A Brief History of the Electrical Units to 1964

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A Brief History of the Electrical Units to 1964

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met o:og aA Brief History of the Electrical Units to 1964

B. W . Petley

Abstract. This paper charts the history of the electrical units up to 1964: from conception through the CGS to theSI via the M KSA system. It incorporates a shortened version of IEC Publication 164 which was published in 1964.

Sections One, Two and Three of IEC Publication 164,which was originally published in 1964, are reproducedin this issue of Metrologia, at the suggestion ofAnders Thor, by kind permission of the InternationalElectrotechnical Commission. The Publication, ofcourse, focuses primarily on electrical matters, andso misses the more muted debate of the 1935-1964era concerning the thermal quantities and other units.It also leaves out much of the electrical debate andcontroversy of that period and consequently some ofthe individual contributions by eminent scientists havealso been lost.At the beginning of the 1930s the necessity fora better system of units had long been recognized.Quite eminent metrologists made mistakes in theirpublications in converting between the emu, esu andpractical units. It was even more of a nightmarefor students: a regular pitfall was the use of thefactor 300 to convert between electrostatic volts andvolts (instead of 299,792 458)! It is conse quentlysurprising that it took so much hard debate andpersuasion to arrive at a compromise between whatwere then two conflicting points of view. IEC 164charts part of the extensive interactions betweenthe International Electrotechnical Commission and theS.U.N. Comm ission of the International Union of Pureand A pplied Physics (which has since been incorporatedinto its successor, the SUN-AMCO Commission).By the time of the 8th Confkrence GCnCrale desPoids et Mesures (CGPM) in 1933, there was aunanimous desire to replace the international electricalunits by the so-called absolute electrical units.Discussions between the two Commissions continuedand a compromise solution was reached by the end ofthe 1930s. Unfortunately, the process of implementingtheir solution internationally was interrupted by globalconflict and the consequent delay inevitably led tosecond thoughts and changes of detail.It is salutary to be reminded that in 1935 thefour-dimensional Giorgi system was known to the

B.W. Petley: Centre for Basic Metrology, National PhysicalLaboratory, Teddington, Middlesex TW11 OLW, UK .

Committee cognoscenti as the MKSX system, whereX was still to be decided. Unfortunately, IEC 164 doesnot reveal how it was decided that the ampere wouldreplace the unknown X of the MKSX system, or howthe rather convoluted SI definition of the ampere wasarrived at.* It is interestin g, however, that the suc cess ofthe SI has been so complete that one of the alternativesystems of units debated in 1935, the Dellinger-B ennettsystem (which was based on the centimetre, lo- gram,second), is almost completely unknown today.Also interesting is that some of the electricalproblems of the MKS system are still waiting in thewings of the SI for a solution. The oersted, for example(the CGS unit of magnetic field strength), was notintroduced into the CGS system until 1930. It mayseem surprising to us today that scientists could haveseen magnetic flux density and magnetic field strengthas being measurable by a common unit, and arguedtheir case so strongly. There is, how ever, still no namefor the SI unit of m agnetic field strength (ampere/metre)and, although it should be clear enough in the SI, manyusers are still sloppy about making this distinction. Themodem technological literature is full of references toa magnetic field of so many tesla, a reminder of thecentury-long confusion between mass and weight: itseems that some things never change! Another earlierelectrical problem (the MKSX) remains close to thesurface of the SI for, despite the definition of the ohmas a derived unit in the SI of today, it is clear that theSI definition adopted for the ampere cunningly allowsus to realize the ohm without physical recourse to theless accurately realized ampere.Today we increasingly use atomic quantities todefine, maintain, or disseminate the SI units. It istherefore useful to be reminded that the internationalampere defined at the turn of the century incorporatedthe electrochemical equivalent of silver (which is

* Paul Vigoureux (personal communication, 1995), recalls themeeting in 1935 when he was with J. E. Sears (NPL) and J . C.MacLenan (Toronto) in a Paris hotel and the finishing toucheswere put to the definition of the ampere which was adoptedby the CCE that same afternoon. Sadly, after the meetingMacLenan was found dead on the train at the Gare du Nord.48 1


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closely related to the Faraday constant), and to lookagain at the problems that this caused metrology in the1930s. One should avoid drawing parallels, however,for the agreed definitions for the Josephson and vonKlitzing constants have been very carefully framed soas to avoid the pitfalls which arose from the adoptionof the international electrical units.IEC 164 certainly serves to whet our appetite formore. For example, it tantalizingly leaves the modemreader wondering why international opinion inclinedtowards one solution rather than an earlier favouredone: in this, the longevity of service of individualson international committees may well be an importantfactor. Earlier compromises are very much part of theSI of today. Derived units such as the siemens, newtonand hertz, which were adopted as units by the IEC

~ ~~

in 1935, have become a vital part of our everyday SIterminology.The success of the S I has been so complete that wetake international accord in metrology very much forgranted. Comparison of todays SI with the systems ofunits discussed in IEC 164 strikingly demonstrates thatour unit system is continuing to evolve. Perhaps, even asyou read this, somew here in the world a next-generationG. Giorgi is being educated who will produce a newsolution to our present and future problems! The overallhistory of metrology encourages us to expect that wewill eventually attain a system of units which is truly(i tous les temps et tous les peuples.As ever, I acknowledge my enduring debt toPaul Vigoureux who continues to provide me withstimulating discussion.

Recommendations in the Field of Quantities and Units Used in Electricity(Excerpt from International Electrotechnical Commission Publication 164, 1964)

Section One: The History of Electricand Magnetic Quantities and Units [l-31Withou t going too far back into the past it is appropriateto mention first and foremost the work of the BritishAssociation for the Advancement of Science, some ofwhose decisions reached in 1862 were republished in1913 [4].It was in 1862 that the British Associationappointed the first Commission entrusted with the taskof studying electrical units. This Commission consistedof physicists with world-wide reputations from variouscountries, which gave it an undeniably internationaland authoritative character.One of the first achievements of this Commissionwas in 1863. It was the result of the initiative ofW. Thomson, later Lord Kelvin, and consisted in theadoption of the system based on the three fundamentalunits: the metre, the gramme and the second. When in1873, the centimetre replaced the metre, the system wasgiven the name C.G.S. system. Its use was universaluntil the introduction at the beginning of the presentcentury of the M.K.S.A. system; and even now it isstill used by many physicists.

After the adoption of the C.G.S. system the sameCommission later decided to adopt the practical unitsthat are obtained by multiplying the absolute C.G.S.482

electromagnetic units by integral powers of ten. Inthis way, the practical unit of resistance became theohm, defined by Maxwell in 1868 as lo7 metres persecond. This ohm was represented by the resistanceof a column of mercury 1 metre long and having1 mm2 cross-section. The Commission also re-definedthe ohm as equal to lo9 electromagnetic C.G.S. units.This value was very close to that of the unit of resistancedetermined at that time in Germany and given the nameSiemens.*The practical unit of electromotive force was thevolt, with a va lue equal to lo8 electromagn etic C.G.S.units; a value very close to that of a Daniel cell,which was very commonly used at that time in variouslaboratories as a unit of electromotive force. Since 1860,Kelvin had been in the habit of using the Daniel cellto-express potential difference.After the very important part played by the BritishAssociation has been recalled it becomes appropriateto enlarge a little on the work of the six InternationalCongresses held between 1881 and 1904. and whichcontributed greatly to the unification of electric andmagnetic units. The last of these, be it noted, occurredbut a short time before the birth of the IEC.

