9 ll t Ch 11 (1991
t n t nd r nt" School Sci. Rev,, 84, 66, - 5 3 f r n 1 pp -91 A lt "On El tr t Ex t d b th M r C nt t f 33. Ibid., pp 1 -13 C nd t n S b t n f ff r nt K nd " Phil. Trans. Roy. Soc,, 34. Ibid., pp 1 -15 800, 90, 3- 31 35 C S dd n " r d Exp r nt n El tr h tr "
3 W h l n nd A C rl l "A nt f th El tr - School Sci, Rev., 64, 46, - 7 l r G lv n App r t f S Al x lt nd Exp r nt 3 f r n 1 p 1 7 rf r d th Same", Nicholson' s J,, 800, 4, 179-1 7 37 S. hn l ll t n "I pr v d l f r th . St nd M Orn d Electrochemistry, Past and r d " J. Chem. Educ., 60, 37 5 Present, ACS W h n t n C 19 9 5 Williams, Michael Faraday, Y r 19 5 p 9 John T. Stock is Professor Emeritus of Chemistry at the C d Gr tth "M rn r r l dd p t n d l University of Connecticut, Storrs, CT 06269. He is interested t d rp ll t nt n d l t n l d d l l tr t in the history of electrochemistry and chemical instrumenta- lv n " Ann, Chim., 80b, 58, 5 -7 tion and is coeditor, along with M. V. Orna, of "Electrochem- 7 v n v d TheCollectedWorks ofSir Humphry istry, Past and Present", Davy, Bart, S th Eld r C nd n 1 p 1 8. Ibid,, p 19 9. Ibid,, p 10. Ibid., p 9 FROM ELECTROCHEMICAL 11 A l v "M r r l - n d ph n n EQUIVALENCY TO A MOLE OF pr nt l l tr t v lt d n p tr v r l ELECTRONS: THE EVOLUTION nd t r l d " Ann. Chim. Phys., 82 , 28, 19 - 1 OF THE FARADAY 1 A l v "An l d r n t n d t r n nt l n t l nt n t d r nt l tr d n n l nt v lt l " Marcy Hamby Towns and Derek A. Davenport, Purdue
Ann. Chim. Phys,, 828, 37, 5 - University 13 A l v " h r h r l d l l tr t v lt l " Mem. Soc. Phys. Géneve, 828, 4, 5 In the 1988 edition of Quantities, Units and Symbols in 1 M r d Experimental Researches in Electricity, J. M Physical Chemistry, (1) we find the following recommended nt nd n 191 p ( h l t d t n f th thr -v l values for the Avogadro constant (L or NA), the elementary t p bl h d b Q r t h nd n 1 39-1 55 charge (e), and the Faraday constant (F): 15. Ibid., p. 15 16. Ibid., p 17 = 13 7(3 x 1 3 l 17. Ibid., p 9 = 1 17733( 9 x 1 - 9 C 1 St nd h n The Development of Instruments = 9 53 9( 9 x 1 C to Measure Electric Current, S n M nd n 19 3 19 W t h "On th El t t f hr d f Gl th S Simple multiplication of the first two of these yields, with f th M t U f l Appl t n f th r p rt t r n l n " suitably arcane adjustments of limits of error, the third, i,e,
Phil. Trans. Roy. Soc., 8 0, 120, 15 - St "S r Ch rl rn n G rd n f th NAe = F
Flame", Bull. Sci. Instr. Soc., 8 , 3 - 1 f r n 1 p 31 Further examination reveals that the recommended values for A "M h l r d r t f El tr - both NA and e are independent of any electrochemical meas- h tr " n r f r n Ch p 3 urement (2), It would seem that the long and fruitful marriage 3 f r n 1 p 9 of electrochemistry and the Faraday has come to an amicable 24. Ibid., p 35 parting of the ways, a parting endorsed by the units of C 25. Ibid., p 5 mol-', A brief history of the "Faraday" will be given in terms 26. Ibid., p of a concept (however named), a value (however measured) 27. Ibid,, p 5 and a name (by whomever dubbed). 28. Ibid., p 53 Faraday's establishment of the law(s) of electrolysis - 29. Ibid,, p "electrochemical equivalents coincide, and are the same with 30. Ibid,, p ordinary chemical equivalents" - has been widely studied (3- 31 S Arrh n "Ob r d t n d r n W r ld t n 8). What has seldom been remarked is the sparsity of examples St ff " Phys. Chem., 88 , I, 31- and the semiquantitative nature of much of the data upon which
ll t Ch 11 (1991 93 his great quantitative generalization was based. In Faraday's table of relative electrochemical equivalents of some 60 anions and cations (including "quinia", "cinchona" and "morphia"!) less than ten were substantiated by direct electrochemical means; the remainder are "chemical results of other philoso- phers in whom I could repose more confidence, as to these points, than in myself' (9). He adds:
I b ll d t xpr h p th t th nd v r ll l b t t t bl f ea , nd n t y o e ica , l tr h l v l nt ; f r h ll l v rr n th f t nd l ll ht nd n n f th n l d l n d r tl n r p th
