B. A. Morrow University of Ottawa Ottawa 2, Canada
On the Discovery of the Electron
M o s t physical chemistry and general chemistry textbooks describe J. J. Thomson's experimental determination of the ratio of the charge to the mass (e/m) of the electron and R. A. Illillikan's experimental determination of the magnitude of the electronic charge e. While both experiments were of enormous importance in the development of the theory of atomic structure, many textbook descriptions do not make adequate mention of the tremendous controversies which these experiments resolved. J. J. Thomson did not set out merely to determine e/m of the cathode rays, but to establish conclusively that they were indeed particles in the first place, and not electromagnetic waves. Similarly, a t the time of Millikan's "oil drop" experiment, several previous investigators had already measured the average charge on the eleetron to within a few percent of the presently accepted value. Millikan established conclusively that the electron always carried a definite and invariant charge. Therefore, Thomson's experiment resolved the controversy concerning the corpuscular or wave nature of the cathode rays and Millikan's experiment resolved the controversy concerning the continuous or discrete nature of electrical phenomena. I n order to show how these controversies arose and why Thomson's and Millikan's experiments were so important, a brief historical account will be given of the early work on the condu'ction of electricity through gases and liquids. Conduction of Electricity through Liquids
The development of the Voltaic pile in 1800 led to a considerable body of research in the first quarter of the 19th century on the chemical and physical nature of the conduction of electricity by liquids. Many early investigators had found that when an electric current was passed through a liquid, the process was usually accompanied by chemical changes in the liquid or a t the metallic terminals at which the current enters and leaves the liquid (I). I n the 1830's Michael Faraday took up the investigation of the quantitative aspects of electrolysis (as he was first to call this process) which subsequently led to the publication in 1833 of what are known as Faraday's two laws of electrolysis. I n his own words, these laws were stated (2) (1) . . .the chemicd power of s. current of electricity is in direct proportion to the absolute quantity of electricity which passes (2) . . .the equivalent weights of bodies are simply those quantities of them which contain equal quantities of electricity
I n spite of Faraday's brilliance in deducing these two experimental laws, he failed to make the decisive step in 584 / Journal of Chemical Education
postulating that a definite and fundamental quantity of electricity was associated with each atom and that during electrolysis each atom received or lost a definite quantity of electricity a t the electrodes (Farac'ay's term for the "metallic terminals"). However, he was essentially correct in his deduction of the nature of chemical binding forces. I11 his Scientific Researches he says (3) That atoms of matter are in some way endowed or associated with electrical powers, to which they owe their most striking qualities, and amongst them their mntual chemical affinity. . . .all the facts show u s . . . t h a t the electric current is only another form of the forces of chemical affinity.. .that, in other words, the forces termed chemical affinity and electricity me one and the same.
This idea had been expressed earlier by Humphry Davy
(4) He [Davyl supposed that there are two kinds of electricity with one or other of which all bodies are united. These we distinguish by the names of positive and negative electricity; those bodies are disposed to combine which possess opposite eleotricities, as they are brought together by the attraction which these electricities have for each other.
J. Clerk Maxwell was probably first to entertain the idea that atoms might possess a definite fundamental unit of electrical charge. Concerning Faraday's laws of electrolysis he stated in 1873 (5) If we.. . m u m e that the molecules of the ions within the eleotrolyte are actually charged with certain definite quantities of electricity, positive and negative, so that electrolytic current is simply a current of convection, we find that this tempting hypothesis leads us into very difficult ground. Suppose we leap over this difficulty by simply asserting the constant value for the molecular charge, and t h t t we cd1 this. . .one molecule of electricity.
lVIaxwell imagined that during electrolysis the cations and anions would liberate "onemolecule" of positive and negative electricity respectively a t the cathode and the anode. However, in the same discussion he eventually rejected this simple model (6) This theory of molecular charges may w v e as a method by which we may remember a good many facts about electrolysis. It is extremely improbable however that when we come to understand the true nature of electrolysis we shall retain in any form the theory of molecular charges, for then we shall have obtained a. secure basis on which to form a true theory of electric currents, and so become independent to these provi~ional theories.
