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The definition of the Langley
SolarEnergy.Vol. 18.pp. 369-370. PergamonPress1976. Printedin GreatBritain
TECHNICAL NOTE The definition of the Langley W. G. DELINGER Department of Physics, Northern Arizona University, Flagstaff, AZ 86001, U.S.A.
(Received 5 June 1975; in revised form 18 February 1976) INTRODUCTION The langley is often used as the unit to express solar radiation energy area--density. This unit was named in honor of Samuel Pierpont Langley (1834-1906) who was secretary of the Smithsonian Institution from 1887 to 190611]. During his tenure at the Smithsonian, Langley performed early solar spectrum studies and established, through high altitude measurements, preliminary experimental limits for the solar constant [2, 3]. Many years after Langley's death, C. G. Abbot, also a secretary of the Smithsonian Institution, suggested the name "langley" be used to represent a solar radiation unit[4]. Such a unit was introduced in 1942 by Linke[5] as 1 langley= lcalcm-2min l; but later, in 1947, a proposal was made by Aldrich et al.[6] to change the dimensions to 1 langley= l c a l c m 2 With the increased use of the International (SI) Metric System[7,8] in scientific work, the langley is gradually being replaced by its equivalent combination of metric units. The langley is still a popular solar radiation unit, however, and it is hoped that this note will help to clarify its definition. First, consider a brief review of the calorie. DEFINITION OF CALORIE The langley as proposed in 1947 was defined in terms of the 15°cal as 1 langley= l cal (fifteen-degree)cm-2[6]. The word "fifteen" in this case is used to convey the fact that this calorie is measured over a specific 1° temperature interval. One 15° cal. is the quantity of heat required to raise the temperature of 1 g, of air-free water from 14.5 to 15.5°C at a constant pressure of 1 (standard)atm. The 15°cal. was adopted by the International Union of Pure and Applied Physics in 1934 and by the Confgrence G6nrrale des Poids et Mrsures in 1950[9]. This latter organization considered the 15°cal. as being the most accurately-determined experimental calorie. The calorie that was often used in very early solar energy work, however, was the mean calorie. The mean calorie is an average over a 100° temperature range. One mean calorie is one-hundredth of the quantity of the heat required to raise the temperature of 1 g of air-free water from 0 to 100°C at a constant pressure of 1 (standard)atm. Normally, heat measurements are referred to work measurements by electrical means. To avoid having to recalculate a large quantity of thermal data every time a new mechanical equivalent of heat is determined, it is standard practice to define the calorie exactly in terms of the joule. The International Table calorie, for example, was set by formal agreement at 1 cal (International Table) = 4.1868 J, exactly. This definition was adopted by the Fifth International Conference on the Properties of Steam (London, July 1956) and was generally regarded as the best definition of the calorie [9]. Similarly, the calorie used in the field of calorimetry as applied by physicists and chemists is called the thermochemical calorie; it has been set at I cal. (thermochemical) = 4.1840 J, exactly[10]. These exact values were chosen so as to keep most of the older thermal data still valid. This now means, of course, that I cal. will not necessarily raise the temperature of 1 g of water exactly 1° in the temperature interval in which it was originally measured; but the calorie has been defined in terms of the SI unit of energy, the joule. All of these preceding definitions for the various calories have been conveniently summarized by Dresner[9]. 369
CONCLUSION The use of 1 cal. in preference to another when converting langleys to other units or vice versa will lead to very small differences. In fact, the differences are only of the order of 0.1 per cent and many times the experimental data are not that accurate. Nevertheless, it seems desirable to have a precise conversion between the langley and the metric units. A survey of the literature shows there are many different conversion factors being used at present. On p. 25 of Duffle and Beckman's book[ll], for instance, the relationship is given as 1 langley = 4.186 J cm -2. Here, they appear to be using the 15° cal. because the experimentally derived value is l cal. (15°)= 4.1858J[8]. For comparison, Appendix A of the NASA publication[12] shows 1 langley=lcal. (mean)cm 2: l cal. (mean) = 4.19002 J. Still other authors give the conversion in terms of the International Table calorie[9, 13] or the thermochemical calorie[8, 14-16]. To eliminate the ambiguity which now exists with regards to these conversion factors, it is suggested that the langley be expressed in terms of the thermochemical calorie in conformity with Refs. [8, 14-16]. Therefore, it is proposed that one langley be defined as a unit of solar radiation energy-density which is equal to one thermochemical calorie of heat energy over one square centimeter of area or in terms of SI units, 1 langley= 4.184 × 104J m -z, exactly. This is the definition that was used in the past by Eppley Laboratories, a solar radiation standards laboratory. Eppley, however, no longer uses the unit of the langley[17]. It should also be noted that, at present, when the solar constant is given in terms of the calorie, the thermochemical calorie is generally used. Thekaekara[12], for example, gives the solar constant at 1.940 cal. (thermochemical)cm 2 rain '. Using the definition proposed in this paper, this is the same as 1.940 langley min-'.