In the prefiguring of this passage in the Diary we find the more admonitory: "I must keep my researches really Experimental and not let them deserve any where the character of hypotheti- cal imaginations" (10), As to precision, the Diary gives the values 59.805, 56,833, 57,9 and 59,57 for the relative electrochemical equivalent of tin (11), (Faraday was charmingly cavalier when it came to significant figures.) In the published paper he states (12): hn r d r n ll (l ft nd M h l r d (r ht r 1 3 It n t ft n I h v bt n d n rd n n n b r [ th th pt d h l v l nt] n r th t I h v j t t d h t n f rd n r tt r r th rd nd f p r r nt t v r f th f r xp r nt v 5 53 th l tr h l th r n n t f f t h h j t f n b l v n th t th v l nt f t n t f tt r r n nd d r t d th l tr l p r t h h th th r t tr n l t nd n t Similarly for lead one finds such varied values as 105,11, th th r t l h l ff n t 97.26, 101.29, 93.17 and 80,51 (13), Admittedly the experi- mental difficulties of working with molten salts were large but However, Faraday, like many of his contemporaries, was at as a later worker in the field somewhat ruefully remarked (14): best a reluctant atomist, a view he passionately believed to be fraught with hypothetical, if not quite horrible, imaginings, h xp r nt p n h h h b d h l f l tr l r n One wonders what he would make of recent work on "electro- nt r t n ll tr t n f th n n ht h h l d r d t chemistry at single-molecule sites" (16). A later passage finds n n t n r l l p n h t t d t b v r r nd him turning aside atomistic temptation (17): n r t d t h h r n h h th th r f th d f n t v l t n nd th "On the Absolute Quantity of Electricity Associated with v l nt d f n t t n f l tr t ntr d nt th t d the Particles or Atoms of Matter- - so runs the heading for the th r f d f n t pr p rt n nd l tr - h l ff n t v r concluding section of Faraday's magisterial Seventh Series of r t A rd n t t th v l nt ht f b d r pl Experimental Researches in Electricity. The opening para- th nt t f th h h nt n l nt t f l tr t graph might seem to raise our Whiggish hopes (15): r h v n t r ll l l tr p r ; t b n th E EC ICI Y h h e e mi es th v l nt n b r ecause t d t r n th h th r f d f n t l tr l t l r l tr h l t n pp r b n n f r Or f d pt th t th r r phr l t t t h d t l p n th a so u e qua i y f l tr t th n th t f b d h h r v l nt t h th r n th r r l tr p r b l n n t d ff r nt b d It p bl rd n r h l t n h v l nt t f l tr t n t r ll p rh p t p n th p nt th t tt n n lf b nd t d th th t I t nf I j l f th t r h t pr nt f t ll t n nd t t ll p bl nd a om f r th h t v r t t l f t t v r d ff lt t p rh p ld b p l t n t t r n p n th bj t Alth h f r l r d f th r n t r p ll h n p nd b d r n n th n f h t n t t nn t r t f r n nd r n d r t n d f ll p rt l h h r pr nt t t th nd; nd th h r n l f n t r t r n r n f l tr t Faraday's electrochemical researches, with their accompa- t b n bl t h th r t p rt l r tt r r tt r r r nying nomenclature, quickly found their way into some of the 9 ll t Ch 11 (1991 more adventurous textbooks of the time (18, 19) but for over th v r r n th t n th pr nt p rf tl f r d t t f r 30 years no one probed much more deeply into their ultimate d b t l tr t th th r f l tr l r n t - significance than had Faraday himself. f t r The closest, and incomparably the most rewarding, reading of Faraday's Experimental Researches in Electricity was that Maxwell makes the characteristic point that: of James Clerk Maxwell. As befits one of the founding fathers of kinetic molecular theory Maxwell was reasonably comfort- th rd n r h l v l nt h v r r th mere n r - able with the concept of a molecule while remaining something l r t n h h th b t n b n h r th l tr h - of an agnostic on the reality of Daltonian atoms. In 1873 (the l v l nt r nt t f tt r f d t r n t n t d year that saw the publication of his A Treatise on Electricity d p nd n n th d f n t n f th n t f l tr t and Magnetism) Maxwell gave a lecture on "Molecules" at the Bradford meeting of the British Association for the Advance- Ah, the physicist's "mere"! He continues: ment of Science (20), After paying tribute to his predecessors - "The lecture in which Democritus explained the atomic It th r f r xtr l n t r l t pp th t th rr nt f th n theory to his fellow citizens of Abdera realized, not in golden r nv t n rr nt f l tr t nd n p rt l r th t v r opinions only, but in golden talents, a sum hardly equalled even l l f th t n h r d th rt n f x d nt t f in America" - Maxwell goes on to propound the conventional p t v l tr t h h th f r th l l f ll t n wisdom of physicists of his time (20): nd th t v r l l f th n n h r d th n l nt t f n t v l tr t Ev r b t n pl r p nd h t n l l If th l l b d v d d t p rt r l l f d ff r nt b t n Maxwell, still an adherent of the "two fluid" theory, then issues r b t n fr th t f h h th h l l l An t the caution (21): f th r h th n t b l l f n l nt r b t n S n th r f r v r l l n t n t b t v r t