* Note: - Since then the name Siemens has been given by the E Cto the unit of conductance in the Giorgi system (1935).


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The scientists who took part in the Congress of1881 acted from then onwards with great caution andavoided any hasty decisions that they might later haveto revoke. At the same time, their courage and foresightwere such that their decisions were never overtaken bythe progress of events. They avoided, on the one hand,the danger of usage coming into conflict with goodsense and, on the other hand, too large a number ofmeasuring instruments being designed on an arbitrarybasis.It should not be forgotten that at the time of theFirst International Electrical Congress in 1881, therewere in various countries:

12 different units of electromotive force,10 different units of electric current,15 different units of resistance.Among the units of resistance it is noteworthy to

mention the kilometre of iron telegraph wire with adiameter of 4 millimetres, and above all the mercuryresistance that Wem er Siemens had just introduced andthat became the prototype for standards used for manyyears.The first International Electrical Congress, asort of precursor to the International ElectrotechnicalCommission, met under the chairmanship of AdolpheCoche ry, Minister of Posts and Telegraphs of the FrenchGovernment, from 15th September to 5th October1881. It was attende d by so me 25 0 delegates from28 countries. Among these were many of the most emi-nent scientists and technologists of their day, who w ereofficially delegated by their respective governments.

Some of the most outstanding names were:From Germany

From AustriaFrom BelgiumFrom the UnitedStatesFrom FranceFrom GreatBritain

From ItalyFrom NorwayFrom theNetherlands

Helmholtz, Clausius, Kirchhoff,Wemer Siemens. Weber, who wasgravely ill and was to die duringthat same year, was unable tocome.Emst Mach.Eric GCrard, ZCnobie Gramme,Gilbert.Rowland.J. B. Dum as, Mascart, PlantC,Marcel Deprez.Sir William Thomson, WilliamCrookes, Hopkinson.* The greatMaxwell, who for 20 years hadbeen the inspiration of the work ofthe B ritish Association for theAdvancement of Science, had died2 years previously.Govi, G alileo Ferraris.Broch.Bosscha.

From RussiaFrom Swe den ThalCn.From Switzerland H. F. Weber.

R. E. Lenz, A. G. Stoletov and P.N. Yablotchkov.

The principal result of this Congress was to giveofficial and interna tional sanction to the proposals of theBritish Association concerning the ohm and the volt. Itwas, moreover, at this Congress that on the proposal ofHelmholtz [ 5 ] the unit of electric current the amperewas defined as the current produced by an electromotiveforce of one volt in a resistance of one ohm andtheoretically equal to lo- C.G.S. electromagnetic unitsof electric current.It was also at this Congress that the practicalunits for quantity of electricity the coulomb, andfor electrical capacitance the farad, were defined interms of C.G.S. units.In addition to the definitions of electrical unitsin terms of conceptual representation, which are, beit well appreciated, the basis of the present system,the same Congress gave its attention to the materialrepresentation of the units. It reached the decision, forinstance, that the best representation of the ohm was acolumn of mercury of 1 mm2 section at a temperatureof OOC, and of a length to be determined by anInternational Commission.This body, presided over by J. B. Dumas, met inParis first in 1882 and then again in 1884.The conclusions of these two meetings areembodied in the following two official resolutions,which were regarded as the basis of a legal system.First resolutionThe ohm is the resistance of a column of mercury of1 mm2 cross-section and 106 cm long at temperatureof melting ice.Second resolutionThe Conference hopes that the French Governmentwill transmit this resolution to the various countriesand recommend its international adoption.

At this second meeting the Conference alsoconfirmed the legal definition of the ampere and volt interms of the absolute C.G.S. units; but w ithout speakingof their material representation, a more delicate matter,and one that did indeed later give rise to someuncertainty.An inconsistency in the decisions of the Congressof 1881, as also of the Comm ission that it set up, isworth noting.The original intention was to avoid any discussionabout the presentation of units in terms of physicalmeasurement. In accordance with this intention, the* Lord Rayleigh helped with preparation work for this Congressbut was prevented at the last moment from attending.

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ohm, the ampere and the volt were all first definedin terms of the C.G.S. units. But the influence ofthose who had founded the existing metrical system andwho w ere much preoccupied with methods of physicalmeasurement came to dom inate their deliberations.A consequence was that they abolished their firstdefinition of the ohm in terms of C.G.S. units, but onlythat of the ohm, and substituted for it a definition thatrequired a physical measurement. It thereby becameanalogous to the definitions of the metre and thekilogramme.The legal ohm was no longer defined as lo9 C.G.S.units but was represented by the resistance of a columnof mercury of specified dimensions.It may be pointed out, however, that this decisiondid not seriously comm it the future, for the legal systemremained in force for only ten years.The Congress of 1889, which also met in Paris,did not have as official a character as the one of1881; none of the Minutes mentioned the presence ofofficial delegates from the various countries as hadbeen done in 1881. But nevertheless this Congressachieved international agreement on the followingpractical units:for work:for power:for the coefficient ofinduction (now calledinductance):

the joule = lo7 C.G.S. unitsthe watt= lo7 C.G.S. units

the quadrant= io9 cm .Mindful of the requirements of practical mea-surement, this Congress also defined the relation thatexpresses the unit of energy in terms of the units inthe legal system of electrical units. This was done byspecifying that the joule was the m echanical equivalentof the heat emitted during one second by a current ofone am pere flowing in a resistance of one ohm.At the third International Congress, which met inFrankfurt in S eptember 1891, the proposal was made togive the name gauss to the practical unit of magneticflux density and weber to the practical unit magneticflux.The C ongress of 1892, which met in Edinburgh,had, like the one of 1889, as chief aim to prepare forthe Congress of 1893 which was to be held in Chicago.The Congress of Chicago ratified the resolutionsof the two unofficial Congresses that preceded it andsubstituted for the legal electrical units that had beenadopted previously, and whose validity was soon toexpire, the international system of units. In addition,they laid down rules for the physical representation ofthe three principal units: the ohm, the ampere and thevolt.The ohm and the ampere were defined in terms

of the C.G.S. electromagnetic system. The internationalohm was defined as equal to lo9 electromagnetic C.G.S.units and represented by the resistance presented to48 4

