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Gravity Measurements and the Standards Laboratory L..,.lldlU!> NBS TECHNICAL NOTE 491 Gravity Measurements II And the Standards Laboratory T 0F /*t* U.S. DEPARTMENT OF COMMERCE ,2.7/ Z ^ National Bureau of Standards Q \ *"»fAU 0» ' - NATIONAL BUREAU OF STANDARDS The National Bureau of Standards ' was established by an act of Congress March 3, 1901. Today, in addition to serving as the Nation's central measurement laboratory, the Bureau is a principal focal point in the Federal Government for assuring maximum application of the physical and engineering sciences to the advancement of technology in industry and commerce. To this end the Bureau conducts research and provides central national services in four broad program areas. These are: (1) basic measurements and standards, (2) materials measurements and standards, (3) technological measurements and standards, and (4) transfer of technology. The Bureau comprises the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology, the Center for Radiation Research, the Center for Computer Sciences and Technology, and the Office for Information Programs. THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United States of a complete and consistent system of physical measurement; coordinates that system with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and com- merce. The Institute consists of an Office of Measurement Services and the following technical divisions: Applied Mathematics—Electricity—Metrology—Mechanics—Heat—Atomic and Molec- ular Physics—Radio Physics -—Radio Engineering '-—Time and Frequency -—Astro- physics -—Cryogenics. THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to im- proved methods of measurement standards, and data on the properties of well-characterized materials needed by industry, commerce, educational institutions, and Government; develops, produces, and distributes standard reference materials; relates the physical and chemical prop- erties of materials to their behavior and their interaction with their environments; and provides advisory and research services to other Government agencies. The Institute consists of an Office of Standard Reference Materials and the following divisions: Analytical Chemistry—Polymers—Metallurgy—Inorganic Materials—Physical Chemistry. THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to promote the use of available technology and to facilitate technological innovation in industry and Gov- ernment; cooperates with public and private organizations in the development of technological standards, and test methodologies; and provides advisory and research services for Federal, state, and local government agencies. The Institute consists of the following technical divisions and offices: Engineering Standards—Weights and Measures — Invention and Innovation — Vehicle Systems Research—Product Evaluation—Building Research—Instrument Shops—Meas- urement Engineering—Electronic Technology—Technical Analysis. THE CENTER FOR RADIATION RESEARCH engages in research, measurement, and ap- plication of radiation tjo the solution of Bureau mission problems and the problems of other agen- cies and institutions. The Center consists of the following divisions: Reactor Radiation—Linac Radiation—Nuclear Radiation—Applied Radiation. THE CENTER FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides technical services designed to aid Government agencies in the selection, acquisition, and effective use of automatic data processing equipment; and serves as the principal focus for the development of Federal standards for automatic data processing equipment, techniques, and computer languages. The Center consists of the following offices and divisions: Information Processing Standards—Computer Information — Computer Services — Sys- tems Development—Information Processing Technology. THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and accessibility of scientific information generated within NBS and other agencies of the Federal government; promotes the development of the National Standard Reference Data System and a system of information analysis centers dealing with the broader aspects of the National Measure- ment System, and provides appropriate services to ensure that the NBS staff has optimum ac- cessibility to the scientific information of the world. The Office consists of the following organizational units: Office of Standard Reference Data—Clearinghouse for Federal Scientific and Technical Information :1 —Office of Technical Information and Publications—Library—Office of Public Information—Office of International Relations. Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing: address Washington. D.C. 20234. : Located at Boulder. Colorado 80302. 1 Located at 5285 Port Royal Road, Springfield, Virginia 22151. UNITED STATES DEPARTMENT OF COMMERCE Maurice H. Stans, Secretary NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director NBS TECHNICAL NOTE 491 ISSUED AUGUST 1969 Nat. Bur. Stand. (U.S.), Tech. Note 491, 10 pages (Aug. 1969) CODEN: NBTNA Gravity Measurements and the Standards Laboratory D. R. Tate Mechanics Division Institute for Basic Standards National Bureau of Standards Washington, D.C. 20234 NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be listed or referred to in the open literature. For sale by the Superintendent of Documents, Government Printing Office Washington, D.C, 20402 - Price 25 cents Contents Page Abstract ------------------------- 1 1. Introduction --------------------- 1 2. Gravity Networks ------------------- 1 3. Absolute Measurements ---------------- 2 4. Survey Measurements ----------------- 3 5. Local Values of Gravity --------------- 4 6. Standard Gravity ------------------- 4 7. The Computation of Forces -------------- 5 8. Summary ---------- _____________ 5 9. References -----_-- _____________ 7 GRAVITY MEASUREMENTS AND THE STANDARDS LABORATORY D. R. Tate The local value of the acceleration due to gravity is a fundamental datum for almost every standards laboratory as it, together with accurate standards of mass, is the basis for the standards involving force. Instruments used as standards in this area include precise deadweight piston gages, deadweight calibrators for force transducers, liquid manometers, and earth field accelerometer calibrators. The practical realization of the absolute ampere and the absolute volt require a knowledge of force. This paper presents the basic information about how gravity measurements are made and outlines procedures for obtaining a suitable value for a given location. It also gives a brief discussion of the background and meaning of the term "standard gravity", and its application in the computation of forces in units of the pound-force and the kilogram-force. Key words: Absolute gravity, deadweight, force, geodetic pendulum, gravity, gravity meter, Potsdam system, standard gravity, units of force. 1. Introduction The local value of the acceleration due to gravity is a matter of considerable importance to almost every standards laboratory as it, to- gether with accurate standards of mass, is the basis of all force measurements. Instruments for the measurement of acceleration, acceler- ometers, have also reached a stage where, for earth field calibrations, the acceleration due to gravity must be known quite accurately. It is the purpose of this paper to present the basic information about how gravity measurements are made and to outline the procedures for obtain- ing a suitable value for a given location. 2. Gravity Networks Gravity measurements have been made by geodesists for many years and a well developed science, with instrumentation and technique, has grown up in the field. Primarily the interest of the geodesist is to gain a better knowledge of the shape and structure of the earth, and the work that has been done has resulted in a network of gravity values . covering the greater portion of the habitable parts of the world. This network, with its excellent coverage in Europe and the Americas, affords a ready-made basis for obtaining satisfactory values in most labora- tories . It is helpful to understand the methods by which gravity networks have been established. The direct measurement of the acceleration due to gravity through the observation of time and distance is a difficult undertaking and not well adapted to survey methods. Consequently the principle has been to establish an absolute value at some one point and to make the survey measurements with instruments that measure the difference in gravity between two points, starting at the absolute site and moving out in the closed-loop technique commonly employed in other types of surveys. The network values in existence today are derived from an absolute measurement made in Potsdam, Germany. 3. Absolute Measurements Absolute determinations of the acceleration due to gravity, in addition to being difficult and time consuming, have in the past been subject to serious systematic errors which were large when compared to the accuracy of differential measurements made with survey type instruments. Consequently it has been desirable to make additional absolute measurements at various points in the world network, even
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