t f n nd th t th l l f th n th n th l l I h ll th rd l l th r n r l t r l tr l t r t ll h r d th rt n d f n t nt t f l tr t p t v nd n t v th t th l tr l t rr nt Later in the lecture he turns to electrolysis but does not pursue pl rr nt f nv t n f nd th t th t pt n h p th the question of a molecular charge (20): l d nt v r d ff lt r nd If n t d f n l l l n d r n bl f l l n t t t n n l tr - W h v n t t d r th n nt n th t t nd rf l h l v l nt f th n th n th t t l h r f ll th l l r t n h h ll d l tr l r n l tr l l h v n n n t f l tr t p t v r rr nt p n thr h d l t d t r nd n x n t n t v pp r t n l tr d nd h dr n t th th r In th p W d n t t n h n l l th r r n n b t n th t r p rf tl l nd t t pp t rr nt f l tr h l v l nt f n b t n b t th l l r th r x n nd f h dr n t b p n thr h t El tr l f h tr h h rr b r t d b n ph l n d r t n th r f r nd f d ff n t d b l tr t v f r pp th t th n b r f l l n n l tr h l v - l nt th f r ll b t n W th r f r n l l r The reasons are not far to seek (20): p l t n th t th n b r f l l n n l tr - h l v l nt n b r n n n t pr nt b t h h h r n th r t f nt t h h t pl n th th rd h r ft r f nd n t d t r n r n b r n l d f th n th r pr n th f r t E h l l th r f r n b n l b r t d fr th t t f r n n r ppr x t n th nd b t nl t f th n t r b n t n p rt th h r h n t d 1/ nd f pr b bl nj t r h r th b l t f l l p t v f r th t n nd n t v f r th n n h d f n t t b l t d t r nd th n b r f l l n b nt t f l tr t h ll ll th l l r h r If t r nt t r n n t ld b th t n t r l n t f l tr t
In the Treatise Maxwell addresses the question of molecular Maxwell's speculations are leading us close to macroscopic/ charge directly in the short chapter on "Electrolysis" (21): microscopic concept of the Faraday, G. Johnstone Stoney is today best remembered for suggest- Of ll l tr l ph n n l tr l pp r th t l l t ing the name "electron" for the elementary charge in 1894. f rn h th r l n ht nt th tr n t r f th l tr rr nt Twenty years earlier he had read a paper "On the Physical b f nd rr nt f rd n r tt r nd rr nt f l tr - Units of Nature" at the Belfast meeting of the British Associa- t f r n nt l p rt f th ph n n n It pr b bl f r tion for the Advancement of Science. This rather idiosyncratic ll t Ch 11 (1991 95 paper was republished in 1881, the year of Helmholtz's famous Faraday Lecture (22). It provides an interesting historical background to the subject of SI units, Having first defined "lengthine", "massine", "timine", and "forcine" Stoney con- tinues (22):
l th l tr n t l tr n r th l tr n t n t nt t f l tr t n th tr r th t nt t f h f th t nd f l tr t h h t b d h r d v r nd n pp - t d r t n l n r n rd r t nt n n t th tr n t rr nt - th rr nt n r n t rr nt b n d f n d th rr nt h h t x t n r tr l n n rd r th t t x rt f r f h p r-d r n p nd r bl tt r t tr d t n h r d th n t f n t
So far all rather academic but later we find (22):
And f n ll t r pr nt n th ph n n n f l tr l th n l d f n t nt t f l tr t h h nd p nd nt f th p rt l r b d t d n th l r I h ll xpr " r d " n th f ll n t r h h I h ll h ll v t pr n v z r h h l b nd h h r pt r d th n n l tr l t rt n nt t f l tr t tr v r th l tr l t , h h th n ll . h d f n t nt t f l tr t I h ll ll El If th r n t nt t f r nn v n l h ltz l tr t h ll pr b bl h v d v r p rt nt t p n r t d f l l r ph n n For our purposes little need be added to what has already been written. Nowhere does Helmholtz estimate explicitly the Crucially, Stoney goes on to estimate Ei using Loschmidt's, elementary (later the electronic) charge, Sir Henry Roscoe was his own, and 'Thomson's estimates of the size of atoms/ in the chair on the occasion of Helmholtz's lecture and his molecules and hence of the approximate number of atoms/ concluding remarks include the passage (25): molecules in a macroscopic "amount of substance", His estimate is within an order of magnitude of today's value. In t r l t r r h n f rth r f r p n r d ll- n n l short, Stoney was the first to interpret a macroscopic electro- f l tr l h h f nd d n l tr - h l th r h h chemical equivalent (of hydrogen) in terms of a microscopic r v l t h t n l n f th t t p rt n t ll charge (positive or negative) carried by an approximately th r lt f th ppl t n f th d rn th r f l tr t known number of microscopic particles, This seems to us the t r d r t xp r nt l l th t th t f v r h l essence of the concept of the "Faraday". l nt l n t d th d f n t nv r n nt t f l tr - In his 1894 paper "Of the 'Electron', or Atom of Electric- t M r v r - nd th t p rt nt - th