G. J. Stoney, however, expressed a belief in Rfax~r-ell's "molecule of electricity" as early as 1874 (7,s) I shall express Fmaday's laws in the followi~rgterms.. .for each chemical bond which is ruptured withi11 a n electrolyte a second quantity of electricity traverses the electrolyte which is the same in all cases. This definite quantity of electricity I
shall call el. If we make this our unit of electricity we shall probably have made a very important step in our study of molecular phenomena.
I n 1881 (8) he estimated the value of this "definite quantity of electricity" to be 3 X lo-" esu about an order of magnitude smaller than the present accepted value of about 4.80 X esu and, in 1891 Stoney first called these charges "electrons" (9). Helmholtz was also in favor of Stoney's electron concept of electricity and wrote in 1881 (10) I f w v wurpr the hypothvsis thnt the rlrmcattnry subrtnwrs are compmerl uf atoms, s r clnrlot e w i d cwwludtng that rlectririty a h is dividrd inro defitotr rlrmrutar? porttons which k h n v r like atoms of electricity.
Conduction of Electricity through Gases
The most direct evidence for the existence of a fundamental unit of electricity came eventually not from a study of electrolysis, but from studies of the conduction of electricity by gases. Throughout the 18th century and in the early part of the 19th century many scientists had observed the brilliant luminescence which accompanies the passage of an electric discharge through rarefied gases but it was again Faraday who, in 1838, carried out the first systematic study of the phenomena (11). During the passage of an electric current through rarefied air he noticed that a purple glow extended from the positive pole but stopped short of the negative electrode. However, the negative electrode itself was covered in a purple glow (the "cathode glow" as it eventually was called) and the intenrening dark space has subsequently been called Faraday's dark space. Faraday was unable to carry the investigation further because of the incapacity of the air pumps then in use to rarefy gases to a higher degree of vacuum. Following the development of a more efficient vacuum pump by Geissler in 1855, Julius Pliicker (1858) took up the investigation of these discharges in highly evacuated tubes and found that as evacuation continued beyond the point reached by Faraday the cathode glow expanded and another dark space developed between the cathode and the cathode glow (12). Eventually the walls of the tube began to phosphoresce, and the position of the phosphorescence on the walls was deflected in a magnetic field. Hittorf (1869) found that a solid body placed between the cathode and the phosphorescence cast a shadow on the walls and inferred that the glow was formed of rays (called glimmstrahlen or "glow rays") which proceeded in straight lines from the cathode to the walls of the tube and caused the phosphorescence ( I S ) . Goldstein subsequently found in 1876 (14) that shadows were also cast when an object was placed near a cathode which was either a point or was an extended flat surface, thereby showing that these "cathode rays" (kathodenstrahlen-a term first introduced by Goldstein) were emitted from each portion of the cathode surface and traveled in a single direction normal to the surface. William Crooks continued with the study of gas discharges and in 1879 (15) found that by increasing the degree of evacuation the dark space around the cathode eventually expanded until it (the "Crooks dark space" as it is now known) enveloped the entire tube. Crooks (16) regarded the cathode rays as a "molecular torrent" of ionized molecules and in the Crooks dark
space he assumed that the mean free path of the ions was so large that collisions could be neglected. I n the ensuing twenty years a number of physicists, including A. Shuster, H. Von Helmoltz, J. J. Tbomson, and E. Riecke developed Crooks' "molecular torrent" idea and assumed that the rays were charged "particles" of the residual gas in the discharge tube (17). The experimental evidence in support of this reasoning was: the rays were deflected by magnetic fields; the thinnest layers of non-metallic solids (including quartz) were opaque to the rays (18); and when the rays impinged on vanes like those in a radiometer the vanes were set in motion (15). Hittorf suggested that the latter of these might not be due to purely mechanical effects but due to secondary thermal effects as occurs when infrared radiation falls upon a radiometer (18). However, several physicists of the German school, namely E. Wiedemann, E. Goldstein, and G. Hertz maintained the view that the cathode rays were merely a disturbance of the aether and were therefore a wave phenomena. They had all found that the cathode rays were capable of passing through thin metal films which were opaque to visible light and i t seemed inconceivable that matter could penetrate through such films. Shuster summed up the controversy when he stated (19) Adoptinp the view rhnt n rurrcrlt of clectriviry simply mratu n flow of uerhrr, i r was trrnprlng to ~ t t r i b u t e11welTe~tr ohsrrvcd under reduced pressure to secondary effects accompanying longitudinal or other vibrations set up by the discharge.
