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J.J. Thomson
The Discovery of the Electron
 
 
 
Dr V B Kamble

 

As early as about 1830, electrical discharges in gases were intriguing a number of experimental physicists in Europe. In 1881, at the Cavendish Laboratory at the University of Cambridge, J.J. Thomson began experimenting with gaseous discharges, and continued to do so for the next 50 years. When Thomson started his research, cathode rays had already been known for about 50 years, but their nature was controversial. As Thomson later wrote in the paper reporting his discovery of the electron, "The most diverse opinions are held as to these rays; according to the almost unanimous opinion of German physicists they are due to some process in the ether to which. no phenomenon hitherto observed is anagous; another view of these rays is that, so far from being wholly ethereal, they are in fact wholly material, and that they mark the paths of particles of matter charged with negative electricity."


Joseph John Thomson was born on December 18, 1856 in Manchester. His father died when he was only 16. Young Thomson attended Owens College in Manchester, where his Professor of Mathematics encouraged him to apply for a scholarship at Trinity College, one of the most prestigious of the colleges at Cambridge University. Thomson finished second in his class in the graduation examination in mathematics in 1880 and won the scholarship. Trinity College gave him a fellowship where he stayed upon. He engaged himself in developing mathematical models that would reveal the nature of atoms and electromagnetic process.

The Cavendish Laboratory at Cambridge was founded in 1871 with James Clerk Maxwell - who developed the basic equations of electromagnetism as the first Cavendish Professor. At 28, the young Thomson was chosen to be the third Cavendish Professor in 1884 following Maxwell and Lord Rayleigh. True, he was inexperienced in doing experiments, but he learned quickly. Supported by his administration and teaching, many important experiments on electromagnetism and atomic particles were performed. Many outstanding Physicists received their early training at the Cavendish, including 7 Nobel Prize winners and 27 Fellows of the Royal Society. Thomson took a keen interest in the work of all the young researchers, daily checking on their progress and making suggestions for improvement.

J.J. Thomson married Rose Paget on January 22, 1890. She was among the researchers at the Cavendish as one of the first generation of women permitted into advanced university studies. She performed experiments of soap films in 1889 after attending some of Thomson's lectures. They had two children: George Paget Thomson who flourished into a prominent Physicist himself and won Nobel Prize for discovery of the diffraction of electron by crystals in 1937. Their daughter John Paget Thomson often accompanied his father in his travels.

In the paper published in Philosophical Magazine 100 years ago in October 1997, Thomson reported that cathode rays were charged particles, which he called "corpuscles". It is hard to recall any discovery since then that has had more impact on not only physics but science, technology and our daily lives. We shall briefly follow the course of history in this article that made it possible.

Cathode Rays

Indeed, as far back as 1705, it had been noticed that sparks from an electrical machine would jump further in rarified air than in air at normal pressures. Watson in 1748 observed an aurora borealis like "arch of lambent (i.e. glowing) flame" in a glass tube of rarified air 32 inches long. In 1838 Faraday sent a current from an electrostatic machine through a glass tube containing air at low pressure and observed a purple glow extending from the positive electrode, or anode, at one end, almost to the negative electrode, or cathode, at the other. The cathode was covered with a glow, and there was a dark space between this glow and the purple column. The dark space has since been called the "Faraday Dark Space." (The colour of the internal glow of such tubes depends on the kind of gas present in the tube. Neon, at pressures approximately one hundredth that of atmospheric pressure, glows with a bright orange colour when current passes through it; helium, a pinkish white; mercury vapour, a light greenish blue.)

Although Faraday observed a number of interesting phenomena, he was limited by the fact that the suction pumps available at the time for reducing the gas pressure were not too efficient. A great step forward was made, about 1854, when H.Geissler, a German glass blower of exceptional skill, not only developed an improved vacuum suction pump, but succeeded in sealing into glass tubes wires attached to metal electrodes. The evacuated Geissler tubes which he made were particularly suitable for the study of the passage of electricity through gases at low pressure, and with them J. Plucker, in Germany, made numerous experiments between the years 1858 and 1862. Among other things, he observed that the tube in the vicinity of the cathode, i.e. the electrode attached to the negative side of the source of potential, emitted a green glow or luminescence. The position of the glow could be changed by bringing a magnet up to the tube.