t th d f n t nt ity" (23) Stoney juxtaposes the second of the above quotations f l tr t tt h d t h t t nd n l nn t n th th to the more famous statement by Helmholtz made in his b n n p r f th t h h d rn h tr t r n- Faraday Lecture of 1881. The circumstances of this lecture are t v l n r f th nt f l tr t b l n n t th n d well known. More so Helmholtz's statement ( t b t n th n t th n th t f th d d t t f th tr d t thr nd n th t t rtl n r lt p rh p f r d l th If pt th h p th th t th l nt r b t n r p d f The future historian of the atomic theory was clearly pleased t nn t v d n l d n th t l tr t l p t v by this marriage of Dalton's atoms and Faraday's laws. Even ll n t v d v d d nt d f n t l nt r p rt n h h though F was not yet the Faraday, N was not yet Avogadro's b h v l t f l tr t A l n t v b t n th Constant. and the electron was not yet discovered, the macro- l tr l t l d h t r n n t d th t l tr v - scopic/microscopic essence of F was now established. l nt r v l nt At th rf f th l tr d d p t n n By its nature, the electrochemical equivalent is a charge-to- t pl f th r ff nt l tr t v p r nd th n th ratio and it is not surprising that it played a key role in t v ff th r l tr h r nd b l tr ll n tr l Thomson's elucidation of the nature of the electron in 1897 and 9 ll t Ch 11 (1991 in Rutherford's identification of the alpha particle in 1905. th n h r f h dr n; ( h l t rr n t th Thomson gave a Friday Evening Discourse at the Royal n h r f h dr n; r (3 n -h lf f th h l t rr n Institution on 30 April 1897. Its title was "Cathode Rays"(26). n l n h r After establishing a value of 1.6 x 10-7 for the mass-to-charge ratio of the electron, Thomson concludes his lecture with the With typical aplomb, Rutherford comes out firmly for the passage (26): second option, We must now turn to a short history of the experimental h v r ll p r d th th v l r f r th r t f th determination of the numerical value of the electrochemical f n t f h dr n t th h r rr d b t If th r lt equivalent. Near the close of his life Faraday purchased the t d b t lf ht th n th t t pr b bl th t r t r first one ohm wire-wound resistance standard offered for sale th n th t h r f [th ] t r th r th n th t l th n by the Committee of the British Association for Electrical th f h dr n t n h v r n nj n t n th Resistance Standards (30), For most of his active research life n rd r lt f r th b rpt n f th th d r th n b r he had had to be content with relative effects, e.g,, relative t f v r th h p th th t th rr r f th h r r electrochemical equivalents, and with semi-quantitative meas- ll r th n th t f h dr n urements based on ingenious ad hoc standards. The problem It nt r t n t n t th t th v l f / h h h v is well-illustrated in one of Faraday's most memorable meta- f nd fr th th d r f th rd r th v l 1 -7 phors (31): d d d b n fr h xp r nt n th ff t f n t f ld n th p r d f th d l ht On r n f t r d l t d t f l t t nd t n ll r r n l tr rr nt t b nt n d f r thr n t nd thr - rt r In Churchill's phrase, this was the electrochemical equiva- f t t ff t t d p t n h h rr nt t b p rf l lent's finest hour, Mention of the Zeeman Effect brings to n h t r t n pl t n r 1/1 f n n h n th n r d-h t mind that an unsuccessful search for this effect was the subject n th r d r n th h l t ; nd f nt rr pt d n h r b of Faraday's last experiment (27), h r l p nt ll pr d v r br ll nt nd n t nt t r f l ht Almost exactly four years later, in another Friday Evening If tt nt n b p d t th n t nt n d h r f l tr t f Discourse, Thomson showed how the charge on a single elec- t n n ll tr t d n th b t f l xp r nt f Mr Wh t t n tron and the value of the electrochemical equivalent yielded a nd t h t I h v d l h r n th r l t n f n nd satisfactory value for Loschmidt's Number (Avogadro's v lt l tr t t ll n t b t h t th t th n r Constant had not yet been so named) without the "not entirely nt t f l tr t l t v r p rf l fl h f l htn n satisfactory" assumptions of Kelvin, Stoney and Loschmidt Y t h v t nd r p rf t nd; n v lv d r t nd pl (28). t t pl r ; nd h n t h p rf r d t f ll r f l tr l z - Shortly afterwards Rutherford was to use similar argu- t n t h nl p r t d th l nt f n l r n f t r ments in pinning down the nature of the alpha particle (29): The establishment of international units in electric science It n n r t n d r h t d d t n n b dr n fr th was effected by one of the earliest, greatest and most influential b rv d v l f / f nd f r th a p rt l h v l f / f r of international collaborations