J. J. Thomson suggested (18) that the apparent transparency of metal films might simply he another aspect of . . .the phenomenon observed previously by Crooks, that 8. metal plate exposed to the full force of these rays becomes a cathode; in Hertz's experiment the films may have been so thin that each side acted like a cathode, and in this case the phosphorescence on the glass would be caused by the film acting like a cathode on its own account. The advocates of the corpuscular theory assumed that the particles would be ionized atoms or molecules and that the discharge phenomenon in rarefied gases was analogous to electrolysis in liquids (18). Indeed, Perrot had shown (20) that in an electric discharge through steam there was a preponderance of hydrogen a t one pole and of oxygen a t the other. It was further assumed that the particles obtained their charge from the cathode and were accelerated initially by electrostatic repulsion. The magnitude of this charge was assumed to he constant (or some simple multiple of a constant charge) or in other words, illaxwell's "molecule of electricity." Shuster resumed the investigation of the cathode rays in the 1880's and says (21) I realized a t a n early stage that in order to demonstrat,e the correctness of the theory of ionic charges i t was necessary to find a proof that the charge is a definite quantity, and a crucial experiment could be devised by observing the magnetic deflection of cathode rays.
If aparticleof mass m, charge e, and velocity u is moving a t right angles through a magnetic field of strength H, then the particle will be deflected through a path of radius r, and the ratio of charge to mass (elm) of the particles will be given (21) by
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Shuster made several attempts to measure the ratio e/m since v, H, and r could he measured, and found a value that was about 500 times greater than the value to he expected (as found by electrolysis) for a nitrogen atom (21). However, Shuster considered this value to he far too large (for he assumed that charged ions were the cathode rays) and he argued that his estimate of the u was too large. I n 1897 Wiechert again attempted to measure this ratio in a modified form of Shuster's experiment (22) and he obtained a value which was about 2000 times greater than that for a hydrogen atom. However, like Shuster, there was a large uncertainty in his estimate of the velocity v of the particles, of which he had only a lower limit and this was rather close to the velocity of light. His experiment did not resolve the particle-wave controversy since, if the rays were waves, they would he moving with the velocity of light. However, Wiechert was first to suggest that if the rays were particles, then m must he very small, for he assumed that e would have the same value as in electrolysis. The definitive experiment which clearly showed that the cathode rays were indeed particles and not a wave phenomenon was carried out by J. J. Thomson later in 1897 (%?).. J. Perrin had previously found in 1895 (24) that when a beam of cathode rays travelled normally from the cathode on to an insulated metal cylinder then a negative charge was imparted to the cylinder, hut when the beam was deflected away, no charge was imparted. Thomson discussed Perrin's work as follows This experiment proves that something charged with negative electricity is shot off from the cathode, travelling a t right angles to it, and that this something is deflected by a magnet; i t is open, however, t,o the objection that it does not prove that the canse of the elect~.ificationin the electroscope [attached to the insulated cylinder] has anything to do with the cathode rays. Now t,he supporters of the aetherial theory do not deny that electrified particles are shot off from the cathode; they deny, however, that these charged particles have any more to do with the cathode rays than a rifle-ball has with tho flash when R rifle is fired.
Thomson modified Perrin's experiment so that the beam of cathode rays would not impinge on the insulated metal cylinder unless deflected in the correct direction by a magnetic field. He followed the path of the rays by observing the phosphorescence on the glass walls of a n evacuated tube and he found that a charge was imparted to the cylinder only when the cathode rays actually fell on the cylinder. Thomson concludes Thus this experiment shows that however we twist and deflect the cathode rays by magnetic forces, the negative electrification follows the same path as the rays, and that this negative electrification is indisaalubly connected with the cathode rays.