a constant electric current by a column of mercuryat the temperature of melting ice, having a massof 14.4521 gramm es, a uniform cross-section, and alength of 106.300cm. The international ampere wasdefined as equal to lo- electromagnetic C.G.S. unitsand represented (with sufficient accuracy for practicalpurposes) by the constant current that, when flow ing ina solution of silver nitrate in water in accordance withan appended specification, deposits silver at a rate of0.001 118 gramme per second. For the internationalvolt, equal to lo8 electromagnetic C.G.S. units, theCongress declared that it would be sufficiently accuratefor practical purposes to define it as the fraction of- f the electromotive force of a Latimer Clark0 0 01434cell. All the other intemational units were regarded asderived, and were defined in terms of the internationalohm and the international ampere.As a consequence of the Congress of Chicago,France suggested that an International Convention becalled for the task of drafting the laws that should beenacted in the various countries in order to make theinternational system mandatory.The U nited States and Grea t Britain did not wait forthe proposed Convention to meet but promulgated therelevant laws on respectively 12th July 1894* and 23rdAugust 1894. The Con vention was thus not required andso never saw the light of day. In France it was decidednot to promulgate a law, but on 24th April 1896, adecree was promulgated instead, which could be morereadily amended. Germany followed by promulgatinga law on 1st June 1898.Once the question of electrical units had beensettled by the Chicago Congress, the succeeding officialCongress, which met in Paris n 1900, concerned itselfmainly with the most contentious question of magneticunits.These had already been the subject of a verythorough study by the American Institute of ElectricalEngineers which, in March 1894, had proposed theadoption for the rationalized C.G.S. magnetic units ofthe following names:

gilbert for the rationalized C.G.S. unit ofmagnetomotive force,gauss for the rationalized C.G.S. unit of magneticflux density,weber for the rationalized C.G.S. unit of magneticflux,oersted for the rationalized C.G.S. unit of reluctance.This Congress proposed that, because of theinconvenient size of these units, the names be given,not to the practical, but to the C.G.S. units.

* Report by F.A. Wolf: Laws concerning electrical units. BulletinNo . 1 of the National Bureau of Standards [ 6 ] .Metrologia, 1994195,31, 81494


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The 1900 Congress, endorsing the proposals fromthe A merican Institute of Electrical Engineers, adoptedthe two following resolutions:First resolution

The Congress recommends that special names be givento the C.G.S. units of magnetic field strength andmagnetic flux.Second resolution

1) T he Congress recomm ends that the name gauss begiven to the C.G.S. unit of magnetic field strength.2) The Congress recommends that the namemaxwell be given to the C.G .S. unit of m agneticflux.

The following Congress met in St. Louis from12th to 17th September 1904 and the InternationalElectrotechnical Commission was initiated at thatCongress. It happened as follows:There had proved to be considerable differencesin the laws promulgated in the various countriesfollowing the Chicago Congress, and an attempt atstandardization was clearly called for. The officialChamber of Delegates adopted therefore, on the basisof a report presented by Mr. Frank Wolff at its meetingon 15th September, the proposal presented by itsSub-committees to form two permanent internationalCommissions with the following duties:

to make a study of electric units and standards,to study the unification of nomenclature and of thecharacteristics of electrical mach ines and apparatus.Two distinct needs were appreciated at the time.Firstly, the Governments saw that it had becomenecessary to take quick, official and common actionabout the very different units that were in use forcommercial transactions.Secondly, it had appeared to be also necessary

to provide a forum that would consist of scientists,and in which technological and learned societies wouldbe represented. Its duty would be to study and setup a terminology for the whole field of scientific andtechnological concepts.From the date of the creation of these twoCommissions, those responsible for the two fieldsof standardization acted separately. The first metin London, where an International Conference tookplace in 1908, to concern itself with elec trical unitsand their physical representation. The Governmentrepresentatives adopted at this Conference:a) a set of fundamental units defined as d ecimalmultiples of the corresponding electromagneticC.G.S. units,

b) a set of international units forming a system of unitsto represent the fundamental units and sufficientlyclose to them to serve for purpose of measurement.These international units had been based on aninternational ohm defined in terms of a column ofmercury and an international ampere defined in termsof the deposition of silver by an electric current.The International Study Committee met in 1910at the National Bureau of Standards, Washington.Four n ations were represented: France, Germany, GreatBritain and the United States. This Committee made acomparison between the standards for resistance and forvoltage that were in use in these respective countries,the four most highly industrialised at that time, andreached agreement on the values to be adopted forinternational units.In 1921, the Sixth General Conference on Weightsand Measures decided to revise the M etre Conventionby extending the field of responsibility of the

International Committee on Weights and Measures toinclude electrical and photometric units.The work of the Laboratory of the InternationalBureau of Weights and Measures became lighter from1928 onwards, and after that year it became possibleperiodically to compare the standards for resistance andelectromotive force that were preserved in the variousNational Laboratories responsible for such standards.In 1927, the Seventh General Conference onWeights and Measures created within the InternationalCommittee for Weights and Measures a ConsultativeCommittee for electricity, and some twenty yearslater, in accordance with a decision reached by theInternational Committee of Weights and Measures on1st January 1948, the absolute electric units (definedas multiples of integral powers of ten of correspondingunits in the electromagnetic C.G.S. system) replacedthe international units.These, which were defined on the basis of physicalmeasurements and on a conventional value for thepermeability of free space, are now in general use.The following conversion factors were provided bythe C.I.P.M. in 1946:1 international ohm: 1,00049 absolute ohm.1 international volt: 1 00034 absolute volt.In 1931, the International Union of Pure andApplied Physics entrusted a specially constitutedCommittee with the responsibility for all questionsconcerning symbols, units and terminology for thequantities used in physics. This Com mittee is called theS.U.N. Comm ittee of the I.U.P.A.P. It works in closecollaboration with the IEC Technical Comm ittee No. 24with the aim of achieving as complete agreement aspossible in this field between physicists and engineers.In addition, as has been mentioned already in

the Introduction, close collaboration has also beenmaintained between I S 0 Technical Comm ittee 12 sinceits creation and Technical Committee No. 24 of the IEC .485


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For clarity of the statement, it is appropriate toturn for a moment from the historical part relatingto names and numerical values given to units of thevarious electric and magnetic quantities and to brieflyrecall the historical aspect of those discussions whichwere deemed necessary in order to clarify the conceptsthemselves for quantities by the choice of clear andprecise designations and definitions for those quantitiesand their units.Work in the field of terminology was undertaken bythe International Electrotechnical Commission, whichwas initiated, as mentioned above in 1904, in St. Louis.It began its work in 1908 under the guidance ofColonel R.E. Crompton in the first Techn ical Committeeto be appointed at that time under the title AdvisoryCommittee on Nomenclature with the Number 1.In the year 1931, during which the S.U.N.Commission was formed, IEC Advisory CommitteeNo. 1 considered that it was convenient to subdividethe group of questions that came within its field ofstudy into three categories as follows:Section A: Vocabulary

Section B: Electric and m agnetic magnitudes andunits*Section C: Letter symbols.In view of the importance of the questions dealtwith by Section B of Advisory Committee No. 1, itwas decided in 1935 to entrust all questions concerningelectric and magnetic magnitudes and units to aspecial Study Committee to which the title Advisory

Committee on Electric and Magnetic Magnitudes andUnits was given with Number 24. This title, by itself,very briefly but clearly summarizes the scope of theTechnical Committee.The early work of the Advisory Committee onNomenclature was conducted under the successivechairmanships of Eric GCrard in Cologne in 1911;L. Lombardi in Turin in 1911; Dr. E. Budde in Paris in1912 and in Cologne and Berlin in 1913. The w ork wasthen interrupted by the First World War, after which itconcentrated on developing as precise a terminologyas possible. It was not until 1927 that the Committeeconcerned itself, under the chairmanship of Dr. C.O.Mailloux, with the study of the various outstandingproblems concerning electric and magnetic quantitiesand units.