The studies of electrical discharge through gases were continued in Germany by Plucker's pupil, W.Hittorf (1869), and by E.Goldstein (1876). From their observations they concluded that the luminescent glow on the tube was caused by "rays" originating at the cathode, which Goldstein consequently called cathode rays. The rays could be deflected by a magnet and were also able to cast a shadow of an obstacle placed in their path, showing that they traveled in straight lines.

Between the years 1879 and 1885 the English scientist William Crookes, who designed improved vacuum dischage tubes, made a very comprehensive series of investigations of the electrical discharge. From these he concluded that the cathode rays actually consisted of a stream of negatively charged particles, which were expelled from the cathode _ the negative electrode _ with extremely high velocities. This view of the nature of cathode rays supported a suggestion made in 1872 by C.F.Varley, but it was opposed by many European physicists, including such eminent men as E.Wiedemann (1880), H.Hertz (1883) the discoverer of radio waves, and (1894) P. Lenard. The latter group thought the cathode rays were an electromagnetic wave motion or vibration, analogous to light waves but of shorter wave length. If the rays are really a stream of charged particles then they should be deflected by passage through an electric field, as well as by a magnetic field, although Goldstein, in spite of several trials, had failed to observe any such effect. But the deflection of cathode rays in the field of magnet was an accepted fact, and this could not be explained if the rays were similar to light waves.

In an apparently decisive experiment, performed by J. Perrin in France in 1895, the cathode rays were allowed to fall on a device known as a Faraday cylinder, connected to an electrometer by means of which the sign and magnitude of electric charge could be determined. It was found that a negative charge collected in the cylinder, and so it was argued that the rays were made up of negative particles. Objection was taken to this conclusion on the grounds that negatively charged particles might well be ejected from the cathode, but there was no proof that they are identical with the cathode rays.

Atomic Models

On the basis of his experiments, J.J. Thomson proposed a model of internal atomic structure according to which atoms consisted of a positively charged substance (positive electric fluid) distributed uniformly over the entire body of the atom, with negative electrons embedded in this continuous positive charge like seeds in a watermelon, or raisins in pudding. Since electrons repel each other but are, on the other hand attracted to the centre of the positive charge, they were supposed to assume certain stable positions inside the body of the atom. Ernest Rutherford (1871-1937) and Hans Geiger together bombard d tbin pieces of gold with alpha particles. Most of the alpha particles passed right through the foil, and the result was exactly what the experimenters expected based on Thomson's model of the atom. But some of alpha particles struck the gold foil and were deflected at a sharp angle often 90ø or more (J J Thomson's Model). This amazed Rutherford, who remarked "It was as though you have fired a 15-inch shell at a piece of tissue paper and it came back and hit you". Early in 1911 Rutherford exclaimed to Geiger, "I know what the atoms looks like!". Rutherford put together a new idea of the atom: what if all the positively charged particles in the atom were not spread like a fluid throughout the atom as Thomson had thought but were lumped together in the centre in one tiny area, or "nucleus"? Most of the atom's mass would be contained in the nucleus, and an equal number of negatively charged electrons would be found in motion somewhere outside the nucleus. Undoubtedly, it was a compelling idea - a sort of tiny planetary system that resembled the larger solar system we are living in.

