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  • 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

  • ContentsPaji
  • 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 the unit of electromotive force (emf), serve in the maintenance of the unit, and are used as standards with which the emf of other cells and systems and IR drops are compared. Together with standards of resistance (/?) they are also used in the measure ment of current, /. When measurements of electric power, P, are made in terms of standards for emf (E) and resistance, the expression for power, P = E 2/R, shows the necessity of knowing E ac curately, since a small error in the standard for E would produce a percentage error twice a& great in the value for the power, P.

    Standard cells are electrochemical systems com posed of two dissimilar electrodes immersed in an electrolytic solution. They are not intended to supply electric current and, therefore, are of dif

    ferent design from those electrochemical systems which are intended for such purpose. Owing to their special use, standard cells are required to meet certain performance criteria and, for precise measurements, to have certain inherent charac teristics. They must be reasonably reproducible, exhibit good permanency, possess low emf-tem- perature coefficients, have a low or moderately low internal resistance, be relatively insensitive to current drains of low magnitude, and, if pos sible, have an emf of convenient magnitude. Since a standard cell is a physical representation of a unit it is obvious why permanency is of prime importance in a standard cell. The precision with which the emf of standard cells is measured, accordingly, exceeds that normally required for other types of galvanic cells.

    2. The Unit of Electromotive Force 2.1. Realization

    The practical unit of emf, the volt, is not an arbi trary one but like the other electrical units is de rived from the basic mechanical units of length, mass, and time using the principles of electromag- netism with the value of the magnetic constant (the so-called permeability of free space) taken as 47T/107 in the rationalized mksa (meter-kilogram- second-ampere) system of units. 1 It has been customary, following the first use of the term by Gauss [9],2 to refer to electrical units based on the basic units of length, mass, and time as absolute electrical units. 3 The transition from arbitrary to absolute units began with the work of Gauss [9] in 1833 and of Weber [2] in 1851, who showed that

    1 This is the Giorgi system [1J, which is a part of the Sys&me International cTUnitts (SI), adopted in a resolution, llth General Conference on Weights and Measures, Paris, October 1960. Other systems for the basic units of length, mass, and time have been used. These include the millimeter-milligram-second system of Weber [2], the meter-gram-second system recommended by the first committee of the British Association for the Advancement of Science appointed to consider electrical units [31, the quadrant-(eleventh-gram)-second or Q.E.S. system of Maxwell [4), the foot-grain- second system used in England for a time [5], and the cgs (centimeter-gram-second system [6] (see Appendix 7). Heaviside also proposed a rationalized system wherein the cgs unit of emf was increased by a factor V4rr (unit of resistance increased by a factor 4ir and the unit of current decreased by the factor V4rr [7]). In all of these, the Giorgi system excepted, the magnetic constant (the permeability of free space) is taken as unity. Regardless of which system is chosen the practical system remains unaffected. Additional information on electrical units is given in reference {8],

    2 Figures in brackets indicate the literature references on page 31.3 It is unfortunate that the name "absolute" has persisted. It is sometimes wrongly

    interpreted to imply that there are no errors involved in the measurements or that per fection has been attained.

    it was possible to measure electrical quantities in terms of mechanical units. Weber pointed out the desirability of making the electrical units con sistent with those used in other branches of science and engineering.

    The electrical units determined in the cgs electro magnetic (em) system are of inconvenient size for practical use. For example, Sir William Thomson (Lord Kelvin) [10] in 1851 showed that the emf of a Daniell cell was about IXlO8 cgs em units. Even so, a Committee of the British Association for the Advancement of Science in 1873 [6] recommended the cgs em system for use both in basic science and practical engineering. However, the practitioners, although they agreed with the desirability of re lating electrical units to mechanical ones, objected to the use of the cgs em system of units in practice both because of the magnitudes involved and be cause they had been using such terms as ohms and volts for their units. Their views prevailed and in 1881 the International Congress of Electricians [11] meeting in Paris adopted the cgs em system of units as the fundamental system and the volt- ohm-ampere system for practical use, with the practical units being made larger or smaller than the correspon