It was particularly the definitions of electromag-netic quantities and their units that gave rise to thefirst considerable difficulties in reaching internationalagreement. This was largely due to certain official

* In 1930, Advisov Committee No. 1 on Nomenclature comprisedtwo Sub-Committees:Sub-committee 1 - VocabularySub-Committee 2 - Magnetic unitsChairman:Dr.C.O. Mailloux,Chairman: Dr.A.E. Kennelly.

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decisions that had been taken earlier, particularly at theParis International Congress of 1900. At this Congress,the decision was taken to give the name gauss to theC.G.S. unit of magnetic field strength, probably as aresult of aq error in translation. This led to the adoptionof the term magnetic field in place of magneticinduction for the English term magnetic field densitywhich had been used in the American proposal.At that time the distinction between the quantitymagnetic field strength and magnetic flux densitywas far from being generally understood.For a large number of physicists and electricalengineers, these quantities were considered at this timeas being of the same nature and therefore calling forthe same unit, and numerous scientists used the namegauss to designate both the C.G.S. unit for magneticflux density and the C.G.S. unit for magnetic fieldstrength. However, some of the electrical engineers whowere w ell aware of the need for two distinct units wouldhave preferred at the Congress of 1900 to give the name

gauss to the unit of magnetic flux density rather thanto that of magnetic field strength, as had been done.Moreover, a large number of them, particularly in theU.S.A., did not adhere to the decision taken at thatCongress and continued the usage according to whichthey expressed in gauss magnetic flux density insteadof magnetic field strength.In consequence, discussions of a theoreticalnature were opened at which very eminent electricalengineers and physicists considered the questionwhether magnetic field strength and magnetic fluxdensity were in fact quantities of the same nature.But disagreement continued and taking note of this,the International Electrotechnical Commission decidedon an effort to remedy the situation.Immediately after the meetings in Bellagio in 1927,the IEC instructed a Sub-committee, composed ofseven eminent personalities, to study the question inreadiness for the next meeting, which was to com mencein Stockholm in June 1930 and finish in Oslo in Julyof the same year.This Sub-committee was presided over by Dr. A.E. Kennelly and comprised the following personalities:

W.H. Eccles Great BritainP. Janet FranceL. Lombardi ItalyV.T. Mitkievitch U.S.S.R.K. Strecker GermanyC.J. Van de Well Netherlands.After intensive correspondence among its members,

this Sub-com mittee arrived at the conclusion that newstudies were necessary. It recommended:1) that international agreement on the definition ofthose new quantities that were important forelectrical engineers should be striven for;

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2) that the question whether the gauss could be usedfor both the unit of magnetic flux density and theunit of magn etic field strength should be consideredagain;3) that the question should be examined whetherit would not be appropriate to select side byside with the C.G.S. system an absolute and

rationalized system for all the practical units.This could be the system proposed by Giorgiin 1901 (metre, kilogramme, second, internationalohm) or the Dellinger-Bennett system (centimetre,gramme, second) both of which would havethe advantage of abolishing the then existingduplicate set of units (electromagnetic units andelectrostatic units) which are found in the C.G.S.system. It would also avoid the need to introduceat every turn the troublesome coefficients c or c2 ortheir reciprocals, c being the speed of propagationof electromagnetic waves in vacuo.The Sub-committee suggested that this last and mostimportant question should be placed on the agenda forthe next international meeting of the Committee onNomenclature.Lastly, the Sub-committee feared that confusioncould arise in the minds of those reading works onelectromagnetism from the proposal submitted by theItalian National Committee at the meeting in Bellagioto rechristen the C.G.S. unit for magnetic flux densityby giving the name maxwell, not to a C.G.S. unit,but to the practical unit of magnetic flux density.Hence the Sub-committee declared itself against thisproposal.The recommendations of the Sub-committee ledto the following decisions of the Committee onNomenclature taken in 1930 in Stockholm and ratifiedin the same year in Oslo:1 ) that the qdestion of the names to be allocatedto magnetic units should not be considered untilgeneral agreement has been arrived at in connectionwith their definitions;2) that the formula B =poH represents the modem

concept of the physical relations for magneticconditions in vacuo, it being understood that, inthis expression po possesses physical dimensions;3 ) in the case of magnetic substances, the aboveformula becomes B = p H , in which p has thesame physical dimension as po. It follows thatthe specific or relative permeability of a magneticsubstance is a number equal to p / p o .

These decisions were reinforced by proposals for thedefinition of som e of the principal magnetic quantities,particularly the following:magnetic field strengthmagnetic flux densitymagnetic flux

magnetomotive forcemagnetic permeability.It would serve no useful purpose to quote here thesuggested wording of these definitions, for they havebeen subjected since then to several modifications.During the same meeting in Stockholm (1930),the Advisory Committee on Nomenclature accepteda proposal of the Sub-committee on Electrical andMagnetic Magnitudes and Units and adopted thefollowing names for the C.G.S. units of the principalmagnetic quantities:

1) the name maxwell for the unit of magneticflux, thereby confirming the decision of the ParisCongress of 1900,2) the name gauss for the unit of magnetic fluxdensity,3) the name oersted for the unit magnetic fieldstrength,4) the name gilbert for the unit of magnetomotiveforce.It took the following decisions concerning electricalquantities and units, that:1) the name var to be used for designating thepractical unit of reactive power,2) provisionally, the term reactive power to be retainedfor the corresponding quantity,3) for a non-sinusoidal system the two quantities,apparent power and power factor, are defined bythe following expressions:Apparent power V . IPower-factor of a single-phase system P/V .14) the proposal to give the name hertz to the unit forfrequency to be referred to the Sub-committee onElectric and M agnetic Quantities and U nits.It is also well to recall that, at this meeting, Sub-com mit tee 2 w hich was dealing with the questionsconcerning magnetic units unanimously adopted:1) on the proposal of Mr. Mitkievitch, supported by

Mr. Lombardi, the designation pramaxwell forthe practical unit of magnetic flux which is equalto lo8 maxwells,2) the prefix pra as a general way of designating apractical unit derived from the C.G.S. system.Later on, recommendations corresponding to boththe above decisions were not followed. It shouldbe mentioned that the procedure in force at theIEC at the time did not provide for submittingdraft recommendations to the N ational Committees forapproval under the Six Months Rule.

