Lmwrs, G. N.,
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Coamsx, R. E., J. Am. Chem. Sac., 55, 2616
(1933).
W ~ s x s n n E. ~ .W.. SMITH. E. R.. Awn FRANDBEN, M.. J . Cham. P h w . KUB, R., AND KRAUBB, A,. Nohrrwias., 22. 119 (1934). DOBTROWKY. I., AND RAYIY,A,, ''Promedings of the International symposium on Isotope Separation'' (Edilora: Krs~sar*mn, J.. Bmmmsnn. J.. A N D NIER.A. 0. C.). Chaoter 26. North-Holland .
Publishing do..~m8terdam; 1958. Cr,oazns, K., Drcar~.G., A N D B ~ o x s aE., , Naturwiss.. 31, 210 (1943). Bmwrcx, H.. AND S C B ~ W., ~ EN&wwiss., , 24,667 (1936). WATBON. W. W.. SIMON, R., A N D W O E R N ~ YD., L.. Phw. Re"., 62. 558 (1942). B u o n ~ w *J. ~ .O., Phys. Rev.. 75, 1332 (1949). C ~ u s m sK.. , AND Mnren, H.. Hela. Chim.Ado.36.2045 (1953). C ~ ~ r s mK., s . S c ~ n M A c h ~E., n , H a n m ~ e nH.. , AND HOBTETTLBR. A. U., 2. Nofurlorsch.. I l a , 708 (1956). W o o ~ n m n aD. ~ , E.. AND SMYTHE, W. R., P h w . RN.. 50,233 (1936). UREY,H. C., ATEN.A. H. W.. JR..AND K B ~ T O A. N , S.. 3. Chem. Phy11,. 4, 622-3 (1936). nr; HEMPTINNE, M.. AND CAPRON. P., 3. PAW. Radium, 10, 171 (1939). Jams, T. F., A N D Lonoox, H.. "Enrichment of W *and '0 Isotopes by Frsotional Distillation of CO." AERE Harwell, Report G/R 661. 1951.
G. Socrates Brunel University London W3, England
'
A n internationally agreed system of units is now coming into general use. In 1960, the international authority on units, the Conference GBn&ale des Poids et Mesures, C.G.P.M., agreed to adopt the S y s t h e Internationale d'unities, or the International System of Units, the agreed abbreviation of which is S I in all languages. This system has also been endorsed by the International Organisation for Standardization, I.S.O., which is the international authority on the standardization of the names and symbols of physical quantities. Both the British Standards Institute, B.S.I., and the International Union of Pure and Applied Chemists, I.U.P.A.C., are members of the I.S.O. At present, some thirty countries have prepared or are preparing the necessary legislation to make S I the only legally acceptable classification of units. By 1975, S I units should very nearly be the only units of measurement employed in the United Kingdom. I t is therefore apparent that SI is destined to be in world-wide use within a few years. Shortly, the whole of Europe will be using only S I units, India and Japan have already changed over to SI. Australia, New Zealand, South Africa, and Pakistan are in the process of changing over. The United States and Canada are considering the desirability and practicability of changing to SI. It does seem very likely that they, too, will eventually change. It is also clear that once the International System of Units has been universally accepted it will be an exceedingly long time before any new system is introduced to replace it. Naturally, the earliest units of measurement which man employed were based on the physical dimensions of the human body. For example, the biblical unit, the cubit, was the length of the forearm from' the elbow to 710 / Journal of Chemical Education
(48) ROTXERFO~O. W. M.. AND KELLER,J. M.. J. Chem. Phua., 44, 723 (1966). , m n HLRTECK. P.. N a t w w i s ~ .28.47 . (1940). (49) G n o ~ nW.. (50) CLnmns. K.. nxo DIOIEG,G., 2. Physik. Cham.. B52, 348 (1942): 2. Physik. Cham.. B53, 178 (1943). K.. GROTR.W.. HARTECH, P.. JBNBBN, H. J. D.. B E ( I O E R O ~ , (51) BETEIILE. G . . FALTINOS, J., SUER.A,. A N D N A N N , E.. Chcm. Ing. Tech.; 21, Q*>
""L
,,O"O,
\A"T",.