The required proof was provided in 1897 by J.J. Thomson (Fig. 1), the famous English physicist, whose work has had a profound effect, both direct and indirect, on the study of atomic structure. In the first place, he repeated Perrin's experiment and confirmed that charged particles are emitted by the cathode. But, in addition, he showed that when the cathode rays are deflected by a magnetic field, as indicated by the change in position of the luminescence they produce, the negatively charged particles are correspondingly deflected. Further, Thomson succeeded, where Goldstein and others had failed, in deflecting the path of the cathode rays by means of an electric field. Previous failures had been due to excessive ionization of the gas still present in the discharge tube, thus offsetting the effect of the electric field. By working at very low pressures Thomson minimized the influence of this ionization and then he was able to observe the anticipated deflection. We shall trace his efforts that established that the cathode rays are actually a stream of particles carrying negative electrical charges. Indeed, this was the result of a number of converging studies by several prominent physicists, which we shall briefly consider.

The Experiments:

First, in a variation of an 1895 experiment by Jean Perrin, Thomson built a cathode ray tube ending in a pair of metal cylinders with a slit in them . These cylinders were in turn connected to an electrometer, a device for catching and measuring electrical charge. Perrin had found that cathode rays deposited an electric charge. Thomson wanted to see if, by bending the rays with a magnet, he could separate the charge from the rays. He found that when the rays entered the slit in the cylinders, the electrometer measured a large amount of negative charge. The electrometer did not register much electric charge if the rays were bent so they would not enter the slit. As Thomson saw it, the negative charge and the cathode rays must somehow be stuck together: you cannot separate the charge from the rays.

All attempts had failed when physicists tried to bend cathode rays with an electric field. Now Thomson thought of a new approach. A charged particle will normally curve as it moves through an electric field, but not if it is surrounded by a conductor, say, a sheath of copper. Thomson suspected that the traces of gas remaining in the tube were being turned into an electrical conductor by the cathode rays themselves. To test this idea, he took great pains to extract nearly all of the gas from a tube, and found that now the cathode rays did bend in an electric field after all (Figure).

Thomson concluded from these two experiments, "I can see no escape from the conclusion that [cathode rays] are charges of negative electricity carried by particles of matter". But, he continued, "What are these particles? are they atoms, or molecules, or matter in a still finer state of subdivision?". This is the famous "duck argument". If it looks like a duck, quacks like a duck and waddles like a duck, then we have good reason to believe it is a duck!

Thomson's third experiment sought to determine the basic properties of the particles. Although he couldn't measure directly the mass or the electric charge of such a particle, he could measure how much the rays were bent by a magnetic field, and how much energy they carried. From this data he could calculate the ratio of the mass of a particle to its electric charge (m/e). He collected data using a variety of tubes and using different gases. The method essentially involved sending the beam into an electrically shielded collector, as in the Perrin's experiment, but in this case making the collector physically small. The beam gave up its charge to the collector and also heated it by mechanical impact. The quantity of heat energy, H, given to the collector in a given interval of time T could be determined from its mass, specific heat, and temperature rise. This charge in temperature could be measured by means of a very light thermocouple attached to the collector. The total charge, Q delivered to the collector could be measured by a sensitive electrometer.

Electron As A Particle

In 1899, Thomson set out to resolve the doubt concerning the significance of the e/m values of the "corpuscles" by determining directly their charge as well as the charge to mass ratio. Unfortunately, this could not be done with the cathode-ray particles, and so he turned to another source. It was well-known towards the end of the 19th century that ultra-violet light falling on certain metals, particularly zinc, was associated with the emission of negatively charged particles, a phenomena known as the photoelectric effect. Thomson determined the en ratio for these particles by means of electric and magnetic fields and found it to be virtually the same as for the cathode ray corpuscles. Charged particles emitted by an incandescent filament, i.e. by the thermionic effect, also had a similar e/m value. His estimate of the electronic charge of the photoelectric particles turned out to be similar to the unit electronic charge. In view of the consistency of the e/m for the negatively charged particles produced in different ways, it was reasonable to conclude that the particles were identical.

In the words of Thomson: "The experiments just described, taken in conjunction with previous ones .... on cathode rays, show that in gases at low pressures negative electrification, though it may be produced by very different means, is made up of units each having a charge of electricity of a definite size; the magnitude of this negative charge is .... equal to the positive charge carried by the hydrogen atom (ion) in the electrolysis of solutions".