Therewith the much discussed question of thedifference between the nature of the quantity H ,Magnetic field strength, and B , Magnetic flux density,487Metrologia, 1994195, 31 , 481-494


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was finally settled, and the Sub-committee on Electricand Magnetic Magnitudes and Units of the AdvisoryCommittee on Nomenclature was able to turn to twoother but not less important questions. They were:firstly, the extension of the existing set of practicalunits into a coherent practical system of physical un its;and secondly, the rationalization of the electromagneticfield equations.As has been mentioned above, the question ofreplacing the C.G.S. system of units by a more practicalsystem was envisaged in 1930 on the occasion ofthe first meeting of Sub-committee 2, on Magneticunits, of the Committee on Nomenclature. In the sameyear, Professor Mitkievitch of the U.S.S.R. proposedat Stockholm that units in this system should have thename of the C.G.S. absolute unit with the addition ofthe prefix pra. This proposal was adopted, but it wasnot until the second meeting of Section B of AdvisoryCommittee No. 1 in Paris in October 1933 that thisCommittee decided to take up actively a study of thereplacement mentioned above. This decision followeda discussion of the resolution taken in June 1933 bythe American Committee of the International Union ofPure and Applied Physics.This discussion led the Committee to submit onits own behalf the following resolution to all NationalCommittees:Section B of the Advisory Committee No. 1 onNomenclature, having heard with great interest thecommunication from Mr. Giorgi on the M.K.S. system,and endorsing the resolution adopted by the Americansection of the International Union of Pure and AppliedPhysics at Chicago in June 1933, decides to invitethe National Committees to give their opinion on theextension of the series of practical units at presentemployed in electrotechnics by its incorporation ina coherent system having as fundamental units oflength, mass and time, the metre, the kilogramme andsecond, and as fourth unit either that of resistanceexpressed as the precise multiple lo9 of the C.G.S.electromagnetic unit or the corresponding value of thespace permeability of a vacuum.

At the following meeting, which was held inScheveningen, in 1935, the Advisory Committeeon Nomenclature took almost unanimously, o n theproposal of its Section Electric and MagneticMagnitudes and U nits, the decision to adopt under thename of Giorgi System, the system with four basicunits comprising the three units: metre, kilogramme,second and a fourth unit to be chosen later.The Committee instructed Professor Lombardi onthe one hand, and Dr. A.E. Kennelly on the other, to getin touch w ith, respectively, the Consultative Committeeon Electricity of the International Committee onWeights and Measures and the S.U.N. Commission ofthe International Union for Pure and Applied Physicsconcerning the choice of the fourth unit.488

At the same meeting, the Committee on Nomen-clature decided, finally, to adopt the name weberfor the practical unit of magnetic flux; and that, inthe new system the three following derived units beadopted as examples and w ith the following provisionaldesignations:a) unit of electric field with the name volt per metre,b) unit for magnetic induction, B, with the nameweber per square metre (myriagauss),c) unit for volume density of energy with the namejoule per cubic metre.

At the same meeting, it was also decided to adopt thename hertz for the unit of frequency and the namesiemens for the unit of conductance, as had beenproposed at the preceding m eeting and submitted to theNational Committees for their opinion.At the same meeting it was finally decided, on arecommendation of Section C for Letter symbols, toadopt the following letter symbols for the units thathave the names of scientists:

Mx for the maxwellGs for the gaussOe for the oerstedGb for the gilbertw b for the weberHz for the hertz.

In 1938, Technical Committee No. 24 on Electricand Magnetic Magnitudes and Units held its firstmeeting in Torquay (England) under the chairmanshipof Dr. Clayton Sharp; its official Chairman Dr. A. E.Kennelly was unable to attend.This meeting was chiefly concerned with theproblem of a choice either of the fourth unit in theGiorgi system or the means of finding a connectinglink between the electrical and the mechanical units ofthe same system.At this meeting, the Committee had before itboth the opinion of the Consultative Committee onElectricity of the General Conference on Weights andMeasures, which was communicated to it by ProfessorLombardi, and the opinion of the S.U.N . Comm ission ofthe International Union for Pure and Applied Physics,which was communicated to it by Professor Koenig.On this basis, it adopted unanimously the followingresolution:The Committee, noting the concordant replies ofthe Consultative Committee on Electricity and of theS.U.N. Committee of the International Union of Pureand Applied Physics as to the choice of a fourth unitin the Giorgi (M.K.S.) System, agrees to recommend,as the connecting link between the electrical andmechanical units, the permeability of free space withthe value of , u ~ in the unrationalized system orpo = 47rThe Committee recognizes that any one of thefollowing practical units, ohm, ampere, volt, henry,

in the rationalized system.

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farad, coulomb, weber, already in use may equally serveas the fourth fundamental unit, because it is possibleto derive each unit and its dimensions from any fourothers mutually independent.This resolution was accom panied by the following note:Note. The necessity for high accuracy has usually ledthe N ational Laboratories to base the realization ofthe absolute units on determinations of the absoluteohm and the absolute ampere.

For purposes of technical measurements, however,the practical standards are generally those ofresistance and electromotive force as they havethe advantages of portability and of simplicity andaccuracy in use. Standards of these two quantitiesare maintained by the Bureau International desPoids et Mesures at S2vres.In addition to the very important decisionconcerning the connecting link between the electrical

and mechanical units in the Giorgi system, theCommittee decided at the same meeting to submit tothe various National Comm ittees, under the Six M onthsRule, the proposal to give the name newton to theunit of force in this system.The Second World War interrupted the work of theTechnical Committee on Electric and Magnetic Unitsof the IEC. This work was not resumed until 1950,when Dr. J. J. Smith replaced, as Chairman, Dr. A. E.Kennelly, who had died in April 1939.At the first post-war meeting, which was held inParis in July 1950, the Committee finally settled thequestion of the choice of a fourth unit by recomm endingthe ampere.Meanwhile the Tenth General Conference onWeights and Measures met in Paris in October 1954and implemented Resolution 6 of the Ninth GeneralConference concerning the setting up of a practicalsystem of measurement for international relations, byadopting as basic units the following:Length metreMass kilogrammeTime secondElectric current ampereThermodynam ic Kelvin degreeLuminous intensity candela

At a m eeting in Paris in O ctober 1960 the eleventhC.G.P.M. adopted Resolution No. 12 which states thatthe system based on the six above m entioned units shallbe given the name Syst8me International dUnitts(International System of Units) and the internationalabbreviation of the name of this system shall be SI.Thereafter nothing more seemed to stand in theway of the general adoption of the M.K.S.A. system,both in industrial, commercial and academic circles.It was at this same first post-war meetingthat T echnical Committee N o. 24 also adopted in

temperature

principle the rationalization of the electromagnetic fieldequations, which had been propo sed in 1 900 by theA.I.E.E. on the occasion of the International Congresswhich was to assemble in Paris the same year.The proposal of the A.I.E.E. had been formulatedin the following terms:That much advantage would accrue to a universalrationalization of the electric and magnetic units, andthat the Congress be requested to consider the meansand advisability of such rationalization.At the m eeting of T echnical Comm ittee No. 24in Paris in July 1950, the following decisions aboutrationalization of electric and magnetic units werereached after long and laborious discussions:1) The total rationalization is adopted for the Giorgisystem.2) The rationalization process shall be re-examinedby the National Committees after the questionhas again been clearly put to them with specialemphasis on its twofold aspect:rationalization by quantities,rationalization by units.