C.. A N D W n ~ s o rW. , W., Phya. Rev., 104,202 (1956). Cbosms. K., A N D HOSTBTTLEB, H. U.,2. Nolurfar8ch.. 1%. 974 (1957). Gnorx, W.. AND HARTEOR, P., Nat(llwi88.. 27.390 (1939). C~usrus.K.. Helu.Phya. Ado. 22,473 (1949). CLUBIUB, K., BUHLER.H. H., HURZELER. H.. AND SCXOMAOAER, E.. 2. Naturforseh.. IOa, 809 (1955). HAVBACX, W. J.. AND GROVE, G . R., ''Stable Gaseous Isotope Beparatim and Purification, July-September 1966." AEC Report MLM1367. April 21. 1967. R n ~ ~ ' S n ~ 0 W. n o .M.. W B ~ E RF.. W.. A N D ECK,C. F., Re". S d . Insh.. 39.94 (January 1968). "Stable Gaseous Isotope Separation and Purification, July-September 1968." A E C Report MLM-1529. February 13. 1969. ''Stable Gaseous Isotope Sepsrstion and Punfloation, Apdl-June 1966," AECRepoit MLM-1353, February 10.1967. HECHTG, E.. A N D VIEHBOCK,F. P., Nuc1.Inslr. Mathefhods,48,323 (1967). Bmm, N.
SI Units the tip of the middle finger. The standard yard was first defined in the fifteenth century as the distance between a man's nose and the tip of the middle finger of his extended arm. These were convenient and readilyavailable units of measurement, even though obviously these specifications were not exact and varied from person to person. The Imperial system of units, which has hitherto been employed in the United Kingdom, was based on the standard yard and standard pound. This system is many centuries old and although it has been developed and extended to meet modern needs, it is an extremely complex system. The pound troy and avoirdupois were both introduced into the United Kingdom in the fourteenth century. By Act of Parliament, the standard yard is now specified as being the distance between two marks on a particular piece of metal bar, and the standard pound as being a particular mass of metal, both of which are in the safe keeping of the National Physical Laboratory. I n fact, there is a replica of the standard yard always on public exhibition in Trafalgar Square, London. The metric system of units was evolved on the advice of French scientists, just after the French revolution. The metric system, which is a denary system, was quickly adopted throughout most of Europe, not only in commerce and industry but also in science and technology. It was originally intended that the unit of length, the meter, should be one ten-millionth of the distance at sea level from the north pole to the equator-passing through Paris, of course. This definition was not practical, and later the meter was accurately specified as the distance between two marks on a particular metal bar kept at S h e s . The unit of mass, the gram, was originally specified as the mass of one cubic centimeter of water a t O°C. The kilogram was later
specified as being the mass of a particular piece of metal. Time has been measured in terms of the four seasons, the phases of the moon, and the daily rotation of the earth. Ingenious devices such as the sun-dial and hourglass were employed for many centuries. At present, a very wide variety of units exists throughout the world. It is only in these recent times that international attempts have been made to standardize the units of measurement. Of course, with the development of more advanced and sophisticated human activities, the need for more and more precise measurements has been made greater. I n this respect, a major disadvantage of the older units is that they are not in any way derivable from scientific formulas or natural constants, whereas most of the basic SI units may be so obtained. For example, instead of defining the meter as the distance between two marks on a particular metal bar, it will now he defined as the length equal to 1650 763.73 wavelengths in vacuum of the radiation corresponding to the transition between the levels 271s and 5ds of the krypton-86 atom. Similarly, the unit of time, the second, will now he defined as the duration of 9 192 631 770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom. On the other hand, the kilogram, the unit of mass, remains equal to the mass of the international prototype of the kilogram kept a t S h e s . It is important to note that the unit of mass is therefore not defined in terms of physical constants. It seems quite possible that the basic unit of mass now known as the kilogram will be renamed the "giorgi," symbol G. This is a unique opportunity to standardize the units of measurement throughout the world. Of course, some units which are in everyday use will remain so, for instance, minute, hour, day, liter, degree Celsius, degree (angle), minute (angle), etc. However, none of these units are to be employed in any scientific context. It should be noted that the liter is now regarded as a special name for the cubic decimeter and no longer according to the old definition as being equal to 1.000028 dma. The term liter should not he used to express results of high precision. At present, there are non-SI units which are to be allowed in scientific studies in conjunction with S I units, such a scnrie, electronvolt, gauss, bar, and barn. It is merely because of their extremely common use that these units are to be allowed. However, their use is to be discouraged and, with time, phased out.