Because the charge on the particles present in the cathode rays, and associated with the thermionic and photoelectric effect, was identical with the elementary electric charge, the name electron originally intended by G. Johnstone Stoney for magnitude of the charge, soon became associated with the actual particles themselves.

Assuming that n particles each of mass m and velocity v hit the collector in time T, and that each particle carries a charge q, then Q = nq (1) provided each particle "sticks" to the collector, and hence deposits its charge to be measured, and does not cause any secondary emission of charged particles. Thomson tried to ensure that these conditions would be met by grounding his shielding electrode. (Even so, if the collector acquires a negative potential from the incoming beam, then it is possible that some of the beam may bounce away from it and be collected by the shield.)

Further, if each particle is collected it will relinquish its kinetic energy to the collector, and this energy will be evident in the form of heat:
H = n(1/2mv2) (2)
Dividing Equation (1) by Equation (2) one finds
q/m = 2Qv2H (3)
It is hence necessary to measure velocity v.

Now, in a magnetic field oriented so as to be perpendicular to the original path of the rays, the resulting path is observed to be part of a circle. The magnetic field, then, must be exerting a centripetal force upon the ray particles. Assuming that each of the particles has a mass m, a velocity v, and charge q, and that they move in a magnetic field of intensity B in a path of radius of curvature R, one can write the following equations:

Magnetic force on particle = Centripetal force for circular motion
Bqv = mv2/ R (4)
Which can be rearranged to give
q/m = v/BR (5)

Hence, combined with magnetic field deflections, the relations (3) and (5) would give values for both q/m and v. Using this method, Thomson found values of v of the order of 2.4x107 to 3.2x107 meters per second (about one tenth the velocity of light), and from 1.0 to 1.4x1011 coulombs / kilogram for the ratio of charge to mass for cathode ray particles.

In the same paper that described the method described above, Thomson reported a different method for getting the needed second relationship between q/m and v. In this second method he used a tube popularly known as the Thomson tube. The schematic diagram for determination of q/m is shown elsewhere. The cathode ray beam could be sent through an electric field produced by the plates A and B in which region there could also be a magnetic field perpendicular to the paper established by external coils. Any deflection of the beam could be measured by the scale S at the end of the tube.

Electron As A Wave

The diffraction and interference properties of radiation necessitate a wave structure, but photoelectric phenomena and the Compton effect imply that radiation consists of particles rather than waves. In Compton effect, an instant X-ray is scattered by a free electron just like in a collision between two rigid spheres. In other words, radiation may be regarded as exhibiting a dual wave - particle behaviour; some of the properties of the radiation may be wave properties where as others are particle properties. By means of Planck's Quantum Theory equation and the mass energy relationship of Einstein, Prince Louis-Victor Pierre Raymond de Broglie (1892-1987) deduced that a particle mass m moving with a velocity v should be associated with waves of length l (lamda), given by

l = h / mv

where h is the Planck's constant. It was calculated that with a moderately high velocity such as could be obtained by passage of an electron through a potential of about 100 to 1000 volts, the de Broglie waves should have a wavelength of the order of 10-8 cms. If this were the case, then crystals should be capable of producing diffraction effects with electrons.

The first definite proof that electrons can be diffracted and consequently exhibit wave, as well as the familiar particle, properties was obtained in the Bell Telephone Laboratories in New York by C.J. Davisson and L.H. Germer in 1927. By studying the reflection and scattering by a nickel crystal, of a beam of electrons, given a specific velocity by passage through a known potential difference, it was found that the electrons behaved like waves rather than particles. Using electrons which had been accelerated by a potential of 54 volts, the experimental results were found to be equivalent to those expected from radiation of wavelength 1.65 Å, in remarkably good agreement with the value of 1.67 Å calculated by means of the de Broglie equation.

Further, evidence for the existence of electron waves was obtained independently in 1927, by George Paget Thomson (1892-1975), son of J.J. Thomson. He passed a stream of fast moving electrons through a very thin sheet of metal and then allowed the resulting beam to fall on a photographic plate. Upon development, the plate showed a diffraction pattern consisting of a series of concentric circles, just as might have been produced by X-rays, indicating that the electrons were manifesting wave properties.