The second part of Thomson's classic experiment, the determination of e/m of the negative particles, is described in detail in many chemistry textbooks and will not be repeated here. The earlier experiments were inconclusive owing to an enormous uncertainty of the velocity of the particles. By balancing the opposing deflection of the cathode rays in magnetic and electric fields, Thomson was able to express the velocity u in eqn. (1) in terms of the precisely known electric field strength. The values of e/m which he found varied slightly (about lo7 amu/g) but were always about 1000 times greater than the value for the ratio e/M for the hydrogen ion in electrolysis. Although he speculated that the value for 586
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e of the cathode rays might be slightly greater than the corresponding "e" in electrolysis, he concluded that the mass m of the cathode rays must be very much smaller than the mass M of a hydrogen ion. Most significantly however, Thomson found no evidence that the value of e/m depended on the gases in the tube, nor on the metal used for the electrodes (Thomson used aluminum, iron and platinum). I n speculation about the nature of these "corpuscles" he suggested that they were some '< primordial" substance which was common to all atoms. If, in the very intense electric field in the neighbowhood of the cathode, the molecules of the gas are dissociated and are split up, not into the ordinary chemical atoms, but into these primordial atoms, which we shall for brevity call corpuscles; and if these corpuscles are charged with electricity and projected from the cathode by the electric field, they would behave exactly like the cathode rays. They would evidently give a value of (elm)which is independent of the nature of the gas and its pressure, far the carriers are t,he same whatever the gas may b e . . .. Thus on this view we have in the cathode rays matter in a new state, a state in which the subdivision of matter is carried very much further than in the ordinary gaseous state: a state in which all matte-that is, mat,ter derived from different sources such as hydrogen, oxygen, etc.,-is of one and the same kind; this matter being the substance from which all the chemical elements are built up.
Therefore, Thomson conclusively established the corpuscular nature of the cathode rays, and showed that these "primordial atoms" (Stoney's electrons) always had within experimental error the same value for elm. Although his conclusion that these corpuscles are common to all matter is correct, only his suggestion that the rays emanated from the residual gas in the tube (and not from the electrode itself as we now know) has not stood up to further experimental study. Determination of the Electronic Charge
The next logical step after Thomson's measurement of elm for the corpuscles, or electrons as they are now called, was a determination of either m or e separately, and from an experimental point of view i t proved more feasible to determine the electronic charge e. Probably the first attempt a t a direct determination of this supposed fundamental unit of electricity was made in 1897-98 by one of Thomson's graduate students, J. S. E. Townsend (25). His experiment is of particular interest historically because i t was the precursor of a number of similar experimental techniques which were used to determine the charges of atomic and subatomic particles. During studies of the gases released during electrolysis, he found that some of the oxygen released from electrolysis of a solution of HzSOn carried a positive charge whereas some of the oxygen liberated during the electrolysis of a KOH solution was negatively charged; in both cases the gas was released from the anode of the cell. Townsend discovered that this charged gas caused the spontaneous formation of a cloud when the gas came in contact with air which was saturated with water vapor. Townsend measured the total charge per cm3 of the cloud with an electrometer and he measured the total weight of the cloud by passing i t through drying tubes and determining the increase in weight of the tubes. He then determined the average size of the drops in the cloud from a measurement of the velocity (v) of descent of the top surface of the cloud which,
assuming that the drops were spherical of radius r would he given by
measured v,, the radius r of the drop could be determined and hence m could he eliminated from eqn. (3) by use of the relation
