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The discovery of the electron: I This article has been downloaded from IOPscience. Please scroll down to see the full text article. 1997 Eur. J. Phys. 18 133 (http://iopscience.iop.org/0143-0807/18/3/002) Download details: IP Address: 130.15.241.167 The article was downloaded on 24/08/2013 at 12:38 Please note that terms and conditions apply. View the table of contents for this issue, or go to the journal homepage for more Home Search Collections Journals About Contact us My IOPscience

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Page 1: The discovery of the electron: I

The discovery of the electron: I

This article has been downloaded from IOPscience. Please scroll down to see the full text article.

1997 Eur. J. Phys. 18 133

(http://iopscience.iop.org/0143-0807/18/3/002)

Download details:

IP Address: 130.15.241.167

The article was downloaded on 24/08/2013 at 12:38

Please note that terms and conditions apply.

View the table of contents for this issue, or go to the journal homepage for more

Home Search Collections Journals About Contact us My IOPscience

Page 2: The discovery of the electron: I

Eur. J. Phys. 17 (1996) 133–138. Printed in the UK 133

The discovery of the electron: I

Nadia Robotti

Dipartimento di Fisica, Universita di Genova, via Dodecaneso 33, I-16145 Genova, Italy

Received 2 October 1996

Abstract. This paper describes the process by which the firststudies on discharges in rarefied gases led to the discovery ofthe electron in 1897. Particular emphasis is laid on the debatebetween the so-called ‘aetherial’ and ‘material’ theoriesregarding the nature of ‘cathode rays’. The paper goes on todemonstrate how the debate was resolved by J J Thomsonwith his proposal of a third hypothesis—the ‘corpuscle’ (or‘electron’ as it became called). The paper closes with ananalysis of the first measurement of the charge of the electronby J J Thomson in 1899.

Resume. Ce travailetablit comment,a partir des premieresetudes sur la decharge dans les gaz rarefies, on put arriver, en1897,a la decouverte de l’electron. On analyse en particulierle debat qui eut lieu entre l’hypothese dite ‘etheree’ etl’hypothese ‘materialiste’ quanta la nature des ‘rayonscathodiques’. Ensuite on explique comment ce debat fut clospar J J Thomson, qui proposa une troisieme hypothese—celledu ‘corpuscule’ (ou ‘electron’, comme il sera ensuitedenomme). L’article se termine avec un analyse de lapremiere mesure de la charge de l’electron realisee parJ J Thomson en 1899.

In 1873 Maxwell made this comment on dischargeprocesses in rarefied gases:

‘These and many other phenomena. . . are exceed-ingly important, and, when they are better understood,they will probably throw great light on the nature ofelectricity as well as on the nature of gases’ [1].

As we will see, history proved Maxwell right. Justa few decades later research of this type led to thediscovery of the electron

1. The first steps

One of the most important challenges faced by physicsin the second half of the 19th century was to understandwhat J J Thomson referred to as the ‘secret ofelectricity’—or the nature of electricity itself and therelationship between electricity and matter. One pathchosen to do this was to study the behaviour of ararefied gas in the presence of an electrical discharge.The reasons behind the choice were twofold. In thefirst instance this type of study would have led toan analysis of the interactions between electricity andmatter in its ‘evidently simplest state’. In the secondplace, for the gaseous state, unlike the other states, therewas a fairly consolidated theory (the kinetic theory)that could be used as a point of reference. In anycase, research of this type could only proceed when(thanks above all to Geissler) vacuum pumps able toproduce pressures of the order of 10−2–10−3 mm Hgbecame available. At these pressures it was observedthat, while the various areas in the gas behaved indifferent ways depending on the nature of the gas, the

type of electrodes, the shape of the tube, etc, there wasalways a dark space near the cathode, independent ofthe operating conditions. Figure 1 below, taken fromPhilosophical Transactionsof 1880 [2], illustrates anumber of discharge phenomena for pressures around10−2 mm Hg.

This dark space near the cathode (which had alreadybeen pointed out by Faraday in 1838 [3]) increased onlyin size as the degree of vacuum increased. At pressuresin the order of 10−3 mm Hg, the space extended beyondthe anode along the entire length of the tube, becomingthe only phenomenon present.

