ABSTRACTS OF ELECTRON JUBILEE LECTURES[ABSTRACTS of four of the lectures delivered in celebration of the 50th anniversary of the discovery of the electron (see 1947, 94,

Part I, p. 340) are given below.]

THE HISTORY OF THE DISCOVERY OF THE ELECTRON AND THE EARLY DEVELOPMENTS

By Professor J. A. CROWTHER.

(ABSTRACT of a lecture delivered at the Royal Institution, 25th September, 1947.)

The importance of the discovery of the electron and thegenius of its discoverer, Sir J. J. Thomson,, can be appreciatedfully only in relation to the physical knowledge and the facilitiesfor research available in the pre-electron era. Victorianscientists were convinced that matter consisted of impenetrableand indivisible atoms which, by definition, were the smallestconceivable particles, and scientific thought was dominated bythe Maxwell-Faraday interpretation of electrical phenomena asa series of stresses in a continuous medium. The existence of adetached atom of electricity seemed at that time to be quiteimpossible.

Although the study of electrical discharges in a vacuumappeared to be the most promising direction of research, progressbefore 1890 had been very slow, largely because of the laboriousmethods of evacuation which had to be used. The first realattack on the problem was made by Prof. Schuster, who, in 1890,determined the magnetic deflection of cathode rays in a measuredmagnetic field. As it was considered certain that the cathodeparticles were atoms of molecules and that atoms were impene-trable bodies, it seemed obvious that the energy given to theparticles by the electric field would be spent in driving themthrough the residual gas in the tube. By assuming the velocityof the rays, Schuster obtained 2 000 as the ratio of the charge eto the mass m. Two important pieces of evidence then followed.Hertz discovered that a cathode ray could penetrate thin sheetsof gold or aluminium, although it was inconceivable that chargedparticles could traverse a conductor without losing their charge,and Lenard, measuring the absorption of the rays in various

substances, found that the distance they penetrated was dependentnot on the chemical nature, but only on the density of thematerial.

Such, then, was the baffling state of affairs when Thomsondelivered his historic lecture on "Cathode Rays" on the30th April, 1897. He explained his recent discovery that themagnetic deflection of the cathode ray in a given magnetic field,and with a constant voltage across the tube, was independentof the nature of the gas in the tube and the metal of the cathode.By analysing Lenard's experiment, Thomson was led to theconclusion that, if cathode rays were charged particles, "the sizeof the carrier must be small compared with the dimensions ofordinary atoms or molecules," and went on to make the revolu-tionary assumption that the atoms of ordinary elements weremade up of "corpuscles and holes," i.e. had a structure, andthat cathode rays consisted of corpuscles charged with negativeelectricity and moving at high speed. Thomson also referred tohis earliest attempt to measure the ratio of the charge to themass of the particle. At first Thomson's great discovery washardly taken seriously. By October, 1897, however, he wasable to describe an entirely new method of determining the ratioe\m for rays by means of the famous electromagnetic balance,and, even at that early stage, to consider how the corpusclesarranged themselves inside the atom. In 1899, Thomson wasable to announce that he had measured both e/m and e for thecorpuscles given out by a zinc plate when irradiated by lightfrom an arc, and had identified the particles with those of cathoderays. From then on, Thomson's views began to be accepted.

ELECTRONS IN MODERN THEORETICAL PHYSICS

By Professor R. E. PEIERLS, F.R.S.(ABSTRACT of a lecture delivered at the Royal Institution, 25th September, 1947.)

Professor Crowther has explained under what conditions theconcepts of electrons and atomic structure were first formulatedby J. J. Thomson. I should like to indicate the effect thesediscoveries had on the development of theories of the behaviourand structure of matter.

Following the work of Thomson, and after Rutherford andBohr had provided more details of the structure of the atom,the question arose as to what prevented the electron of thehydrogen atom from falling into the nucleus. The explanationwas found by means of the quantum theory, which had beenformulated by Planck in connection with radiation and lightquanta. The first quantum theory still retained ideas of classicalmechanics and failed to resolve incongruities of the behaviourof atoms. These problems were solved by quantum mechanics,or wave mechanics, which showed that the difference between

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waves and corpuscles was not as fundamental as had beenassumed, as a particle could in some respects behave as a waveand in other respects as a corpuscle. These ideas were con-firmed by the discovery of electron diffraction by Sir GeorgeThomson and by Davisson and Germer. When Uhlenbeckand Goudsmit had postulated the idea of electron spin, i.e.the rotation of electrons about their own axis in addition totheir motion about the centre of force within the atom, the basictheory of atoms was virtually complete, but it had yet to beapplied to explain many chemical and physical phenomena,such as the properties of metals, the conduction of electricity,electrical resistance and magnetism. The problem of super-conductivity still remains to be explained.

New complications arise when electrons are moving at speedscomparable to that of light. Thomson had discovered that