ELECTRICAL RADIATIONS The electric sparks which pass through the air between op positely charged conductors when they are brought near together attracted attention in very early times. When Franklin showed the identity of atmospheric electricity and ordinary electricity lightning came to be regarded simply as enormous electric sparks.
The invention of mercurial air-pumps about 185o enabled higher vacua to be obtained, and the electric discharge through gases was soon examined in much higher vacua than hitherto by Geissler, Plucker and Hittorf. It was found that as the pressure was reduced the negative glow became thicker and moved away from the negative electrode, leaving a well defined dark space between the electrode and the glow. This dark space, which is now called the Crookes dark space, becomes wider as the pressure is reduced, and, when its boundary gets near to the glass walls of the discharge tube, the glass emits a greenish light. The distribution of this light on the glass is altered when a magnet is brought near. Hittorf in 1869 placed a solid body in the Crookes dark space, between the cathode and the glass, and found that it cast a sharply defined shadow in the light emitted by the glass. He concluded that the cathode emits rays in straight lines, which cause the glass to glow where they fall on it. These rays are known as the cathode rays. Goldstein in 1876 showed that the cathode rays proceed from the cathode in directions perpendicular to its surface, so that, for example, a plane cathode emits a parallel beam of the rays. Cromwell Varley in 1871, suggested that the cathode rays consist of negatively charged particles.
The cathode rays were investigated with great skill by Sir Wil liam Crookes about 1879. He reduced the pressure in his dis charge tubes until the Crookes dark space filled the whole tube, and devised several beautiful experiments to illustrate the prop erties of the rays. Crookes showed that the rays can be concen trated on a small area, by using a concave cathode, and that a body placed near the centre of curvature of the cathode becomes intensely heated. He found that the rays cause bright phosphor escence of many substances, notably zinc and calcium sulphides and calcspar. He showed that a narrow beam of the rays can be obtained by passing them through a slit. Such a narrow beam is deflected by a transverse magnetic field, just as a flexible con ductor, carrying a current flowing towards the cathode, would be deflected. Crookes adopted Varley's view that the cathode rays are rapidly moving negatively charged particles.
Hittorf had found that cathode rays can pass through very thin metal foil, and their penetrating power was thoroughly investi gated in 1894 by Lenard, who obtained results of fundamental importance. He found that the mass per sq.cm. of a layer which stops one half of the rays is nearly the same, whatever the nature of the matter of which the layer is made. For example the mass is nearly the same for a layer of air and for one of gold. Lenard found that the rays could be passed out into the air through a small window of aluminium foil, in the wall of the discharge tube, and could penetrate through air at atmospheric pressure for sev eral centimetres. If the cathode rays consist of negatively charged atoms it is hard to see how they can have such penetrating power, so Lenard and other observers were inclined to regard them as some form of aethereal vibration analogous to light.
Crookes' celebrated experiments on cathode rays aroused general interest, among physicists, in the passage of electricity through gases, and attempts were soon made to formulate a general theory of the nature of gaseous conduction of electricity. Faraday's laws of electrolysis show that the charge associated with the ions in solution is always a multiple of the same quantity of electricity, the ionic charge, and in 188i Helmholtz revived this idea of Faraday's, and pointed out that, if it is admitted that elementary substances are composed of atoms, we can scarcely avoid the conclusion that electricity is also made up of equal atoms. It was natural to suggest that conduction through gases is analogous to conduction through salt solutions, and that conducting gases con tain positively and negatively charged particles, or ions, which move in opposite directions in an electric field, so producing a current. W. Giese of Berlin in 1882 definitely put forward such a theory, and applied it very successfully to the explanation of the electrical conductivity of the gases from a flame. He supposed that some of the molecules in a flame dissociate into two parts, one positively charged and the other negatively charged, and that the conductivity of the flame and the gases coming from it is due to the presence of these ions. The gradual disappearance of the conductivity as the gases moved away from the flame, Giese at tributed to the recombination of the ions, and he showed that, when the conducting gas was passed through a strong electric field, its conductivity disappeared. He explained this by supposing that the positive ions were attracted to the negative electrode, and the negative ions to the positive electrode, so that the ions were re moved from the gas as it passed through the field.
Similar ideas were developed by Schuster in 1884 to explain the conductivity of gases at low pressure. He showed that, when a dis charge is passed between electrodes near one end of a discharge tube, the gas near the other end of the tube becomes conducting, and he attributed this to the diffusion of ions formed in the dis charge throughout the gas in the tube. In 1887 Hertz discovered that ultra-violet light facilitates the passage of sparks between two oppositely charged electrodes. It was soon found that the effect is due to the light which falls on the negative electrode, and that, when ultraviolet light falls on a negatively charged conductor, the negative electricity escapes from the conductor through the sur rounding gas. The light has no action on a positively charged conductor. (See PHOTOELECTRICITY.) X-rays.—In 1895 W. C. Rontgen of Munich discovered a new kind of radiation now known as X-rays, or Rontgen rays. Rontgen found that when cathode rays strike a solid body the new radiation is emitted. The X-rays travel in straight lines and are remarkably penetrating. They affect a photographic plate, and cause certain substances to fluoresce brightly. They pass through light bodies, like wood or aluminium, with little absorption and without appre ciable deviation. The absorption of the X-rays is nearly propor tional to the density of the absorbing substance. If, for example, the X-rays from a point source are passed through a man's hand, and then fall on a fluorescent screen, a shadow of the hand is ob tained, in which the bones can be seen sharply outlined.