With no electrical field applied, the magnetic field can deflect the beam upward or downward as shown, for example, by the dotted path in the figure. The beam travels in a straight line except in the region of the magnetic field, where its path is an arc of a circle of radius R, as in Equation 5. Neglecting the fringe effects, it is fairly easy to calculate R from the measured deflection of the beam and the geometrical constants of the tube. Thus this tube may be used, as other tubes had been, for measurements with the magnetic field alone, and under these circumstances Equation 5 would apply.

But suppose that while the magnetic field is present, a potential difference V is applied to the two deflecting plates. If the plates are a distance D apart, then the resulting electric field strength between the plates will be
E = V/D (6)

(If V is measured in volts and D is measured in meters, then E will be found in newtons per coulomb). A charge q in this field experiences a force Eq, up or down depending on the sign of q and the direction of the field E. If we arrange the applied potential difference to have a value such that the force Eq is numerically equal, but opposite in direction, to the force on the particles due to the magnetic field (which is Fmag = Bqv, in which v is the velocity of the particles), we would then have

Felec = Fmag
or Eq = Bqv and hence (7)
v = E/B (8)

(It is easy to show that if E is measured in newtons / coulomb, and B in webers per square meter, then v is in meters per second). In practice, the potential V is varied until the beam is observed to be at a position of no net deflection. Zero deflection implies that the electric and magnetic forces are equal. V is then measured with a voltmeter, and B is determined by use of a search coil and ballistic galvanometer. One can then put the numerical value of v thus found back into Equation 5 to determine the value of q/m. Thomson found, in a series of experiments, values for q/m which, when averaged, came to 0.77x1011 coulombs / kilogram. This value disagreed with the one he published from his heating effect experiments, a disagreement he attributed primarily to possible systematic errors in the latter experiments. In his writings for the next few years he usually gave the value of q/m as "approximately 1011 coulombs / kilogram".

Thomson boldly announced the hypothesis that "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 matter is of one and the same kind; this matter being the substance from which all the chemical elements are built up". Thomson remarked that this surprising result might be due to the smallness of m or to the bigness of e. He argued that m was small, citing Philipp Lenard, who had shown that the range of cathode rays in air (half a centimeter) was far larger than the mean free path of molecules (10-5 cm). Lenard was awarded the Nobel Prize in Physics in 1905 for studying the cathode rays. If the cathode ray travels so much farther than a molecule before colliding with an air molecule, it must be very much smaller than a molecule. Thomson concluded that these negatively charged particles were also constituents of atoms.

From 1897 onward, thanks largely to the experiments of Perrin and Thomson, the corpuscular model for cathode rays received general consent. Thomson's view that the cathode ray particles were the fundamental building block, or even a fundamental building block of atoms was not, however, received with much enthusiasm. Several other lines of research, notably in the fields of analysis of spectra and of radioactive phenomena, had to converge before the real role of Thomson's corpuscles within the atom could be understood and generally accepted.

Thomson's achievements were honoured in numerous ways, and mark him as among the most accomplished physicists of his era. In 1906 he was awarded the Nobel prize in physics for his researches into the discharge of electricity in gases. In 1918 he was chosen Master of his old college, Trinity, and the next year he resigned the Cavendish Professorship. He guided Trinity with his usual common sense and benevolence until shortly before his death in August 30, 1940.

"e" by Millikan's method

Millikan's apparatus consisted of two horizontal metal plates about 22 cm diameter and 1.6 cm apart as indicated by A and B. The plates were supported in a closed vessel containing air at low pressure, and were connected to the poles of a high voltage (10,000 volts) battery, V. In the upper plate, there were a number of small holes as represented by C. By means of a atomizer, a fine spray of a non-volatile oil was introduced into the vessel. As a result of friction in the atomizer, the droplets of oil so obtained were electrically charged. From time to time, one of these droplets would pass through the hole C, and then it could be observed by means of a telescope (not shown in the figure). By using the illumination of a powerful beam of light, entering the window W (at left), the droplet appeared as a bright star on a dark background.