where pis the density, g the acceleration due to gravity, and 7 the coefficient of viscosity of air (Stokes law). He therefore obtained the average mass and volume of the drops and hence calculated the total numher of drops. By assuming that every ion had a drop of water condenscd on it Townsend evaluated the average charge per gaseous ion to he 2.8 X esu for the positively charged ions, and 3.1 X 10-'0 esu for the negatively charged ions. He concluded that the fundamental unit of positive and negative charges were equal within his experimental unccrtainty, with a value of about 3 X 10-'" esu. However, the range of values found by Townsend was large, as were the experimental uncertainties, some of these being, the assumption of single charges per drop, the assumed lack of evaporation of the aqueous drops, and the assumption that Stokes law was valid for very small drops. Therefore, the experiment in no way proved the existence of a discrete value for the fundamental unit of electrical charge. Later in 1898 (26) J. J. Thomson used a modified form of the falling cloud method to obtain a value of the negative charge produced by the action of X-rays on moist air. C. T. R. Wilson (27) (Wilson cloud chamber) had just discovered that when dust free moist air was subjected to a t least a 1.25 fold expansioninvolume (corresponding to a four fold increase in supersaturation) drops of water vapor spontaneously formed from the cooled supersaturated gas. However, if the moist air bad been previously exposed to X-rays then a dense cloud formed after a 1.25 fold expansion, and the iudividual droplets were negatively charged. Thomson determined the numher of ions per cm3 of the gas by measuring the current produced in this gas when a known electromotive force was applied. Then, using Townsend's method of measuring the rate of falling of the cloud under the influence of gravity in order to determine the size of the drops he was ahle to deduce a value of e which he found to he independent of the nature of the gas in which the ions were produced. Thomson's value for e of about 6.5 X esu was the mean value of many determinations which gave results for e within the range of 5 to 8 X esu. I n 1902-03 Thomson repeated his experiments (28) as he suspected that his earlier experimental technique was a t fault and he now found a value fore of about 3.4 X 10-lo esu. In 1903 H. A. Wilson (29) modified the falling cloud technique by subjecting the falling charged droplets to a vertical electric field and he measured the velocity of descent of the top surface of the cloud in the absence (v,) and in the presence (vz) of the electric field. If the average mass of a drop in the top surface of the cloud is m, then the ratio of the velocities of descent will be given by
After substitution of the known values of p and 7 , eqn. (3) could he rearranged to give the following expression fore
+
where m y represents the gravitational force, and 7ng Ee represents the gravitational and electrical force when an electric field of strength JTis applied to a drop with a sinqle charge e. Using Stokes law (eqn. (2)) with the
Wilson's method was in principle an improvement over other methods since i t was now somewhat justifiable to assume that in the presence of the electric field the drops a t the top of the cloud would indeed contain only a single charge, the other drops being forced down more rapidly. However, this technique still had most of the other disadvantages of Townsend's and Thomson's method with the added assumption that successive c l o u d ~ f o the r measurements in and out of the electric field-were identical. Therefore, it is not surprising that Wilson again found a wide range of values for e, with a mean of 3.1 X 10-In esu. An estimate of the electronic charge e was made in 1900 by Planck (SO,31) who had just derived an expression which would fit the experimental black body spectral distribution curve. I n the course of this work Planck introduced and evaluated two new universal constants, hand k, now known as Planck's constant and Boltzmann's constant respectively. Planck correctly interpreted k as being the gas constant for one molecule, and since R was known (the gas constant per mole) he was able to determine Avogadro's number No from the relationship Knowing the macroscopic Faraday constant (F) from electrolysis, Planck was then ahle to determine the electronic charge from the relation Planck's value for No was 6.175 X loZamolecules/mole, and fore was4.69 X esu (31). Rutherford and Geiger (32) at Manchester in 1908 developed a method for counting the numher of alpha particles emitted per second from radium (in an early version of the Geiger-Muller counter) and were ahle to determine that alpha particles were helium atoms with a positive charge of 9.3 X 10-lo esu. They argued that these particles were most probably doubly charged and not triply charged and that Thomson's and Wilson's measurements of e were probably too low. This being the case, they then proposed that the value for the fundamental unit of positive and negative charge should he 4.65 X esu. I n support of this value, Rutherford and Geiger said in a footnote It is of interest to note that Planck deduced a value of e = 4.69 X 10WLoesu from a general optical theory of the natural temperature radiation.