Once the dark space had been identified as a constantin discharge processes, it became the focus of attentionin an attempt to identify its physical properties. From1860 onwards, thanks above all to the work of Plucker,Goldstein, Varley and Crookes, it was established thatthis dark space was the transit area for ‘something’deriving from the cathode, which invisible until itmet an obstacle, after which it became evident andcould be perceived. This ‘something’, independentlyof the material used for the cathode, had the followingproperties:

(i) it caused phosphorescence in the glass or on anyphosphorescent object placed in its trajectory;

(ii) it was emitted perpendicularly to the cathode andtravelled in a straight line, independently of the positionof the anode;

(iii) it produced chemical reactions, exerted amechanical effect, was deflected by a magnetic field,and created a shadow from any object placed in its path.

So far as concerns the nature of this something,from 1870 two opposing theories were developed.On the one hand, there was the ‘aetherial’ theory

0143-0807/96/030133+06$19.50 c© 1996 IOP Publishing Ltd & The European Physical Society

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Figure 1. Electric discharge phenomena in different operating conditions.

supported by most physicists of the German schoolstudying the question. Under the terms of this theory,the phenomenon was regarded as an electromagneticprocess, i.e. a wave of small wavelength (this is thereason for the name ‘cathode rays’). On the otherhand, there was the ‘material’ theory supported bymost physicists of the British school, who, using ananalogy with the electrolytic processes studied for sometime, considered the ‘cathode rays’ to be negativelyionized atoms or molecules (the sign of the chargewas established by the curvature in a magnetic field).Clearly these two theories succeeded in explainingin part the properties of ‘cathode rays’ discoveredup to that point. It is no coincidence that therewas an immediate conflict between the two theories,above all in experimental terms. Every experimentdesigned and implemented to highlight new propertiesof ‘cathode rays’ was set up to be able to distinguishbetween these two theories. Despite the objectiveof the various experiments performed, no experiment,taken individually, was sufficient to resolve the debatebetween the two theories, and in fact the debatecontinued at an experimental level until 1897 [4].

Without entering into details, I shall indicate the mostdifficult phase of the conflict. In 1887 Hertz devised aseries of experiments in order to test the basic hypothesisof the ‘material’ theory, according to which the ‘cathoderays’ were electrical in nature [5]. In particular, heverified whether or not they carry a charge and weredeflected by an electric field.

Figure 2 illustrates the apparatus used by Hertz todetect an eventual charge. The vacuum tube was placedinside two coaxial cylinders,β andγ , both connected tothe electrometer. The inner cylinder acted as a chargedetector and the external tube as a screen. Accordingto Hertz, if the ‘cathode rays’ carried a charge itshould have been communicated by induction across theinner cylinder to the electrometer and detected by it.However, contrary to every prediction of the ‘material’theory, no charge was signalled.

To verify the eventual action of an electric field,Hertz used an apparatus (similar to the one shown infigure 4) in which the ‘cathode rays’ were made topass through two metal plates connected to the polesof a battery. If the ‘cathode rays’ were sensitive to theelectric field, they should have been deflected. Contrary

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Figure 2. Hertz’s apparatus to detect an eventualcharge of the ‘cathode rays’.

to this prediction of the material theory, there was noperceivable shift of the ‘rays’.

Hertz’s experiments did not, however, end the debatebetween ‘material’ and ‘aetherial’ theories. Quite tothe contrary, the debate not only continued, but nowfocused principally on the problems touched on byHertz’s experiments, and eventually led to diametricallyopposed results. Supporters of the ‘material’ theorywere encouraged by the deflection of the ‘rays’ in amagnetic field, and at the same time they were able toaccount for Hertz’s results in terms of their own theoryby saying that an unexpected effect or an as yet unknownphenomenon had in some way masked the electrostaticcharacteristics of the ‘cathode rays’. It is from this pointof view and with the precise purpose of eliminatingspurious effects present in Hertz’s experiments, that inmy opinion we must interpret, for example, Perrin’sexperiment of 1895 [6].

Figure 3. Perrin’s apparatus to detect an eventual charge of the ‘cathode rays’.