in science (32, 33). There were th h dr n n n th l tr l f t r n n t b v r n rl two essential components to the task: (a) relating the various 1 h h dr n n pp d t b th h dr n t th electrical units to the more fundamental units of mass, length p t v h r th t th v l f / f r th h dr n t 1 and time, e.g,, resistance has the dimensions of a velocity, (b) h b rv d v l f / f r th p rt l 5 l x 1 3 r n r nd developing practical and transportable standards incorporat- n b r n h lf f th t f th h dr n t h d n t f h l ing these fundamental units. Following theoretical contribu- h b n f nd t b 1 9 t th t f h dr n nd fr b rv - tions of Gauss and of Weber and prompted by the "progress t n f th v l t f nd n h l t h b n d d d th t and extension of the electric telegraph", a particularly impor- h l n t r th t n l d d th t th h l tant role was played by the Committee of Electrical Standards t h n t ht 3 9 If h l t rr th of the British Association for the Advancement of Science. h r th h dr n n th v l f / f r th h l t The original committee of 1861 consisted of Williamson, h ld n ntl b b t 5 x 1 3 If th t th Wheatstone, Thomson (Kelvin) and Jenkin. They were shortly p rt l rr th h r th h dr n n th f th joined by Siemens, Maxwell and Joule. All of these illuminati p rt l t th t f th h dr n t W r h r nf rt - were working members and it is scarcely surprising that n t l nfr nt d th v r l p b l t b t n h h t progress was rapid, The choice of the (as yet un-named) ohm d ff lt t d f n t d n as the first target of opportunity was dictated partly by the h v l f / f r th p rt l b xpl n d n th importance of resistance measurements in telegraphy and pt n th t th p rt l (1 l l f h dr n rr n partly by the realization that the unit could be manifested in a ll t Ch 11 (1991 97
pl t r l standard such as a specified column of mer- However, Richard's value for the electrochemical equivalent cury that could then be matched with conveniently transport- of silver differed significantly from that of Rayleigh, able wire-wound resistors, International agreement was rati- In spite of all the experimental ingenuity subsequently fied in 1881. expended on the silver voltameter (or silver coulometer as The next step - the establishment of units and standards for Richards preferred to call it), nagging discrepancies remained. current/quantity and/or for electromotive force - was more As a consequence, alternate chemical systems were investi- complex. As Rayleigh was later to state in his classic 1884 gated. The first of these was the iodine coulometer perfected paper "On the Electrochemical Equivalent of Silver, and on the by Washburn and Bates (39). This obviates the weighing of Absolute Electromotive Force of CLARK Cells" (34): silver deposits (possibly containing occluded liquid) and has the further advantage of internal referencing since the reac- h pl t l t n f th pr bl f b l t l tr l r - tions at the anode and cathode can be monitored by identical nt nv lv h v r nd d t r n t n l r n nd b t analytical methods: t nd p nd nt f th f r t In dd t n t r t n r r t n th r l tr l nt t h rr nt r l tr t v 2e- + I3-( — 31 ( f r S f r r r ll th th d pl d f r th p rp d f n n th f r t n t n n l tr l rr nt; b t 31- ( — I3- ( + rr nt nn t l r t n b b d d n n t r l t nd rd f r f t r th r lt f th r nt t b r rd d n t r Differences of 0,02% in the value of the Faraday calculated f ff t h v r l b rv r h v d t r n d th nt t from the iodine and the silver voltameter remained, though f lv r d p t d r th nt t f t r d p d b th many years later it was shown that these differences could be p f n n rr nt f r n n t In th th largely reconciled (39). Other systems studied included ben- d f n t n r l t n t h t l tr rr nt t l tr nt t zoic and oxalic acids (40), and 4-aminopyridine (41) coulome- ters. A major advance in precision was also achieved when the Rayleigh had inherited a tradition (and even some requisite silver coulorneter was changed from the silver-deposition to equipment) for advancing electrical standards from his prede- the silver-dissolution mode, cessor as Cavendish Professor, James Clerk Maxwell. His As we shall see, the various electrochemical determina- experiments, in which he was aided by Mrs. Sidgwick, were tions of the Faraday gradually converged over the years, carried out in the same room where, 15 years later, his succes- Increasingly, however, they were challenged by non-electro- sor, J, J, Thomson, was to discover the electron (35), chemical methods, Given