They were also aware that Millikan and Begeman had reported a preliminary value for e of 4.06 X 10-lo esu (33). When Iilillikan took up the investigation of the electronic charge in 1907 he immediately tried to duplicate Wilson's experiment but found that i t was extremely Volume 46, Number 9, September 1969
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difficultto follow the diffuse top layer of the cloud as it fell (34). Therefore, apart from the experimental uncertainties associated with the falling cloud method in general (35), some of which have already been mentioned, i t was intrinsically difficult to make a reproducible measurement. However, a more serious objection to the previous work was that none of the experimental methods used thus far to determine e , however accurate they might be, were capable of showing whether the clectron had a fixed charge. They all relied on some measurement of the average charge of a macroscopic collection of particles and therefore could not demonstrate the cxistence of a discrete and fundamental unit of charge. In other words, they did not show whether the value obtained for e (or the charge on an ion) was a fixed quantity, or a statistical mean of a range of possible values. By a stroke of luck, Millikan discovered a method which would allow him to resolve this dilemma. I n attempting to improve upon Wilson's method he decided to use a much higher electric field than was used by Wilson. With the field applied so as to oppose the downward motion of the cloud he hoped to be able to hold the charged cloud in suspension between the plates. He anticipated that he would then be able to determine the rate of evaporation of the charged drops, and so eliminate this uncertainty. However, when the electric field was applied the charged cloud immediately dispersed, showing that the previous assumption of uniform charge on the drops was far from correct. However, he did notice that a few individual drops remained, those which just happened to have the correct mass to charge ratio to remain stationary in the field. By viewing the motion of a drop through a telescope, both in and out of the electric field, he could determine the exact charge on an individual drop. This was a vast improvement over the previous methods and Millikan later referred to the technique as the "balanced drop" method for determining e (34). Most elementary textbooks give an adequate general description of Millikan's experimental arrangement and further details are given in several books written by NIillikan (34-36). Millikan's early "balanced drop" experiments with water were only moderately successful because the drops would not remain stationary owing to the evaporation of the water (33). However, in 1909 he hit upon the idea of using a low vapor pressure oil instead of water for forming his drops, thereby circumventing this difficulty, and in his subsequent experiments he was able to establish conclusively that an individual drop always carried an exact integral multiple of some precise unit of electrical charge. He was further able to determine what this unit of charge was, and to demonstrate the intrinsic accuracy of his method, his value of e determined in his later experiments is within 0.1% of the currently accepted value for e when the latest data on the viscosity of air is used in his equa-
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tions and with his data. Thus, although Planck and Rutherford had determined average values for the supposed fundamental unit of electric charge that were within 2% of i\'lillikan's later value, only n'Iillikanls experiment conclusively established that this was indeed a fundamental and invariant unit of charee. iaeain., Stoncy's concept of the electron) and this is the major significance to be attached to his work. -
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Literalure Cited (1) DAMPIRR,W. C., "A History of Science," Cambridge Universit,y Press, 1961, p. 214. M., "Experimental Researches in Electricity," (2) FARADAY, Dover Publication Inc., New Yark, 1965, see. 821, sec. 869. (3) Reference ($1, see. 852, sec. 918. (4) M ~ R C E J., T , "Conversations on Chemistry," London, 1809, Vol. 1, p. 23; WILLIAMS, L. P., ".Michael Faraday," Chapman and Hall, London, 1965, p. 20. (5) M n x w n ~ ~J., C., "A Treatise on Electricily and Magnetism," Clarendon Press, Oxford, 1873, sec. 259, sec. 260. (6) Reference (6),sec. 260. G. J., British A~soci&tion,Belfast Report, 1874. (7) STONEY, ( 8 ) STONEY, G. J., Phil. Mag., 11, 384 (1881); Scientific Proceedings of the Royal Dublin Society, Fcb. 1881. (9) STONEY, G. J., Sci. Trans. Roy. Dublin Sac., 4, 563 (1891). (10) YON HELMHOLTZ, H., "On the Modern Development of Faraday's Concept of Electricily," Faraday Lectures, 18691928, (London), 1928, p. 145. (11) Reference (d), sec. 1526. (12) P L ~ ~ C K J., E RAnn. , der Phys., 103,80, 151 (1855R); 104,113, 622 (1858); 105, 67 (1858); 107, 77 (1859). (13) HITTORF,W., Ann. der Phys., 136, 1, 197 (1869). (14) GOLDSTEIN, E., Berlin Monalsberichle, p. 279 (1876). (15) CROOKS, W., Phil. Trans., 170, 135, 641 (1879). (16) CROOKS, W., Phil. Mag., 7, 57 (1879). (17) W H I T T - I KE., ~ , "A History of the Theories of Aether and Electricity," Thomas Nelson & Sons Ltd., London, 1953, Vol. 1., D. 352-365.
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THOMSON, J. J., "Electricity and Magnetism," Clarendon Press, Oxford, 1893, sec. 117, 118, 209. S H U S T ~A., , "The Progress of Physics," Cambridge University Press, Cambridge, 1911, p. 56. PI:RROT, A,, Ann. de Chimi?, (3) 61, 161 (1861). Reference ( I # ) , p. 62-65. WII:CHORT. E.. Sehrifln d. vhvsik.-oca. GES.ZU. Kmios~~