In 1895 Perrin repeated Hertz’s experiment on chargecarrying, but with an important variation. The twocoaxial cylinders (β and γ ), which in the arrangementadopted by Hertz were placed outside the vacuum tube,in this case were placed inside the tube to eliminate thescreening effect of the glass. The apparatus used byPerrin is illustrated in schematic form in figure 3.

The two metal cylinders ABCD and EFGH had twosmall openings,α andβ, to allow the ‘cathode rays’ toenter them. The cathode was formed of an electrodeN, and the anode of the protection cylinder EFGH.With this apparatus Perrin observed that when the beamof ‘cathode rays’ entered cylinder ABCD, ‘invariablythe cylinder became charged with negative electricity’.If, however, the equipment was placed in a magneticfield, so that the ‘cathode rays’ could no longer entercylinder ABCD, ‘the cylinder was not charged’. Perrinhence concluded: ‘Cathode rays are therefore chargesof negative electricity’.

Despite Perrin’s results, the ‘material’ theory stillhad to come to terms with Hertz’s objection, when hedemonstrated that the ‘cathode rays’ were not deflectedby an electric field. Several supporters of the ‘material’theory had repeated Hertz’s experiment but had alwaysobtained the old result. In the meantime, yet anotherobjection to the ‘material’ theory had arisen.

In 1894 Lenard, one of the most strenuous supportersof the wave nature of ‘cathode rays’, used an observationmade by Hertz in 1892 [7]—according to which‘cathode rays seemed to be capable of passing throughthin metal films’—to design a new type of vacuumtube, in which the wall of the tube opposite the cathode(which in normal conditions blocked the ‘cathode rays’)was replaced with a metal film, having small enoughthickness to be passed through by ‘cathode rays’ (e.g.a thickness of about 0.003 mm for aluminium). Thisisolated the ‘cathode rays’ from the discharge tube,enabling them to be studied under a wide range ofconditions. Lenard [8] proceeded with a systematicstudy of the absorption of these ‘rays’ by the variousmaterials. As ‘cathode rays’ could pass through metalfilms that were impenetrable to atoms (they were oftenused to separate hydrogen or other gases, on one side,from a good vacuum, on the other), according to Lenardthey could not be considered atoms but had to beregarded rather as waves. This was the state of thedebate on ‘cathode rays’ when Roentgen announced hisdiscovery of ‘x-rays’ in 1895 [9].

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Figure 4. Thomson’s apparatus to detect the deflection of the ‘cathode rays’ by an electric field.

2. The ‘corpuscle’

At the beginning of 1896, when Roentgen’s firstpapers began to circulate among English physicists, J JThomson (then director of the Cavendish Laboratory inCambridge) was studying the conduction processes ofelectricity in gases. The first thing he did on obtaininga copy of the x-ray apparatus devised by Roentgenwas to ‘verify the effect of these rays on a gas’. Herealized that the x-rays ionized the gas, turning it intoa good conductor of electricity. At this point the waywas open to the discovery of the electron. Thomsonchecked whether ‘cathode rays’ had the same propertiesas ‘x-rays’. If this were the case, it would have beenpossible to consider the lack of deflection of the ‘rays’in the presence of an electric field—observed duringexperiments—as due to the ionization of the gas, whichin some way masked the electric field present. In otherwords, the deflection of the ‘rays’ was zero becausethe electric field strength was reduced to zero. Bymeans of a series of experiments aimed at establishingthe eventual link between the conductivity conferred onthe gas by the ‘cathode rays’ and the gas pressure,Thomson arrived at the conclusion that there was aconductivity of the gas, which disappeared very rapidlyas the exhaustion was increased [10]. At this pointThomson repeated Hertz’s experiment of 1887, butat reduced pressure (see figure 4), and he observedthe deflection of the ‘rays’, even ‘when the potentialdifference (created between D and E) was as small as2 V’. Thomson commented [11]: ‘It was only whenthe vacuum was a good one that the deflection tookplace’. After verifying that the ‘cathode rays’ werecarrying a charge and were deflected by a magnetic fieldas well as by an electric field, Thomson concluded asthe only possibility ‘that they are charges of negativeelectricity carried by particles of matter’. Then he askedthe new question: ‘What are these particles? Are theyatoms, or molecules, or matter in an still finer stateof subdivision?’ To answer this question, Thomsondetermined the mass/charge ratio of these particles byusing two different experimental methods.