the simple identity F = Ne, it is Rayleigh's was by no means the first determination of the obvious the knowledge of any two quantities can be used to electrochemical equivalent of silver but it set a standard (in calculate the third. This relationship was first implicitly several senses) for thoroughness and exquisite attention to employed as we have seen by Stoney and was later used by J, detail that lasted until the middle of the 20th century, It is not J, Thomson to show that it yielded a plausible value for N, or difficult to recognize the experimental skills that were later to rather for the Loschmidt Number, With the progress of X-ray enable Rayleigh to sniff out the presence of argon in the crystallography, increasingly accurate values for N became atmosphere from a less than one half of one per cent discrep- available and the presently accepted value cited at the begin- ancy in the density of nitrogen (36), As R. J. Strutt proudly ning of this article is based on this method (42), Precision points out in his biography of his father, Rayleigh's value for measurements of the absolute charge on the electron by Mil- the electrochemical equivalent of silver (0.00111794 g,/am- likan and others followed a more chequered path (43,44). As pere-second corresponding to F = 96488) stood the test of time a consequence, in 1949 Sommer and Hippie still felt justified extraordinarily well. In 1893 it was to become the basis of the in claiming (45): international ampere, Many others were to attempt to refine Rayleigh's value, In h v l f th r d h b n d t r n d b ph l th d a paper titled "The Universally Exact Application of Faraday's h r nt p rt l rl n f nt b th n Law", T. W. Richards showed that (37): th d nt r l d ff r nt fr th l l tr h l d r v - t n lv n rr nt d p t nt ll th nt f lv r fr l t n f r nt n tr t n th r [ d nd p t ] The method measures the Faraday directly and nv lv deter- n tr t t 5 °C t d fr n l t n t 5°C th n minations of the proton rest mass, the gyromagnetic ratio of the 5 p r nt n n nn t n th pr v r f h rd proton, and the proton magnetic moment in nuclear magne- C ll n nd rnr d th r lt h th t r d l n t tons. In a 1968 summary paper, Zielen gave the comparative r ppr x t n b t r th r t b r n d n th t pr values shown in Table 1 for the electrochemical Faraday (46), nd n r l f th l f n t r Since then the electrochemical methods have been made 9 ll t Ch 11 (1991
bl 1 C p r t v v l f th r d n W h ll n t fr xpl r t n l b / v l nt And th nd f ll r xpl r n W ll b t rr v h r t rt d And n th pl f r th f r t t S lv r d l t n l t r 9 ± I d n l t r (n r r l l t d 9 5 ± 3 When did the value for the electrochemical equivalent (of I d n l t r ( ld 9 9 7 ± 1 9 silver) become known as the Faraday? Later than one might Ox l t 9 1 ± 3 expect it seems. The name "ohm" for the unit of electrical El tr n t 9 7 ± 1 3 resistance was adopted in 1862. Five years later the unit of electrical capacitance was dubbed the "farad", "Volt", "cou- lomb", and "ampere" were adopted at the First International vastly more precise to yield 96,486,00 ± 0,10 (47). In addition Electrical Congress held in Paris in 1881, Others had been the 4-amino pyridine coulometer of Diehl et alia yields quick to capitalize on the Faraday name, In A Practical 96,486,05 ± 0,72 (48), Problems of interpretation and corre- Treatise on the Medical and Surgical Uses of Electricity, the lation still remain as is apparent from the following wry second edition of which was published in 1875, we find index comment by Diehl (49): entries for: "Faradism", "Faradization", "Farado-contractal- ity", "Farado-electrolyza- h Cr lv r d l t n tion", "Farado- puncture or v l th pt d v l Electropuncture with the fr 19 n r l l t d Faradic current (not much v l f r th h ft f used)", and "Farado-suscep- th t ht l t r- tibility"(50), As is typical b n-1 f r t h n n th with that most faddish of d f n t n f th p r f r professions, few of these d t r n t n f th t p pseudo-treatments and ef- r t n th lv r d f r fects survive. h n n th d f n t n f th It is probably largely a v lt nd f r r n r coincidence that the words t t t l tr t nt th n Cr "mole", "Avogadro's Con- v h n d t h ph - stant" and "Faraday" all t nt r t d n th v l f entered the scientific litera- th v r f nd nt l n- ture during the ten years t nt v n v l b t- following the discovery of t r v l f r v r ph l the electron. The term nt t bt n d th n f - l f th r d v r th r h r v l nd r f r n "mole" was introduced