It was emphasized that the designation Giorgisystem in the first resolution was used w ithout prejudiceto the rationalization process.In face of the impossibility of arriving atsatisfactory international agreement concerning con-crete and definite proposals, Technical CommitteeNo. 24 appointed a Committee of Experts under thechairmanship of Mr. M. Landolt (Switzerland) toundertake a study of the method of the rationalizationthat was referred to as total.After three meetings, held respectively in Opatijain 1953, in Paris in O ctober 1953, and in Philadelphiain September 1954, the Com mittee of E xperts, afterlong and difficult discussions, submitted to TechnicalCommittee No. 24 concrete proposals concerning theprocess of rationalizing the electromagnetic fieldequations that should be recom mended by the IEC.The proposals of the Committee of Experts wereexamined at the meeting in Philadelphia of TechnicalCommittee No. 24, and were approved by it; they werefinally unanimously adopted by the IEC in July 1956.The adopted text prescribes the form in which theprincipal equations for the electromagnetic field are tobe w ritten and introduces into them the magnetic perme-ability po and the permittivity EO of free space, definingthe values to be used for them. These values are:

po = 47r henry/metre for magnetic perme-abilityfor permittivity of free space, whichis approximately 8.85 x lo-*f a ra dmetre.

At the first post-war meeting of TechnicalCommittee No. 24 in 1950, the Committee definitelyadopted the name newton for the unit of force in the489

1Eo =20 PO


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Giorgi system. It also decided to submit to NationalCommittees the question of giving the name of ascientist to the unit for magnetic flux density in theGiorgi system to replace the term weber per squaremetre (myriagauss) which had been adopted at theScheveningen meeting in 1935.At the second meeting of the Comm ittee of Expertsof Technical Committee No. 24, which took place inOpatija on the 30th June 1953, opinions were expressedby several members concerning the proposal to find aname for the unit of magnetic flux density in the Giorgisystem. After an exchange of views, the Committeeof Experts decided to ask the Secretariat of TechnicalCommittee No. 24 to consult all National Committeesin order to find out if they could agree to the nametesla as the unit of magnetic flux density in theGiorgi system.The invitation was accompanied by the followingcomment:

Note on the reasons justifying g ivinga scientists name to the unit of magnetic Jiux densityin the Giorgi systemA current definition of magnetic flux density isthe density per unit area such that the expressionWb/m2 (weber per square metre) might appear to besufficient to designate the unit in the Giorgi system(M.K.S.A.). Another definition is based on the lawof the phenomenon of induction which gives the unitV.s/m2 (volt.second/square metre).Now the definition given in the second edition ofthe International Electrotechnical Vocabulary (Term 05-25-035) is based on the law of the force F to whicha conductor of length I , in which a current I flows,is subject when placed in a field of magnetic fluxdensity B. This definition is more direct and leads tothe representation of the unit in the Giorgi system bythe expression N/m - A (newtodmetre ampere) which,containing three units, is too long and inconvenient foruse in teaching and for those persons who must makewide use of it in their work.Thus the need has made itself felt - as was felt inthe electromagnetic C.G.S. system, in which the name

of the unit of magnetic flux density is the gauss - ofa new name for the units of magnetic induction in theGiorgi system.It should be recorded incidentally that as early as1930 Paul Janet, who was at that time Chairman ofthe French Technical Committee on Nomenclature, hadexpressed practically the same opinion with referenceto the decision taken by the Congress of 1900 to givethe name gauss to the C.G.S. unit of magnetic fieldstrength.The text of his contribution is as follows:It seems to me a t once clear that engineers have an

urgent need for a unit for flux density. It is true that themaxw ell per square centimetre is a correct term; strictly49 0

speaking it would suffice but it is a long expression andthere is an advantage in having a short name.As a result of the replies received from variousNational Committees and in accordance with arecommendation of its Comm ittee of Experts, TechnicalCommittee No. 24 decided almost unanimously duringthe meeting in Philadelphia of loth and 1 th September1954 to submit the following resolution to the NationalCommittees for approval under the Six Months Rule:The IEC recommends the adoption of the nametesla for the unit of magnetic flux density in theGiorgi system.This recommendation was finally adopted by theCommittee of Action at its meeting in Munich in July1956.Apart from the questions already listed and notyet settled, in particular such as the rationalizationof electric and magnetic units, Technical CommitteeNo. 24 found itself confronted after the meeting inPhiladelphia with the study of several other questionsof which some were very important.These questions are mentioned below and gave riseto long and difficult discussions during the meetingsof Technical Committee No. 24 in Stockholm in July1958 and in Madrid in July 1959, both under thechairmanship of Professor C. Chambers (U.S.A.) whosucceeded Dr. J. J. Sm ith (U.S.A.) after the Philadelphiameeting (September 1954).The following are the more important of the furtherquestions:1) The name to be given to the M.K.S.A. system of

units.2) The sign to be given to reactive power.3) The name to be given to units in the Giorgi systemfor certain electric and magnetic quantities thathave not yet received official names.4) The name to be given to the unit of magnetic fieldstrength in the Giorgi system.5 ) Change in the terms designating the quantitiespermeability of free space and permittivity offree space.6) Relation between the units in the Giorgi system andin the C .G .S . system for the quantity of magneticfield strength.7) Definitions of the basic concepts that are concernedwith quantities and units.8) Comparison of systems of measurement with threeand four dimensions.Discussion of the two first questions gave rise to definitedecisions by Technical Committee No. 24 at its meetingin Madrid (July 1959).The corresponding recommendations, which arementioned in Section Two, were later approved by theCommittee of Action of the IEC.

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A Brief History o j the Electrical Units to 1964

During the same meeting in Madrid in July 1959,Technical Committee No. 24 adopted a rider to the formof words constituting their recommendation about therationalization of the electromagnetic field equations.This rider included a table showing the m ore importantelectric and magnetic quantities; the conversion factorsrequired for multiplying the measures of quantitiesin the electrostatic, electromagnetic and symmetricalC.C.S. systems in order to obtain the measures of thesesame quantities in the Giorgi system.

The questions 3, 4 and 5 in the above list werediscussed, but no decisions were reached about them.They are retained on the agenda for further study byTechnical Committee No. 24.The last three questions on the list (6 , 7 and 8)are more or less closely linked with the questionof rationalization, which was regarded as no longerjustifying discussion within the IEC. Hence theCommittee decided to drop these, at least provisionally,from its programme of future work.