With the battery V disconnected, the droplet fell slowly under the influence of gravity, and the rate of fall was measured. This rate (or velocity) represented by v1 is dependent on the mass m of the droplet and is given by the equation v1 = kmg where g is the gravitational acceleration (981 cm/sec2) and k is a proporationality constant which is related to the viscosity of the air and the size of the oil droplet. The high voltage battery was then switched on, thus producing the electric field, the direction being such as to make the charged droplet move upward, against the force of gravity. If E is the strength of the electric field, i.e., the voltage of the battery divided by the distance between the plates, then the upward force acting on the droplet is Een , where en is the charge carried by the droplet. Since this is opposed by the gravitational force mg, the net upward force is Een - mg. The upward velocity v2 of the oil droplet which is measured, is then represented by

v2 = k (Een - mg)
The proportionality constant k has the same significance as in the previous equation.
From the above two equations, it is easy to show that
en = mg (v1 + v2) / Ev1

Since the quantities v1, v2, g are available, it is possible to calculate the charge en carried by the oil drop if the mass m were known. Using Stokes Law, applicable to small spherical drops falling under the influence of gravity, it could be shown that

v1 = 2gr2d / 9h

Here h is the density of the oil of which the drops are made. Since v1 has been determined, as described above and g, h and d may be regarded as known the radius r of the drop could be determined from the above equation. It is now easy to determine the mass of the oil drop which is given by m = 4pr3d / 3, inserting this result into the equation for en, together with the measured velocity, v1 and v2, the magnitude of the charge en carried by the oil droplet can now be determined.

As a result of a large number of measurements, Millikan found that the charge en was always an integral i.e. a whole number, multiple of a definite elementary charge, which was presumably the electronic charge. After applying numerous corrections to the foregoing equations, Millikan concluded in 1917 that the most reliable value of the unit charge was 4.774 X 10-10 esu which is very close to the modern value 4.803207 X 10-10 esu.

Millikan and his oil drops

Robert Andrews Millikan (1868-1953) was the son of Silas Franklin Millikan a congregational preacher, and Mary James Andrews, a graduate of Oberlin who had been Dean of Women at a small college in Michigan. Raised in Maquoketa, Iowa, where his family moved in 1875, young Millikan enjoyed a story book, Midwestern American boyhood, fishing, farming, fooling and learning next to nothing about science. In 1886, he enrolled in the preparatory department of Oberlin College and in 1887, in the classical course of the college itself. At the end of his sophomore year, he was asked to teach an introductory physics class. Millikan plunged into the subject, liked it and soon decided to make it his career.

Millikan graduated from Oberlin in 1891 and continued to teach physics to the preparatory students. He was awarded an M.A. for his achievement of successfully pursuing a course of instruction in Dynamic Electric Machinery in 1893. Millikan entered Columbia University on a fellowship as the sole graduate student in physics. He was impressed by the experimental deftness of Michelson, under whom he studied at the University of Chicago in the summer of 1894. After receiving his Ph.D. in 1895, Millikan went to Europe for post-graduate study. He heard PoincarŠ lecture at Paris, took a course from Planck at Berlin, and did research with Nernst at Gottingen. In 1896, the excitement of the discovery of X-ray still fresh in his mind Millikan joined the faculty of the University of Chicago as an assistant in physics. This is where he met Greta Irwin Blanchard, the daughter of a successful manufacturer from Illinois whom he married in 1902. He spend a large fraction of his energies into the development of the physics curriculum, especially in introductory courses and wrote/co-authored a variety of text books which quickly became standards and sold in large numbers. Mainly because of his outstanding pedagogical achievements, Millikan was promoted to an Associate Professorship.