by ntl l r v l h d - n b r f r th p nt n th r ph r 1884: 9 9 9 (3 ; 1902: Wilhelm Ostwald in the r p n pp 9 53 9 (5 ; 1912: 9 53 (3 ; 1929: 9 9 (57 ; 1941: 9 5 1 1900 edition of his Grund- f r t r t r th n (5 ; 1953: 9 9 ( 5 ; 1968: 9 7 ( ; 1968: 9 5 ( ; linien der anorganischen th t t d n rt nt n th 1980: 9 33 (47);1983: 9 5 ( 1 1986: 9 5 3 9 (1 Chemie (51). The concept l tr h l v l nd t n and name are introduced t th t t d n rt nt n th l l t d v l S nf d nt almost in passing in a section titled "The Molar Weight of h d th ph t b b 1973 th t th f lt t n r t r j t Hydrogen Peroxide". Understandably there is no mention of th Cr l tr h l v l tr ht "b n bj t t - the associated (in our eyes) Avogadro Constant or Loschmidt r rr r" It th n r f t n h nt t th h n number, for Ostwald was at that time the most visible apostate th I St t Un v r t (ISU v l b d n l tr t tr t n from atomic theory; indeed the suggestion has been made that f - n p r d n dv n d n 197 r n n t pl nt the coinage of the word mole was a consequence of this nd rpr n f h n th th Cr v l ( 9 apostasy (52), In this connection it is of interest to read Ostwald's Faraday Lecture to the Chemical Society (53) Today both physical and chemical methods seem to be ineluc- delivered "in the Theatre of the Royal Institution on Tuesday, tably and asymptotically approaching the "true" value, a value April 19th 1904". It expresses a profound scepticism concern- that appears astonishingly close to that put forward by Ray- ing the existence of atoms, One wonders if the ghost of Faraday leigh in 1884, One is reminded of T, S. Eliot's lines: murmured his approval. The use of the term "Faraday" for the electrochemical ll t Ch 11 (1991 99 equivalent seems also to have arisen in Germany, In 1904 we h tr C nt xt v l p Kn l d " n St find Lehfeldt writing in the opening chapter of his Electro- nd M O d E ec oc emis y as a ese , A r n chemistry (54): Ch l S t W h n t n 19 9 pp 3 - 9 St " h th t th f El tr l " th h f nd nt l nt t f l tr t h h r n t ntl n ll r t n n l tr - h tr ll d b th G r n "f r - 9 M r d E e ime a esea c es n E ec ici y, l I d " t r h h n En l nd v r ll d pt S r II Q r t h nd n 1 39 p r r ph 5- 7 1 M rt n d a a ay s ia y, l ll nd n 193 The name soon took hold in England and elsewhere, p r r ph 1 7 p 1 All threads of our story seem to come together in Jean 11 f r n 1 p r r ph 1175 11 1 1 7 s classic paper of 1909, "Mouvement Brownien et 1 f r n 9 p r r ph 79 Réalité Moléculaire" (55): 13 f r n 1 p r r ph 11 5 11 7 1 5 1 57 nd 1 14. E. W W hb rn An I o uc io o e i ci es o ysi An t r - l l nt n th n b r f l l h ca C emis y, M Gr - ll Y r 1915 p 5 nv r bl n b r n v r l n t nt h h ppr pr t l 15 v r n 9 p r r ph 5 b d n t d Av dr C n t nt 1 M M jd nd l z " f n t n l M n l l r l tl f th n f r d v n t th nt t f l tr t n r- l d tt l t El tr d El tr h tr t S n l
(9 55 l b h h p n th d p t n f 1 r - M l l G S t " J. Am. C em, Soc,, 1991, 113, 5464 - 5466, l l f h dr hl r d t n n th t th d p t n f 17 f r n 9 p r r ph 9 n th r r - l l p n d b th p f h l 1 n ll A I o uc io o e S u y o C emica n b r f f r d nd n n n th t n n rr h l i oso y, J. W. r r nd n 1 39 pp 5 5-55 n b r f t th h r n th h dr n n h h r e th 19 W r p r A e ook o C emis y, rp r Y r l pp r nd v bl nd n t t t th t f l tr t r 1 pp 1 9-13 th l tr n ( l h ltz C M x ll "M l l " Nature, 1873, 8, 37- 1 It t bt n th n v r l n t nt f th r f th n t nt 1 C M x ll A ea ise o E ec ici y a Mag e ism, r [ 3 / ] n n S n th r - t f h dr n n Cl r nd n r Oxf rd 1 91 pp 375-3 th n t t th t t t f h dr n rr n f r d G St n "On th h l Un t f t r " i . Mag., th n n r l 1881, 11, 381 - 390, 23. G. J. St n "Of th El tr n r At f El tr t " i . All that remained was to improve the accuracy and precision Mag., 1894, 8, 1 - with which N and/or e and/or F was known. l h ltz "On th M d v l p nt f r d C n pt n f El tr t " J. Chem. Soc. Trans., 1881, 39 77-3 References and Notes 5 " r f r l h ltz r d t r " t r 1881, 23, 540, 1. I. M ll Cv t K nn K ll nd K K h t 26. J. h n n r nd G rt r d "C th d Qua i ies, U i s a Sym o s i ysica C emis y , 19 p 1 " e oya I s i u io i a y o Scie ce: e ysica E C h n nd l r " h 19 Adj t nt f Scie ces, l 5 Appl d S n nd n 197 pp 3 - 9 th nd nt l h l C n t nt " CO A A u ,., 1986, 63, 7 f r n 1 p 5 1- 9 h n n r nd G rt r d " h Ex t n 3 rtl n r nd G rt r d "M h l r d f d S ll r th n At " e oya I s i u io i a y o nd El tr h tr " e oya I s i u io i a y o Scie ce: e Scie ce: e ysica Scie ces, l 5 Appl d S n nd n ysica Scie ces, l 9 Appl d S n nd n 197 pp 3 - 197 pp 5- 1 1 9 E th rf rd " h t r nd Ch r f th rt l fr W ll Mic ae a a ay, In d t v S b t n " a u e, 1908, 79, 1 -15 Y r 19 pp 7- 73 pr nt d b C p r 19 7 3 S rd o s o e , e ise o E ec ici y, r 5 v r " r d El tr h tr nd t r l h l - M nn p l M 19 r - p ph " n G pp rn ll nd W tbr d Se ec e o ics i 31 f r n 9 p r r ph 53 e is o y o E ec oc emis y, h El tr h l S t r n- 3 n n d e o s o e Commi ee o E ec ica S a t n NJ, 197 pp 15 -1 1 S l v r A i i y a Ma e , a s, Sp n nd n 1 73 Cl r nd n r Oxf rd 1971 pp -1 33 f r n 3 p S M G r ln " h C nt xt f r d El tr h - 3 rd l h nd Mr S d "On th El tr h -
