours, and days of the year-- in the code format of the Aeronautics and Space Administration. For more precise determi UT2, both WWV and WWVH broadcast in International Morse Code ap corrections based on astronomical data from the U. S. Naval Obs Washington, DC.

(c) At WWVL a carrier frequency of 20 kHz is provided a shared with others near 20 kHz. Also these carrier frequencies a from nominal, hence timing information derived from their phase al conformity with the UTC system. Station WWVB broadcasts on the radio frequency of 60 kHz without offset, emitting time marker pu second apart, as determined by the NBS cesium clock. The phase pulses is re-set every few months by 200 ms adjustments as recomm the Bureau International de 1'Heure in order to approximate U about 100 ms. This is the coordinated system known as Stepped Ato (SAT).

(d) The U. S. Naval Observatory (USNO) also broadcasts stan time and time interval (frequency), using both astronomical a data. It coordinates a number of Navy and Coast Guard stations ar globe and also monitors the NBS broadcasts. Both USNO and NB broadcasts from selected selected laboratories in foreign countries periodic comparisons of reference signals.

a

*For sale by the

Superintendent of Documents, U. S. Government

Office, Washington, DC 20402. 15 cents.

**See G. E. Hudson, Some Characteristics of Commonly Used Time Proceedings, Institute of Electrical and Electronic Engineers, Vol 6, pp. 815-821, June 1967.