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Standard Cells: Their Construction, Maintenance, and Characteristics NBS MONOGRAPH 84 Standard Cells Their Construction, Maintenance, And Characteristics U.S. DEPARTMENT OF COMMERCE NATIONAL BUREAU OF STANDARDS THE NATIONAL BUREAU OF STANDARDS The National Bureau of Standards is a principal focal point in the Federal Government for assur- ing maxinmm application of the physical and engineering sciences to the advancement of technology in industry and commerce. Us responsibilities include development and maintenance of the national standards of measurement, and the provisions of means for making measurements consistent with tliose standards; determination of physical constants and properties of materials; development of methods for testing materials, meciianisms, and structures, and making such tests as may be neces- sary, particularly for government agencies; cooperation in the establishment of standard practices for incorporation in codes and specifications; advisory service to government agencies on scientific and technical problems; invention and development of devices to serve special needs of the Government; assistance to industry, business, and consumers in the development and acceptance of commercial standards and simplified trade practice recommendations; administration of programs in cooperation w ith Lnited States business groups and standards organizations for the development of international standards of practice: and maintenance of a clearinghouse for the collection and dissemination of scientific, technical, and engineering information. The scope of the Bureau's activities is suggested in the following listing of its four Institutes and their organizational units. Institute for Basic Standards. Electricity. Metrology. Heat. Radiation Physics. Me- chanics. Applied Mathematics. Atomic Physics. Physical Chemistry. Laboratory Astrophysics.* Radio Standards Laboratory: Radio Standards Physics; Radio Standards Engineering.** Office of Standard Reference Data. Institute for -Materials Research. Analytical Chemistry. Polymers. Metallurgy. Inorganic Materials. Reactor Radiations. Cryogenics.** Office of Standard Reference Materials. Central Radio Propagation Laboratory. ** Ionosphere Research and Propagation. Tropo- sphere and Space Telecommunications. Radio Systems. Upper Atmosphere and Space Physics. Institute for Applied Technology. Textiles and Apparel Technology Center. Building Re- search. Industrial Equipment. Information Technology. Performance Test Development. Instrumentation. Transport Systems. Office of Technical Services. Office of Weights and Mea- sures. Office of Engineering Standards. Office of Industrial Services. *NBS Group, Joint Institute for Laboratory Astrophysics at the University of Colorado. **Located at Boulder, Colorado. UNITED STATES DEPARTMENT OF COMMERCE • Luther H. Hodges, Secretary NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director Standard Cells Their Construction, Maintenance, and Characteristics Walter J. Hamer National Bureau of Standards Monograph 84 Issued January 15, 1965 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C., 20402 Price 35 cents Library of Congress Catalog Card Number: 64-60065 Foreword Accurate measurement of electromotive force is important in many areas of science and technology. Physical standards for such measurement are provided by standard cells, which are long-lived electrochemical systems of highly stable electromotive force. This publication gives the origin and derivation of the unit of electromotive force and outlines the procedures by which the National Bureau of Standards maintains and disseminates this unit by means of standard cells. Information is also given on the construction, maintenance, and characteristics of standard cells as well as a history of their development. Emphasis is placed on the precision and accuracy of electromotive force measurements; the stability of standard cells, especially those of the National Reference Group; and efforts made to construct standard cells of high quedity. A. V. ASTIN, Director Library of Congress Catalog Card Number: 64-60065 Foreword Accurate measurement of electromotive force is important in many areas of science and technology. Physical standards for such measurement are provided by standard cells, which are long-lived electrochemical systems of highly stable electromotive force. This publication gives the origin and derivation of the unit of electromotive force and outlines the procedures by which the National Bureau of Standards maintains and disseminates this unit by means of standard cells. Information is also given on the construction, maintenance, and characteristics of standard cells as well as a history of their development. Emphasis is placed on the precision and accuracy of electromotive