The first method exploited the deflection of ‘rays’both in an electric and in a magnetic field. The apparatusused is shown in figure 4, where a magnetic fieldB(of fixed strength) was applied perpendicularly to the

path of the ‘rays’, while a variable electric fieldE wascreated between the plates D and E. Thomson variedEto that valueE0 at which the ‘cathode rays’ returnedto the undeflected position. Under these conditions thevelocity v of the ‘rays’ took on the value

v = E0/B. (1)

Then the electric field was removed and the radius ofcurvatureR of the ‘rays’ observed and calculated. Onapplying equation (1), Thomson found an expression form/e that contained only observable quantities, namely

m/e = RB2/E0. (2)

In the second method, Thomson exploited only thedeflection in a magnetic field. The velocityv of the‘rays’ was determined, assuming that all the kineticenergy can be transformed into heat, from making useof the formula:

m/e = RB2Q/2W (3)

where Q represented the charge passing through asection of the beam in the time unit andW was thekinetic energy associated with it. These quantities weremeasured by Thomson using three different tubes, allof the type devised by Perrin (see figure 3). Thequantity W was measured using a thermocouple ofa known thermal capacity placed behind the centralopening.

These two methods enabled Thomson to obtain avalue for the ratiom/e of the order of 10−7 g/emu.This value proved out to be independent of the materialused for the cathode, the gas employed, and the pressureapplied. He emphasized, in particular, that it was ‘verysmall compared with the value 10−4’, which was thesmallest value known so far for the mass/charge ratio ofan ion, the hydrogen ion.Per se, this value was rela-tively insignificant unless it was separated into mass andcharge, separately. Thomson, however, succeeded indeducing information from this ratio thanks to Lenard’smeasurement of 1894 of the absorption of cathode raysin air; at a pressure of 0.5 atm, he obtained the meanfree path of the ‘cathode rays’ and compared it to themean free path of a molecule of air under the same con-ditions. In the case of ‘cathode rays’ the value came outto be about 0.5 cm, while for air the value was about2×10−5 cm, that is ‘a quantity of a quite different order’.

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According to Thomson, ‘cathode rays’ were new parti-cles of a much smaller mass than ordinary molecules.

Given a constant mass/charge ratio, independently ofthe materials used, it became impossible for Thomsonto avoid not only the conclusion that the atom was acomplex structure made of constituents—this hypothesishad already been adopted by a number of chemists andThomson himself the previous year to explain the lawof absorption of x-rays [12]—but also by the hypothesisthat the ‘cathode rays’ represented one of theseconstituents (i.e. a component of the atom with negativecharge) that had left the atom. This atomic constituent(which was to become ourelectron) Thomson called‘the corpuscle’. He thus concluded the debate between‘aetherial’ and ‘material’ theories by stating:

‘We have in the cathode rays matter in a new state,a state in which the subdivision of matter is carriedvery much further than in the ordinary gaseous state: astate in which all matter derived from different sourcessuch as hydrogen, oxygen, etc, is of one and the samekind; this matter being the substance from which allthe chemical elements are built up. [· · ·] If, in thevery intense field in the neighbourhood of the cathode,the molecules of the gas are dissociated and are splitup, not into the ordinary chemical atoms, but intothese primordial atoms, which we shall for brevity callcorpuscles; and if these corpuscles are charged withelectricity and projected from the cathode by the electricfield, they would behave exactly like the cathode rays.They would evidently give a value ofm/e which isindependent of the nature of the gas and its pressure,for the carriers are the same whatever the gas may be’.

What remained was to measure the charge or the massof these corpuscles separately. As we will see, thistask Thomson performed in 1899; he thus confirmedthe hypothesis of the existence of a negatively chargedatomic component.

3. The charge of the ‘corpuscle’

In 1874 Stoney [13], interpreting the laws of Faraday onelectrolysis in the light of the valence theory proposedby Kekule a few years before, managed to identify theexistence of a ‘defined quantity of electricity’ by meansof which atoms seemed to combine chemically. This‘defined quantity of electricity’ (later called ‘electron’by Stoney) was estimated by Stoney to be equal to1.03× 10−21 emu. Other estimates in the framework ofelectrolysis arrived at values between 4.31 and 4.71×10−20 emu [14].