Section Two: Recommendations of the IECConcerning Electric and Magnetic Q uantitiesand Units Adopted up to DateThe IEC Recommendations presented in the presentdocument have been classed in four categories viz.:I - Recommendations concerning terminology andusage for the different systems of unitsI1 - Recommendations concerning the relation ofquantities and units among themselves.I11 - Recommendations concerning rules for theuse of certain quantities.IV - Recommendations concerning the names tobe given to the units of certain quantities.The date is shown on w hich the resolution to makeeach Recommendation was taken.Those Recom mendations concerning the definitionsof quantities and units and their letter symbolswhich are given in other IEC publications, suchas the International Electrotechnical Vocabulary andPublication 27 containing the Letter Symbols used inconnection with Electricity, are not reproduced in thepresent publication.I - Recommendations concerning terminologyand usage for the different systems of unitsa ) Desirability of extending the existing series ofpractical units into a complete comprehensive system.Text of the RecommendationSection B of the Advisory Committee No. 1 onNomenclature, having heard with great interest thecommunication from Mr. Giorgi on the M.K.S. system,and endorsing the resolution adopted by the AmericanSection of the International Union of Pure and AppliedPhysics at Chicago in June, 1933,* decides to invitethe National Committees to give their opinion on theextension of the series of practical units at presentemployed in electrotechnics by its incorporation in acoherent system having as fundamental units of length,mass and time, the metre, kilogramme and second, andas fourth unit either that of resistance expressed as the

precise multiple lo9 of the C.G.S. electromagnetic unitor the corresponding value of the space permeability ofa vacuum. (Paris 1933) [3].After this resolution had been adopted, TechnicalCommittee No. 1 asked Giorgi to prepare a memoran-dum in which the general principles for a coherentsystem in accordance with the conditions formulated inthe text of the Recommendation were presented [7].b) Adoption of a system based on four fundamentalunits (M.K.S.X.) (1935).Text of the RecommendationThat the system w ith four fundamental units proposedby Professor Giorgi be adopted subject to the fourthfundamental unit being eventually selected.The system with four fundamental units shallreceive the designation Giorgi system. (Scheveningen1935) [31.This Recommendation was confirmed in 1959 byTechnical Committee No. 24 and in 1960 by the IECin the following form:Text of the RecommendationThe IEC recommends the adoption of the name Giorgisystem for the system of units based upon the followingfour basic units:the metre,

the lulogramme,the second,the ampere.

* Text of the resolution passed in Chicago (24th June 1933) by theAmerican Section of the International Union for Pureand Applied Physics:The existing series of practical electrical units (ohm, volt, ampere,coulomb, farad, henry, joule and watt) could w ith advantagebe extended to form a co herent system of practical units inphysics with the help of the metre, kilogramme and second(M.K.S. system) or the centimetre, gramme (-seven)** second(C.G.S. system). Of the two systems, the M.K.S. system ispreferred as a subject of study.** The bracket must be interpreted in the sense: unit of mass:IO- e .

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The basic unit kilogramme is the unit of massof the system.Nevertheless, the name M.K.S.A. system isequally accepted.The IEC recognizes that this is consistent withthe following decision by the ComitC International desPoids et Mesures, to which the Conference GCnCraledes Poids et Mesures had given full powers for thechoice of the name to be given to the system of unitsto be used in international transactions, i.e.:To adopt the name Systkme International dUnitCs(International System of Units) for the system of unitsbased on the six basic units adopted by the TenthConference GCnCrale des Poids et Mesures, namely:the metre,the kilogramme,the second,the ampere,the candela,the degree Kelvin. (Madrid 1959) [3].

The provisional adoption at Philadelphia of theseRecommendations gave rise to the following comm ents:The Technical Committee No. 24 requests theCentral Office to verify that the Gio rgi system is used inall IEC documents and publications. However, during atransitional period, which should be as short as possible,in certain special cases, certain units of the C.G.S.system may be used when this appears to be absolutelynecessary. (Philadelphia 1954) [3].c) Adoption of the ampere, which has been approvedby the Ninth ConfCrence GCnCrale des P oids e t Mesuresas the fourth fundamental unit (1950).Text of the RecommendationTechnical Committee No. 24 considers it convenientto base the Giorgi system for electrical quantities onfour principal units, and recommends that, for thepurpose of developing the definitions of the units, thefourth principal unit should preferably be the ampere,as defined by the ConfCrence Gtnerale des Poids etMesures.The definition of the ampere adopted by theConference GCnCrale des Poids et Mesures can betranslated as follows:Ampere (unit of electric current)The constant current that, maintained in twoparallel straight conductors infinitely long, all ofnegligibly small circular cross-section and separated inempty space at a distance of o ne metre, exerts betweenthe conductors a force of 2.10-7 newton per metre ofconductor length.11 - Recommendations concerning the relationof quantities and units am ong themselvesa) Relation between magnetic flux density B an dmagnetic field strength H (1930).49 2

Text of the RecommendationThe E C considers that the formula B = @OHrepresents the modem view concerning physicalrelations for magnetism in a vacuum, it beingrecognized that in this expression po has physicaldimension s. For magn etic substances, the above formulabecomes B = p H in which p has the samephysical dimensions as po. It follows that the relativepermeability of a magnetic substance is a number equalto: pIp0. (Os10 1930) [3].In 1935, as a consequence of the aboveRecommendation, the IEC resolved on the followingsupplementary Recommendation:Text of the RecommendationThat the symbol po for the magnetic permeability ofa vacuum be m aintained and introduced in all relevantformulae. (Scheveningen 1935) [31.b) Ratio nalization of the electromagnetic field equations(1956 and 1961).Text of the Recommendation 1 ) The rationalization of the equations of theelectromagnetic field recommended by the IEC ischaracterized by the following principal equationswhose form remains the same whether the symbols areregarded as representing the physical quantities cominginto play or are regarded as representing their numericalvalues:

1 H d s = Zl 11 DdA=CQ@= / IdA @ = I /dAB = p H D = E E

These equations apply to isotropic media. The totalcurrent (XI) ncludes in the general case thedisplacement current. The permeability of free space( p o )has the numerical value of 4.rr where the unitis the henry per metre. The permittivity of free space

or approximately the numericalEO ) has the value-value 8.85-10-23where the unit is the farad per metre.(Munich 1956) [3].2) As the classical equations of electromagnetism arenot rationalized, the factor 47r appears in the conversionfactors applicable to the measures of certain electricand magnetic quantities. For the different quantitiesappearing in the equations in the text for this resolution,the following table contains:

14 Po

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Table 1.No . Name of the quantity Classification number Letter Name and symbol Conversion factors

according to the I.E.V. in the I.E.V. (second symbol of the unit in the(secon d edition) edition) Giorgi system k, ks km(M.K.S.A.)