By 1909 Millikan was deeply involved in an attempt to measure the electronic charge. No one had yet obtained a reliable value for this fundamental constant, and some any-atomistic continental physicist were insisted that it was not the constant of a unique particle but a statistical average of a diverse electrical energy. His famous experiment of determination of the electronic charge is described elsewhere in the article. Of and on all the while Millikan had continued his exploration of the photoelectric effect and by 1950 had confirmed the validity of the Einstein's equation of photoelectric effect in every detail.

Millikan held many important posts and membership of several eminent Academies and Societies. In 1921, Millikan accepted appointment as the Chairman of the Executive Council and Director of the Norman Bridge Laboratory at the California Institute of Technology. Employing the photonic interpretation of cosmic rays, Millikan developed a theory of their origin in 1928. To find a measure of cosmic ray energies, he put Carl Anderson, a young research fellow at Caltech to work with a cloud chamber set in powerful magnetic field which ultimately lead to which detection of the negative electron - also called Positron, in 1932.

How the "Positive Electron" or the "Positron" was Discovered

The English mathematical physicist P.A.M. Dirac (1902-1984) in 1928 presented theoretical arguments indicating that a particle similar in mass to the electron but carrying a positive charge may exist. His discussion based on relativistic wave mechanics was of a highly abstruse character. However, the proof of the existence of the long-sought positive electron was obtained by C.D. Anderson (1905-1991) at the California Institute of Technology in 1932. In order to study the so-called cosmic rays, which appears to come from outer space, Anderson in conjunction with R.A. Millikan, had constructed an apparatus known as a cloud chamber which was placed in a very strong magnetic field. In this cloud chamber, the path of an electrically charged particle could be rendered visible and also photographed. A cloud chamber is based on the fact that whenever an electrically charged fast-moving particle passes through the air (or any other gas), it produces ionization along its tract. If the air through which these particles pass is saturated with water vapour, the ions serve as the centres of condensation for tiny water droplets, and we see long thin tracks of fog stretching along the particle's trajectories. The intensity of the track provided information concerning the mass of the particle, and the direction in which it was bent in the magnetic field indicated whether the charge was positive or negative. Numerous tracks were observed due to charged particles resulting from the impact on matter of the very highly energetic cosmic rays. A lead plate of 6 mm thickness was placed across the chamber with the object of depriving the particles of some of their energies. Anderson stated in one of his lectures: "The degree of curvature in the magnetic field shows a difference depending on the amount of energy lost in the plate. Measurements made on the track of a particle before and after it has passed through the plate, together with observations of the density of the track itself, give definite information about the mass of the particle and the magnitude of the electric charge it carries" (Figure).

The photograph shows one of the numerous photographs obtained in this manner - a photograph of historical significance, for its interpretation by Anderson lead to the discovery of the positive electron. Since the curvature of the track is less below the plate than above, the energy of the particle is greater below the plate. Hence the particle must have been moving upward. Knowing the direction of the magnetic field and the direction of the motion of the particle, the curvature of the track to the left immediately showed that the particle must be positively charged. The density of the track was less than would be expected for a proton, but its length was greater. "Photographs of these positively charged particles could be understood only if the particles were assumed to have a mass approximately equal to that of the ordinary electron of negative electric charge, and thus the first evidence for the existence of the positive electron .... was obtained", Anderson said.

Other cloud chamber photograph examined in the light of new discoveries provided further proof that positive electron were produced by the action of cosmic rays. Some photographs showed charged particles to fall into two groups, one being deflected in one direction and the other in the opposite direction by the magnetic field, representing negative electrons and the positive electrons respectively. Anderson suggested the name "Positron" for the positive electron, and this immediately became into general use.

Millikan was an able populariser and lecturer and after he won the Nobel Prize in 1923, he became perhaps the most famous American scientist his days. He was an outspoken, religious modernist. Even after his retirement from his professorship in 1946, he remained active as a public lecturer and spoken frequently on the subjects of science and religion. By the time of his death, he had been awarded numerous medals even from honorary degrees and professorship of 21 foreign scientific Societies, including the R