Wiechert [15] used the thus ‘defined quantity ofelectricity’ in January 1897 to interpret the nature of‘cathode rays’. Although he had only been studying thistopic for a short time, he adopted a ‘material’ conceptionof these rays, and hypothesized that they consisted ofnegatively charged particles having a charge value equalto ‘1 electron’. To establish if these particles were‘chemical atoms, or groups of atoms, or some otherbody’, Wiechert attempted to estimate their mass, from

deflecting them in a magnetic field. He obtained, inparticular, the following expression (which was lateralso used by Thomson) for a magnetic fieldB placedperpendicularly to the ‘rays’:

m = Bre/v (4)

wherem, e, v, and r denote, the mass, the charge, thevelocity, and the radius of curvature of the materialcharged objects, respectively. Sincee had been fixeda priori, in order to obtainm from (4) it was necessaryto measurev. Wiechert managed to estimate only anupper limit and a lower limit forv, and thus derived arange of values form. To estimate the upper limit ofvelocity he assumed that all the energy obtained fromthe electric field in the discharge was transformed intokinetic energy. Then he obtained a maximum velocityof 108 m s−1 and, with equation (4), a lower limit forthe mass, namely 1/4000 of the mass of the hydrogenatom. The minimum velocity followed from havingthe cathode rays interact with an electromagnetic wave(produced by the same alternating current used to obtainthe ‘cathode rays’). Wiechert measured the time it tookthe ‘cathode rays’ to transit over a distance of 20 cm. Inthis way he concluded that the velocity of the ‘cathoderays’ was greater than 3× 10−7 m s−1; consequently,their mass was less than 1/2000 of the mass of thehydrogen atom. He therefore concluded:

‘So far as the cathode rays are concerned, they cannotbe atoms as they are known in chemistry, as their massis 2000–4000 times smaller than that of hydrogen, orlighter than the known chemical atoms’.

In judging about Wiechert’s conclusion, whichpreceded Thomson’s conclusions by several months, itmust be remembered that it rested on the assumptionthat the charge of the ‘cathode rays’ was identical withthat of the ‘electron’ of electrolysis. In 1897 such anassumption cannot be considered to be evident. In anycase, Thomson, having obtained the mass/charge ratio ofthe ‘cathode rays’ and concluded that they were particlesmuch smaller than the hydrogen atom, embarked on aprogram of research aimed at measuring their charge, inorder to establish ‘how much smaller than the hydrogenatom’ they were.

This program was not an easy one, as Thomson im-mediately encountered serious difficulties in performingthe necessary measurements on ‘cathode rays’. Indeed,he had to look for another phenomenon, which he finallyfound in the photoelectric effect: he first gave a newinterpretation to this effect in terms of ‘emission of cor-puscles’; second, he obtained—by means of a method Ishall not explain here—a value for the mass/charge ra-tio of these ‘corpuscles’ having the same order of mag-nitude as that for ‘cathode rays’; finally, in 1899 hemanaged to devise a system to measure the charge [16].It should be added that this procedure was based onThomson’s experiences since 1890 (as director of theCavendish Laboratory) on the problem of discharge ingases, in which he had involved a number of young re-search associates including C T R Wilson, E Rutherford,J S Townsend and J Zeleny.

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The methods devised by Thomson were based on thefact that ‘corpuscles’, when produced by means of thephotoelectric effect, emitted into a gas saturated withwater vapour and subjected to a process of expansion,behaved like condensation nuclei and each of them couldbe isolated in water droplets. Thomson counted, on theone hand, the total number of droplets observed, whichcorresponded to the number of ‘corpuscles’ present; onthe other hand, he measured the total charge present.From both results he estimated the charge of a single‘corpuscle’.

In this way he finally obtained a value for the chargeof 2.3 × 10−20 emu: hence it assumed the size ofthe ‘unitary charge’ in electrolysis that had been called‘electron’. By inserting this value into the mass/chargeratio previously measured, Thomson found the mass ofthe ‘corpuscle’ to be 3× 10−26 g. After many years oflabour on the problem of discharge in gases, he finallyachieved what he called in a letter to Rutherford the‘ . . .direct proof of the existence of masses only 1/1000of the mass of the hydrogen ion’ [17].