Acknowledgements--The author wishes to thank Dr. John S. Hall and Dr. William R. Willis for their valuable comments on this manuscript. Thanks are also given to Miss Jacque Hayden and to the Smithsonian Institution library staff for their work in locating references. The author gratefully acknowledges the helpful correspondance from Dr. R. P. Hudson of the National Bureau of Standards Heat Division. REFERENCES 1. P. H. Oehser, Sons of Science, The Story of the Smithsonian. Greenwood Press, New York (1968). 2. C. G. Abbot, The Solar Constant of Radiation. Annual Report Smithsonian Institution, U.S. Gov. Printing Office, Washington, D.C. (1910). 3. K. L. Coulson, Solar and Terrestrial Radiation. Academic Press, New York (1975). 4. B. Goldberg, Smithsonian Institution, private communication (1976). 5. F. Linke, Handb. Geophys. 8, 30 (1942). 6. L.B. Aldrich, H. Wexler, S. Fitz, I. F. Hand, A. Court and W. P. Mellen, Science 1 ~ , 225 (1947). 7. C. H. Page and P. Vigoureux, (Ed.), The International System of Units (SI), a translation of Le Syst~me International
370
W.G. DELINGER
d'Unit6s (SI) Nat'l. Bur. Stds. Spec. Publ. 330 (1974). 8. R. A. Hopkins, The International (SI) Metric System and How It works, 3rd Edn. Polymetric Services, AMJ Publishing, Tarzana, Calif. (1975). 9. S. Dresner, Units of Measurement, An Encyclopaedic Dictionary of Units both Scientific and Popular and the Quantities they Measure. Harvey Miller and Medcalf, Aylesbury, England (1971). 10. E. U. Condon and H. Odishaw (Ed.), Handbook of Physics. McGraw-Hill, New York (1%7). 11. J. A. Duffle and W. A. Beckman, Solar Energy Thermal Processes. Wiley-Interscience, New York (1974).
12. M. P. Thekaekara, Solar Electromagnetic Radiation. NASA SP-8005 (1971). 13. Metric System Guide, Vol. V--Metric Definitions and Terminology. Keller and Associates, Neenah, Wisc. (1975). 14. E. A. Mechtly, The International System of Units, Physical Constants and Conversion Factors. NASA SP-7012 (1973). 15. Metric Practice Guide. ASTM Publ. No. E280-74 (1974). 16. F. Daniels, Direct Use of the Sun's Energy. Yale University Press, New Haven, Conn. (1%4). 17. J. Hickey, Eppley Laboratories, private communication (1976).
TECHNICAL NOTE The definition of the Langley W. G. DELINGER Department of Physics, Northern Arizona University, Flagstaff, AZ 86001, U.S.A.
(Received 5 June 1975; in revised form 18 February 1976) INTRODUCTION The langley is often used as the unit to express solar radiation energy area--density. This unit was named in honor of Samuel Pierpont Langley (1834-1906) who was secretary of the Smithsonian Institution from 1887 to 190611]. During his tenure at the Smithsonian, Langley performed early solar spectrum studies and established, through high altitude measurements, preliminary experimental limits for the solar constant [2, 3]. Many years after Langley's death, C. G. Abbot, also a secretary of the Smithsonian Institution, suggested the name "langley" be used to represent a solar radiation unit[4]. Such a unit was introduced in 1942 by Linke[5] as 1 langley= lcalcm-2min l; but later, in 1947, a proposal was made by Aldrich et al.[6] to change the dimensions to 1 langley= l c a l c m 2 With the increased use of the International (SI) Metric System[7,8] in scientific work, the langley is gradually being replaced by its equivalent combination of metric units. The langley is still a popular solar radiation unit, however, and it is hoped that this note will help to clarify its definition. First, consider a brief review of the calorie. DEFINITION OF CALORIE The langley as proposed in 1947 was defined in terms of the 15°cal as 1 langley= l cal (fifteen-degree)cm-2[6]. The word "fifteen" in this case is used to convey the fact that this calorie is measured over a specific 1° temperature interval. One 15° cal. is the quantity of heat required to raise the temperature of 1 g, of air-free water from 14.5 to 15.5°C at a constant pressure of 1 (standard)atm. The 15°cal. was adopted by the International Union of Pure and Applied Physics in 1934 and by the Confgrence G6nrrale des Poids et Mrsures in 1950[9]. This latter organization considered the 15°cal. as being the most accurately-determined experimental calorie. The calorie that was often used in very early solar energy work, however, was the mean calorie. The mean calorie is an average over a 100° temperature range. One mean calorie is one-hundredth of the quantity of the heat required to raise the temperature of 1 g of air-free water from 0 to 100°C at a constant pressure of 1 (standard)atm. Normally, heat measurements are referred to work measurements by electrical means. To avoid having to recalculate a large quantity of thermal data every time a new mechanical equivalent of heat is determined, it is standard practice to define the calorie exactly in terms of the joule. The International Table calorie, for example, was set by formal agreement at 1 cal (International Table) = 