In 1881 J. J. Thomson pointed out that, according to Max well's electromagnetic theory, the sudden stopping of rapidly moving charged particles, such as the cathode rays were supposed to be, should produce electromagnetic waves like light, and he suggested that the fluorescent light emitted by glass, when Struck by the cathode rays, was caused by such electromagnetic radiation emitted by the cathode rays. Soon after the discovery of X-rays Schuster, Wiechert and Stokes independently suggested that they are very short light waves produced in the manner predicted fifteen years earlier by J. J. Thomson. This theory has since been abundantly verified, and the study of X-rays has developed into one of the most fruitful and interesting branches of Optics. (See X-RAYS.) In 1896, J. J. Thomson found that, when X-rays are passed through any insulator, they render it conducting. J. J. Thom son and E. Rutherford, working together, investigated the con ductivity of gases due to X-rays, and they showed that it could be explained in the same way as the conductivity of the gases from flames had been explained by Giese in 1882. It was supposed that the X-rays caused the gas molecules to dissociate into posi tively and negatively charged particles, or ions, which moved through the gas in opposite directions in an electric field, so caus ing an electric current through the gas. Thomson and Rutherford further supposed that the positive and negative ions in a gas will attract each other, and so tend to meet and combine, so disap pearing. This process is called recombination. The number of pairs of ions which disappear in this way is proportional to the product where is the number of positive ions per cu.cm. in the gas, and the number of negative ions. It may be put as where the constant a is called the coefficient of recom bination. Thomson and Rutherford showed that the relation between the current i and the potential V agreed nearly with the equation where d is the distance between the parallel plates used as elec trodes, and k, and k, are the mobilities of the ions, defined so that the velocities of the positive and negative ions in a field of strength F are k,F and respectively. Ingenious methods of measuring a, k, and were soon devised by Rutherford, J. Zeleny, P. Langevin and many others, and the ionic theory of gaseous con duction became firmly established in a few years. (See ELEC TRICITY, CONDUCTION OF, In Gases.
The problem as to the nature of the cathode rays was finally solved in 1897, with epoch-making results. The idea, derived from Faraday's work on electrolysis, that electricity consists of equal indivisible atoms was generally believed to be probably true, and it was supposed that the charges on gaseous ions were equal, like the charges on ions in solutions, to small multiples of the charge of one electric atom. The ratio of the quantity of electricity, required to deposit any mass of an ion in electrolysis, to this mass is equal to the ratio of the charge to the mass of one ion. It was known that 96,50o coulombs is required to deposit one mol of any univalent ion. The ratio of the charge to the mass for any ion of molecular weight, M, is therefore equal to 9,65o/M electromag netic units of charge per gram. For an hydrogen ion, the lightest atom known, the ratio is 9,65o since for hydrogen M= I approxi mately. According to the view that cathode rays are negatively charged particles, and that electricity is atomic, we should expect the ratio of the charge, e, to the mass, m, of cathode ray particles to be not much greater than e/m for hydrogen ions, or 9,65o elec tromagnetic units per gram. Schuster, about 1884, had tried to estimate e/m for cathode rays from their deflection by a magnetic field, and some of his results indicated values much larger than 9,65o, but he believed the rays to be charged atoms, and supposed the large values of e/m to be erroneous.
In 1897 reliable methods of estimating e/m for cathode rays were devised and carried out independently by E. Wiechert, W. Kaufmann and J. J. Thomson. Kaufmann measured the potential difference, P, between the anode and cathode of his discharge tube, and deflected the cathode rays by means of a uniform mag netic field of known strength, H. The kinetic energy of the rays, is equal to Pe, and, since the transverse force on a charged particle moving perpendicular to a magnetic field H is equal to Hey, we have where r is the radius of the path of the rays in the field. The equations Pe= and He=mv/r give e/m= and v= 2P/Hr. Kaufmann measured P, H and r, and so determined e/m and v. He found that e/m was about 2X I electromagnetic units per gram, and that the velocity of the rays was of the order of one-tenth that of light. J. J. Thomson obtained similar results by several methods. (See ELECTRICITY, TION OF, In Gases; and ELECTRON.) He showed that e/m is the same, with cathodes of different metals and with different gases in the discharge tube. Shortly afterwards J. J. Thomson and Lenard independently measured e/m for the electrically charged particles emitted by metals when exposed to ultra-violet light, and J. J. Thomson measured the ratio for the negative charge escaping from hot bodies in a vacuum. In both cases it was found nearly equal to that for cathode rays.