References

[1] Maxwell J C 1873Treatise on Electricity andMagnetism(Oxford: Oxford University Press)section 57 (3rd edn, 1904)

[2] De La Rue W and Muller H W 1880 Experimentalreasearches on the electric discharge with the chlorideof silver batteryPhil. Trans.171 65–116

[3] Faraday M 1838 Experimental researches in electricityPhil. Trans. R. Soc.128 125

[4] On the subject see for example: Anderson D L 1964TheDiscovery of the Electron(Princeton, NJ: PrincetonUniversity Press) Van Nostrand Momentum Book

Whittaker E T 1989A History of the Theories of Aetherand Electricity2nd edn (New York: Dover)

Falconer I 1987 Corpuscles, electrons and cathode rays:J J Thomson and the discovery of the electronBr. J.History Sci.20 241–76

Owen G E 1956 The discovery of the electronAnn. Sci.11 172–82

Pais A 1986Inward Bound(New York: OxfordUniversity Press)

Feffer S M 1990 Arthur Schuster, J J Thomson and the‘Discovery of the Electron’Hist. Studies Phys. Biol.Sci. 20 33–61

Chayut M 1991 J J Thomson: the discovery of theelectron and the chemistAnn. Sci.48 527–44

Price D J 1956 Sir J J Thomson, OMFRSNuovoCimento4 (suppl) 1609–29

[5] Hertz H 1896 Experiments on the cathode dischargeMiscellaneous Papers(London: MacMillan) (transl.from 1883Ann. Phys., Lpz.19 782–816)

[6] Perrin J 1895 Nouvelles proprietes des rayonscathodiquesC. R. Acad. Sci., Paris121 1130–5

[7] Hertz H 1892 Ueber den Durchgang derKathodenstrahlen durch dunne MetallschichtenAnn.Phys., Lpz.45 28–32

[8] Lenard P 1894 Ueber Kathodenstrahlen in Gasen vonAtmospharischem Druck und im aeussersten VacuumAnn. Phys., Lpz.51 225–67

[9] Roentgen W C 1895 Preliminary communicationSitzungsberichte der Physikalisch-medizinischenGesellschaft zu W¨urzburg (Published in: 1896 On anew form of radiationThe Electrician36 415–7)

[10] Thomson J J 1897 Cathode raysThe Electrician37104–11

[11] Thomson J J 1896 Cathode raysPhil. Mag. 44 293–316[12] Thomson J J 1896 Les rayons RoentgenRevue Sci.6

289–95[13] Stoney G J 1874 On the physical units of NatureReport

of the British Association, 44th Meeting (Belfast)(Published in: Stoney G J 1881Phil. Mag. 11381–90)

[14] Lodge O J 1885 On electrolysisReport of the BritishAssociation, 55th Meeting (Aberdeen)pp 723–64

Richarz M F 1891Sitzungsberichte derNiederrheinischen Gesellschaft zu Bonn(Reported inEbert H 1894 Heat of dissociation according to theelectrochemical theoryPhil. Mag. 38 332–6)

[15] Wiechert E 1897 1. Ueber das Wesen der Elektricitat.2. Experimentelles uber die KathodenstrahlenSchriften der Physikalisch-oekonomischenGesellschaft (K¨onigsberg)vol 38, pp 1–16

[16] Thomson J J 1899Uber die Masse der Trager dernegativen Elektrisierung in Gasen von niederenDruckenPhys. Z.1 20–2

Thomson J J 1899 On the existence of masses smallerthan the atomsReport of the 69th Meeting of theBritish Association for the Advancement of the Science(Dover) (Published in 1899Phil. Mag. 48 547–67)

[17] Thomson J J toRutherford E, 23 July 1899(E Rutherford Correspondence, CambridgeUniversity Library, Cambridge). See on this topic,e.g., Robotti N 1995 J J Thomson at the CavendishLaboratory: the history of an electric chargemeasurementAnn. Sci.52 265–84