4.1868 J, exactly. This definition was adopted by the Fifth International Conference on the Properties of Steam (London, July 1956) and was generally regarded as the best definition of the calorie [9]. Similarly, the calorie used in the field of calorimetry as applied by physicists and chemists is called the thermochemical calorie; it has been set at I cal. (thermochemical) = 4.1840 J, exactly[10]. These exact values were chosen so as to keep most of the older thermal data still valid. This now means, of course, that I cal. will not necessarily raise the temperature of 1 g of water exactly 1° in the temperature interval in which it was originally measured; but the calorie has been defined in terms of the SI unit of energy, the joule. All of these preceding definitions for the various calories have been conveniently summarized by Dresner[9]. 369
CONCLUSION The use of 1 cal. in preference to another when converting langleys to other units or vice versa will lead to very small differences. In fact, the differences are only of the order of 0.1 per cent and many times the experimental data are not that accurate. Nevertheless, it seems desirable to have a precise conversion between the langley and the metric units. A survey of the literature shows there are many different conversion factors being used at present. On p. 25 of Duffle and Beckman's book[ll], for instance, the relationship is given as 1 langley = 4.186 J cm -2. Here, they appear to be using the 15° cal. because the experimentally derived value is l cal. (15°)= 4.1858J[8]. For comparison, Appendix A of the NASA publication[12] shows 1 langley=lcal. (mean)cm 2: l cal. (mean) = 4.19002 J. Still other authors give the conversion in terms of the International Table calorie[9, 13] or the thermochemical calorie[8, 14-16]. To eliminate the ambiguity which now exists with regards to these conversion factors, it is suggested that the langley be expressed in terms of the thermochemical calorie in conformity with Refs. [8, 14-16]. Therefore, it is proposed that one langley be defined as a unit of solar radiation energy-density which is equal to one thermochemical calorie of heat energy over one square centimeter of area or in terms of SI units, 1 langley= 4.184 × 104J m -z, exactly. This is the definition that was used in the past by Eppley Laboratories, a solar radiation standards laboratory. Eppley, however, no longer uses the unit of the langley[17]. It should also be noted that, at present, when the solar constant is given in terms of the calorie, the thermochemical calorie is generally used. Thekaekara[12], for example, gives the solar constant at 1.940 cal. (thermochemical)cm 2 rain '. Using the definition proposed in this paper, this is the same as 1.940 langley min-'.
Acknowledgements--The author wishes to thank Dr. John S. Hall and Dr. William R. Willis for their valuable comments on this manuscript. Thanks are also given to Miss Jacque Hayden and to the Smithsonian Institution library staff for their work in locating references. The author gratefully acknowledges the helpful correspondance from Dr. R. P. Hudson of the National Bureau of Standards Heat Division. REFERENCES 1. P. H. Oehser, Sons of Science, The Story of the Smithsonian. Greenwood Press, New York (1968). 2. C. G. Abbot, The Solar Constant of Radiation. Annual Report Smithsonian Institution, U.S. Gov. Printing Office, Washington, D.C. (1910). 3. K. L. Coulson, Solar and Terrestrial Radiation. Academic Press, New York (1975). 4. B. Goldberg, Smithsonian Institution, private communication (1976). 5. F. Linke, Handb. Geophys. 8, 30 (1942). 6. L.B. Aldrich, H. Wexler, S. Fitz, I. F. Hand, A. Court and W. P. Mellen, Science 1 ~ , 225 (1947). 7. C. H. Page and P. Vigoureux, (Ed.), The International System of Units (SI), a translation of Le Syst~me International
370
W.G. DELINGER
d'Unit6s (SI) Nat'l. Bur. Stds. Spec. Publ. 330 (1974). 8. R. A. Hopkins, The International (SI) Metric System and How It works, 3rd Edn. Polymetric Services, AMJ Publishing, Tarzana, Calif. (1975). 9. S. Dresner, Units of Measurement, An Encyclopaedic Dictionary of Units both Scientific and Popular and the Quantities they Measure. Harvey Miller and Medcalf, Aylesbury, England (1971). 10. E. U. Condon and H. Odishaw (Ed.), Handbook of Physics. McGraw-Hill, New York (1%7). 11. J. A. Duffle and W. A. Beckman, Solar Energy Thermal Processes. Wiley-Interscience, New York (1974).
12. M. P. Thekaekara, Solar Electromagnetic Radiation. NASA SP-8005 (1971). 13. Metric System Guide, Vol. V--Metric Definitions and Terminology. Keller and Associates, Neenah, Wisc. (1975). 14. E. A. Mechtly, The International System of Units, Physical Constants and Conversion Factors. NASA SP-7012 (1973). 15. Metric Practice Guide. ASTM Publ. No. E280-74 (1974). 16. F. Daniels, Direct Use of the Sun's Energy. Yale University Press, New Haven, Conn. (1%4). 17. J. Hickey, Eppley Laboratories, private communication (1976).