1 2 3 4 5 6* 7 81 Element of arc of curve d3 metre m 10-2 10-2 10-22 Element of surface dA square metre m2 10-4 10-4 10-43 Electric curren t 05-20 -040 I ampere A lO/CO I O4 Electric charg e 05 -15-010 Q co ulo mb C lO/CO 106 Displacement (**) 05-15-130 D7 Electric field strength 05-15 -045 E volt per metre V/m 1 0 - 6 ~ ~ 108 Permittivity 05 - 15-120 E farad per metre F/m 1 o l l / 4 ~ ~ ; 1O1/4T9 Magnetic Aux 05-25-035 @ weber wb 10-8co10 Magnetic flux density 05-25-030 B tesla T 104CO

5 Electric flux 05-15-135 i c ou lo mb C 1 0 / 4 T ~ 1 0/ 4Tcoulomb per ks = k,square metre C/m2 1 0 5 / 4 ~ ~ 1 0 5 / 4 ~

10 3 / 4 aI 4 a / 1 0 7zk, = k,11 Magnetic field strength 05-25-020 H ampere per metre A/m 1 0 3 / 4 ~ c o12 Permeability 05-25-045 11 henry per m etre W m 4 ~ ~ ; / 1 0 ~* CO is the numerical value of speed of propagation of electromagnetic waves in vacuo, measured in cm/s.** In the electrostatic, electromagnetic and symmetric C.G.S. systems, the quantity D (displace men t) must be interpreted deviating fromMaxwells original definition, and gives the expressiontotal electric charge Q; in this case the quantity D is sometimes called electric induction.

D d A = 4 ~ r Qor the flux of displacement across a closed surface embracing asColumn 1: the number.Column 2: the name of the quantity as given in thesecond edition of the International ElectrotechnicalVocabulary.Column 3: the classification number in the second

edition of the lntemational Electrotechnical Vocab-ulary.Colum n 4: the letter symbol of the quantity appearingin the 1953 edition of IEC Publication 27concerning Letter Symbols used in Electricity.Column 5 : the name of the M.K.S.A. unit of therationalized system.Column 6: the conversion factor k, by which one mustmultiply the numerical value of a quantity evalu-ated in the electrostatic C.G .S. unit to obtain the nu-

merical value of the corresponding quantity evalu-ated in the M.K.S.A. unit of the rationalized system.Column 7: the conversion factor k, by which one mustmultiply the numerical value of a quantity evalu-ated in the symmetric C.G.S. unit to obtain the nu-merical value of the corresponding quantity evalu-ated in the M.K .S.A. unit of the rationalized system.Column 8: the conversion factor k by which onemust multiply the numerical value of a quantityevaluated in the electromagnetic C.G.S. unit toobtain the numerical value of the corresponding

quantity evaluated in the M.K.S.A. unit of therationalized system.

111 - Recommendations concerning rulesfo r the use of certain quantitiesa) Sign of reactive power (1959).Text of the Recommendation

The IEC recommends that the positive sign begiven to the reactive power absorbed by an inductor.Note. The IEC considers that reactive power canbe produced, transmitted and absorbed with theconvention that a capacitor produces it and aninductor absorbs it. In consequence, the use of theadiectives inductive and capacitive is no longernecessary in connection with the term reactivepower. (Madrid 1959) [3].b) Triangular power diagrams (1935).Text of the Recommendation

The IEC recommends that, in triangular vectorialpower diagrams, the reactive power supplied to areactance coil should be represented by a vectordirected downwards (direction of the susceptance inthe admittance diagram of the coil), and the activepower supplied to the coil should be represented by avector directed to the right. (Scheveningen 1935) [3].Note. In so far as convention treats a vector directeddownwards as representing a quantity with anegative sign, the resolution of Scheveningen 1935

is rendered invalid by the resolution of Madrid1959. (Bucharest 1962) [31.493etmlogia, 1994195, 31, 4 8 1 4 9 4


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IV - Recommendations concerning the names tobe given to the units of certain quantitiesAs has been mentioned above, definitions and lettersymbo ls of units are not quoted in the present docum ent.a) Units in the electromagnetic C.G.S. system [3].units:oerstedgaussmaxwellgilbertb) Units in the electrostatic C.G.S. system.give the name of scientists to units in this system.

The IEC recommends the following names forfor the unit of m agnetic field strength(1930)for the unit of magnetic flux density(1930)for the unit of magnetic flux (1930)for the unit of magnetomotive force(1930).

The IEC has not made any recommendations to

c) Units in the Giorgi system (3).ampere per metre for the unit of magnetic field strength (1954)tesla for the unit of magnetic flux density (1956)weber for the unit of magnetic flux (1935)volt per metre for the unit of electric field strength (1935)siemens for the unit of conductance (1935)V U for the unit of reactive power (1930)newton for the unit of force (1938)joule per cubicmetre for the unit of density of energy (1935).

d) Units common to several systems [3].hertz for the unit of frequency (1935).e) Particular units [3].kilohm to designate lo00 ohms (1933).

Section Three: Bibliography1. International Electrotechnical Vocabulary, IEC Pub-lication 50(05) - 1954.2. International Letter Symbols used in connection withelectricity, IEC Publication 27 - edition 1953.3. Minutes of meetings of Technical Committees andof Committees of Experts of the IEC that deal withquestions of electric and magnetic magnitudes andunits:RM 77 Stockholm and Oslo (1930)RM 97 London (193 1)RM 105 Paris (1933)RM 118 Scheveningen (1935)RM 173 Torquay (1938)RM 229 Paris (1950)RM 315/CE 24 Exp. Opatija (1953)RM 319/CE 24 Exp. Paris (1953)RM 354/CE 24 Exp. Philadelphia (1954)RM 355 Philadelphia (1954)RM 510 Stockholm (1958)RM 547 Madrid (1959)RM 719 Bucharest (1962)4. Report of the Electrical S tandards Committee ofthe British Association published by the CambridgeUniversity Press (1913).5 . Minutes of the International Electrical Congress of1881.

6. Historical Outlines of the Electrical Units, by A.E.Kennelly - Journal of Engineering Education - Vol.XIX, No. 3, p. 229, November 1928.7. Memorandum on the M.K.S. System of practicalunits, by G. Giorgi M.I.E.E, published by the C entralOffice of the E C in September 1954.8. The M.K.S. System of Giorgi as adopted by IEC inJune 1935, by A.E. Kennelly - Journal of EngineeringEducation - Vol. XXVII, No. 4, p. 290, December1936.9. Procbs-verbaux of the Cornit6 International des Poidset Mesures, 1946, Vol. XX, Page 132.

Acknowledgement. I am pleased to thank the IntemationalElectrotechnicd Commission for their permission to reprintthis extended excerpt from Publication 164. It was preparedby the Technical Committee I E W C 24, Electric and magneticmagnitudes and units. Later IEC/TC 24 was merged withIECRC 25 , Letter symbols and signs, to form the newTechnical Committee E C/ TC 25 , Quantities and units, and theirletter symbols. In 1993 IEC/TC 25 decided to withdraw IECPublication 164, because it concerned the history of quantitiesand units in electricity and had neither the form nor the contentof an Intemationd Standard. This is a unique document and it isgood to have the chance once more to bring it to the attentionof those who are interested in the development of metrology.The differences in presentation between the 1964 publication andpresent-day practices have been retained since they are of historicalinterest.

D. A. Blackburn

Documents

A Brief History of the Electrical Units to 1964