Basic Units The following are the basic units of the S I system. Physical Quanliiy length ma88 time electric current temperature luminous intensity plane angle solid angle
Unit meter kilogram second ampere kelvin candela radian steradian
Symbol m kg
1K ed rad sr
The mole has been recommended by the I.S.O. as a basic unit but this remains to be accepted by the C.G.P.M.
Derived Units Derived units with special names are as follows. Physical Quantity energy force power electric charge electricsl resistance electrical potentid difference electric capacitance magnetic flux magnetic flux density inductttnce luminous flux illumination frequency
Name joule newton watt coulomb ohm
Symbol
J N W C
n
V F Wb T H Im Ix Hz
volt farad weber teala henry lumen lux hertz
Prefixes I t is recommended that prefixes to be used shauld differ from a unit in steps of loa. Prefix tera mega kilo 'hecto 'deca "deci *centi milli micro nano piw femto alto
Symbol T
Fador
G M
loQ 10' 108 10' 10' 10-
k b da. d o
m M
n P f a
lo'1
lo-¶ lo-' lo-* 10-lz
Ezamples
1 000 000 Y = 1 megavolt = 1MV 0.000 000 001 s = 1 nanosecond = 1ns 1000 M = 1 kilometer = 1 km
Special Notes on the Use of 51 Only the singular form of units is to be used, e.g., km not kms Full stops a t the end of abbreviations are to be omitted, e.g., km not km. The decimal sign is to be a point or a comma on the line, e.g., 2.5 or 2,5 andnol2.5 Digits shauld be grouped in threes about the decimal point in order to facilitate the readine of lone numbers. Cammas shauld not be used to space digits in numben, e.g., 16 543 211.133 45 not 16,543,211.133,45 The degree sign is to he omitted when the kelvin scale is employed, e.g., 273K and not 273°K The solidus isnot to he encouraged, but where used, nomore than one salidus should be employed, e.g., J/kg K not J/kg/K
Only one prefix should be used for a. given unit. Ezamples 10-% 1 000 000 m 1000 kg 10-0 kg
= 1 ns not 1 mps = 1 Mm not 1 kkm = 1 Mg not 1 kkg = lpg not 1 nkg
The power to which a unit is raised applies to the whole unit including the prefix. Examples kma = (km)' = (1000 m)l = 10emPnot 1000 ma pma = (prn)= = (10-'m)* = 10-l8 m%ot 10- ma
Volume 46, Number 1 1, November 1969
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71 1
Physical Constants
Conversions Physical Quantity length
area volume ma3S force
pressure
energy
power radioactivity magnetic flux density kinematic viscosity dynamic viscosity
Unit mile *nmtieal mile, international *barn *square inch cubic inch 'pound *dyne poundd pound-force *kilogram-force 'atmos~here 'bar torr psi. *calorie international calorie 15'C 'cdorie thermochemical B.t.u. electmnvolt *ere u horse power *curie
SI Equivalent 10-lo m 0.0254m 1.609 34 km
An asterisk indicates that the convenions are exact.
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Journal of Chemical Education
Gas constant Molar volume of ideal gss st 101 325 Nm:s (1 atm) and 273.15 K Avogadro constant F a r d a y constant Planck constant velority of light char1:e of electron mas:: of electron
Further Reading Suggested further reading in connection with S I Units is (1) ANDERTON, P., AND BIGG,P. H., "Changing to the Metric System," (3rd ed.), H.M.S.O., London, 1969. (2) MCGLASHAN, M. L., "Physico-Chemical Quantities and Units," The Royal Institute of Chemistry, London, 1968. (3) SOCRATES, G. AND SIPPER,L. J., "SI and Metrication Conversion Tables," Newnes-Butterworth, London, 1969. Publications giving - the latest values of constants are reference (3)and , (1963). (4) J. &EM. E ~ u c .40,642 (5) Information Bulletin No. 32, I.U.P.A.C., August (1968)