l " sis, 1979, 0, 59 - 75. l E v l nt f S lv r nd On th Ab l t El tr t v r f 7, F. A. J, "M h l r d r t f El tr - C A K C ll " i . a s. oy. Soc., 1884, 175(11), 11- 1 ll t Ch 11 (1991
35 Str tt e i e o o Wi iam S u i a o 19 1 th t l n th l t f ht Onl " e o s o ay eig , 0 M., . . S ., Un v r t f W n n M d n WI 19 og ess i ysics, 1941, 8, 9 -13 pp 1 -1 7 S p 11 3 f r n 35 pp 1 7- 5 37 W h rd W St ll " h Un v r ll Ex t Appl - Derek A. Davenport is Professor of Chemistry at Purdue t n f r d " oc. Am. Aca . A s Sci,, 1902, 8( , University, West Lafayette, IN 47907, where he teaches a 9- 13 course on the history of chemistry. A former Chair of the 3 E W W hb rn S t " h I d n C l t r nd th Division of the History of Chemistry of the ACS, he has, in l f th r d " . Am. C em. Soc,, 1912, 4, 13 1-13 addition to the Faraday symposium, organized historical 39 A l n "A l l t n f th I d n r d " . symposia on Priestley, Dalton and G, N, Lewis, and is cur- E ec oa a . C em., 1968, 8, 9-9 rently putting together a symposium on C, K. In gold. Marcy G M r n n K l r "El tr h l E v l nt f Hamby Towns is completing her doctoral degree in chemical nz nd Ox l A d" A a . C em,, 1968, 40( , 1 5-1 51 physics at Purdue and has served as a teaching assistant for 1 hl nd " h n t f -A n p r d n Dr. Davenport's history of chemistry course. d n Cr t l nd r n W h n " a a a, 1983, 0, 9 - 9 l tt nd th r " t r n t n f th Av dr C n t nt" ys, e . e ., 1974, (8 , 3- 3 K r n e ise o o e Mi ika , C rn ll Un v r- A BIOGRAPHICAL CHECKLIST t r Ith 19 p "Int rd p nd n nd th I p rt n f Err r The following is a checklist of biographies of Faraday, For n Ch tr th S r h f r S n l Err r d t x n - teachers and students looking for a brief, accessible introduc- t n f th W r f v b l r t nd v d l f r tion, Thomas (1991) is highly recommended. For a more C rt n nd nt l C n t nt " _C em. E ., 1991, 68(4 , 73- 7 detailed biography, Williams (1965) is still the standard and is 5 S r nd A ppl E ec oc emica Co s a s, currently available in an inexpensive paperback reprint, Though NBS C r 5 W h n t n C 1953 pp 1- long out of print, the volumes by Bence Jones (1869) and f r n 39 p 9 Thompson (1898) are also very worthwhile provided one is 7 E r nd S v " h El tr h l E v - lucky enough to come across a copy. l nt f r S lv r - A l f th a a ay", . es. S, 1980, 8 ( , 175-191 E Andr Mic ae a a ay, 86 , Wh t n Ex t r f r n 1 p 97 1937 9 hl " h r n C l tr nd th l f th A a a ay as a a u a i oso e , Un v r t f r d " A a . C em., 1979, ( , 31 A-33 A Ch Ch 11 1971 5 An n A ac ica ea ise o e Me ica a Su gica Appl rd A i u e o Mic ae a a ay, C n t bl nd n Uses o E ec ici y, W ll W d nd C p n Y r 1 75 1931 51 W O t ld G u i ie e a o ga isc e C iemie, En l- E W A h r ft a a ay, r t h El tr l nd All d M n f t r r nn pz 19 pp 1 3 1 7-1 A t n nd n 1931 5 G l n " h El v M l " E uc. C em., 1991, 28, n n e i e a e e s o a a ay, l 1 3-1 n n Gr n nd C nd n 1 9 53 W O t ld "El nt nd C p nd " . C em. Soc., r Mic ae a a ay a e Mo e Wo , E A rr 1904, 8 , 5 -5 W nd n A b 1991 5 A hf ldt e ooks o ysica C emis y: E ec o r nd S n d Cu iosi y e ec y Sa is ie : c emis y, a I, Ge e a eo y, n n Gr n nd C p n a a ay s a e s i Eu o e, 8 8 , r r n nd n 1991 nd n 19 p 3 M r ph Mic ae a a ay, r ht Y r Y 55 M rr n "M v nt r n n t l t M lé l r " 199 A ,C im. ys., 1909, 8, 1 -19 tr n l t d n M rr n ow ia W r Mic ae a a ay, W l n C nf r n Off Mo eme a Mo ecu a ea i y, l r nd r n nd n 191 nd n 1 77 p 1 G C nt r Mic ae a a ay, Sa ema ia a Scie is : A S u y 5 W h rd nd G W r d "On th A r f th o Scie ce a e igio i e i e ee Ce u y, M ll n I pr v d lt t r" oc. Am. Aca . A s Sci., 1902, , 1 nd n 1991 57 r " r b bl l f th G n r l h l G C nt r G d n nd A Mic ae a a ay, C n t nt " ys, e ., 1929, ( , 1- M ll n n t 1991 5 r " h G n r l h l C n t nt f A t G C Mic ae a a ay, l nd n 19 3