force measurements; the stability of standard cells, especially those of the National Reference Group; and efforts made to construct standard cells of high quality. A. V. ASTIN, Director Contents Foreword iii 1. Introduction 1 2. The unit of electromotive force 1 2.1. Realization ] 2.2. History 4 2.3. Maintenance 5 2.4. Dissemination 8 2.5. International comparisons 10 3. Early standard cells 11 4. The Clark cell 13 5. The Weston (or cadmium sulfate) cell 15 5.1. General 15 5.2. Preparation and properties of materials 16 5.3. Containers for standard cells 19 5.4. Assembly and mounting of standard cells 20 5.5. Electromotive forces of newly made cells 21 6. Effect of variations in components on the electromotive force of stand- ard cells 22 6.1. Concentration of solution 22 6.2. Acidity of solution 22 6.3. Composition of amalgam 23 6.4. Crystal phases of cadmium sulfate 23 7. Characteristics of standard cells 23 7.1. Emf-temperature coefficient 23 7.2. Emf-temperature hysteresis 26 7.3. Temperature range 27 7.4. Emf-pressure coefficient 27 7.5. Internal resistance 28 7.6. Effect of current 28 7.7. Effect of light 30 7.8. Effect of shock 30 7.9. Effect of vibration 30 8. Life of standard cells 30 9. References 31 10. Appendix 1 33 11. Appendix 2 34 12. Appendix 3 35 13. Appendix 4 35 14. Appendix 5 36 15. Appendix 6 37 16. Appendix 7 37 iv Standard Cells Their Construction, Maintenance, and Characteristics Walter J. Hamer This Monograph contains information on the construction, maintenance, and characteristics of standard cells. The effects of temperature, pressure, electric current, light, shock, and vibration on standard ceUs are discussed. A history of the realization and maintenance of the unit of electromotive force is also included. A record of international comparisons of the unit of electromotive force is pre- sented as well as information on the constancy of the National Reference Group of Standard Cells. 1. Introduction Standard cells are physical representations of ferent design from those electrochemical systems the unit of electromotive force (emf), serve in the which are intended for such purpose. Owing to maintenance of the unit, and are used as standards their special use, standard cells are required to with which the emf of other cells and systems and meet certain performance criteria and, for precise IR drops are compared. Together with standards measurements, to have certain inherent charac- of resistance {R) they are also used in the measure- teristics. They must be reasonably reproducible, ment of current, /. When measurements of electric exhibit good permanency, possess low emf-tem- power, P, are made in terms of standards for emf perature coefficients, have a low or moderately (£") and resistance, the expression for power, low internal resistance, be relatively insensitive P = E^IR, shows the necessity of knowing E ac- to current drains of low magnitude, and, if pos- curately, since a small error in the standard for E sible, have an emf of convenient magnitude. Since would produce a percentage error twice a» great a standard cell is a physical representation of a unit in the value for the power, P. it is obvious why permanency is of prime importance Standard cells are electrochemical systems com- in a standard cell. The precision with which the posed of two dissimilar electrodes immersed in an emf of standard cells is measured, accordingly, electrolytic solution. They are not intended to exceeds that normally required for other types of supply electric current and, therefore, are of dif- galvanic cells. 2. The Unit of Electromotive Force 2.1. Realization The practical unit of emf, the volt, is not an arbi- it was possible to measure electrical quantities trary one but Uke the other electrical units is de- in terms of mechanical units. Weber pointed out rived from the basic mechanical units of length, the desirability of making the electrical units con- mass, and time using the principles of electromag- sistent with those used in other branches of science netism with the value of the magnetic constant and engineering. (the so-called permeability of free space) taken as The electrical units determined in the cgs electro- 47r/10^ in the rationalized mksa (meter-kilogram- magnetic (em) system are of inconvenient size for second-ampere) system of units. ^ It has been practical use. For example. Sir William Thomson customary, following the first use of the term by (Lord Kelvin) [10] in 1851 showed that the emf of 10^ Gauss [9],^ to refer to electrical units based on the a Daniell cell was about 1 X cgs em units. Even basic units of length, mass, and time as absolute so, a Committee of the British Association for the electrical units. ^ The transition from arbitrary to Advancement of Science in 1873 [6] recommended
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