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 maximum application of the physical and engineering sciences to the advancement of technology in industry and commerce. Its responsibilities include development and maintenance of the national standards of measurement, and the provisions of means for making measurements consistent with those standards; determination of physical constants and properties of materials; development of methods for testing materials, mechanisms, 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 with United 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 quality. A. V. ASTIN, Director 111 Contents Paji<' Foreword..................................................................................... iii 1. Introduction............................................................................. 1 2. The unit of electromotive force.................................................... 1 2.1. Realization..................................................................... 1 2.2. History........................................................................... 4 2.3. Maintenance................................................................... 5 2.4. Dissemination.................................................................. 8 2.5. Internationa] 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. Hairier 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 cells 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 (/?) 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 (E) and resistance, the expression for power, low internal resistance, be relatively insensitive P = E 2/R, 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