Lecturing at the Royal Institution on April 3o, 1897, J. J. Thomson pointed out that it is impossible to explain Lenard's results on the penetrating power of cathode rays by supposing that they are charged atoms, and that it is therefore necessary to assume that they are much smaller than atoms. If we assume that the charge on a cathode ray particle is equal to that on a monovalent ion in solution, i.e., to one atom of electricity, it follows, since for cathode rays, and e/m=9,65o for hydrogen ions, that the mass of the cathode ray particles is only of that of one hydrogen atom. The cathode rays therefore appear to consist of negatively charged particles much lighter than the atoms of any element. They can be obtained from different kinds of matter, and must therefore be one of the con stituents of ordinary matter. That the cathode rays carry a nega tive charge was proved by Perrin, and by J. J. Thomson, by col lecting them in an electrode, which received a negative charge. It was also shown that they are deflected towards the positive electrode in an electric field. J. J. Thomson's views on the nature of cathode rays have been abundantly confirmed by subsequent researches. Wiechert and Kaufmann put forward similar views at nearly the same time, and so share with J. J. Thomson the honour of the great discovery which may be said to be the starting point of a new era in modern physics. FitzGerald pointed out that J. J. Thomson's cathode ray particles might be regarded as the negative electrons of H. A. Lorentz' electron theory; this sugges tion has proved to be correct and cathode ray particles are now usually called electrons.
Since matter contains these electrons, it is natural to suppose atoms are built up out of them and positive electricity. J. J. Thomson rapidly developed a theory of the constitution of atoms, according to which an atom consists of a definite amount of positive electricity, together with enough electrons to make the atom electrically neutral. He supposed the electrons to describe orbits in the atom, or to oscillate about positions of equilibrium. The number of electrons in an atom, J. J. Thomson supposed, increases with the atomic weight. He supposed that the electrons are arranged in groups, or layers, and that the chemical properties of the atom are largely determined by the number of electrons in the outermost layer. The chemically inert atoms helium, neon, argon, xenon and krypton, J. J. Thomson supposed, contain especially stable arrangements of electrons. An atom with one electron less than an inert atom tends to attract an additional electron, so becoming negatively charged, while an atom with one more electron easily loses an electron, so becoming positively charged. Atoms which easily lose an electron combine readily with atoms which tend to acquire an electron. In this way J. J. Thomson was able to explain many of the properties of atoms, and especially the way in which their properties vary periodically with the atomic weight. The general validity of these ideas has since been abundantly confirmed, and J. J. Thomson is generally regarded as the founder of the modern theory of the constitution of atoms. (See ATOM.) Positive Rays.—Goldstein in 1886 had observed that, if a small hole is made in the cathode of a highly exhausted discharge tube, rays pass through the hole, moving in the opposite direction to the cathode rays. These rays were shown by Wien, in 1897, to consist of positively charged particles. The value of e/m for these positive rays was found to be of the same order as e/m for the ions in solutions. For example, in hydrogen, Wien got positive rays for which e/m was about 9,65o as for hydrogen ions. These positive rays are therefore regarded as positively charged atoms, or atoms which have lost one or more electrons. They have since been thoroughly investigated by Wien, J. J. Thomson and F. W. Aston, and results of immense importance have been obtained. Aston in 1919 developed a very accurate instrument, called the mass spectrograph, by means of which it is now possible to deter mine e/m for positive rays to within one part in ten thousand. He has determined the ratio m/e for the positive rays of many ele ments, and finds that, taking m/e for oxygen equal to 16, the values for other elements are always equal to integers, or, in the case of rays which have lost more than one electron, to integers divided by the number of electrons lost. It thus appears that all atomic weights are almost exactly integers, when 0=16 is taken as the standard. Elements for which the chemical atomic weight is not an integer are mixtures of atoms having identical chemical properties, but different atomic weights. For example chlorine is a mixture of atoms of atomic weights 35 and 37, its chemical atomic weight being 35.46. Atoms having the same chemical properties but different atomic weights are called isotopes. (See and POSITIVE RAYS.) Formation of Clouds on Ions.—In 1887 Helmholtz showed that the gas, through which an electric discharge has just been passed, causes condensation of supersaturated steam. In 1896, C. T. R. Wilson found that the ions, produced in moist air by X-rays, give rise to a cloud when the air is suddenly expanded (to produce supersaturation of the water vapour in it). The cloud consists of minute drops of water formed by condensation on the ions. In this way it is possible to study the distribution of the ions in the air, and C. T. R. Wilson has obtained many re sults of extraordinary interest. For example, when a narrow beam of X-rays is passed through the air, it is found that the ions formed are not uniformly distributed along the beam, but lie on narrow curved tracks. These tracks begin in the X-ray beam, and curve about in an irregular manner, finally ending in a cluster of ions. It is supposed that the X-rays cause the ejection of an elec tron, from an atom, with high velocity, and that this electron moves through the air, ionizing the atoms which it strikes until it is brought to rest. Thus C. T. R. Wilson's method enables the path of a single electron through the gas to be examined and photo graphed. (See ELECTRICITY, CONDUCTION OF; and NUCLEUS.) The formation of clouds on the ions in a gas makes it possible to determine the charge on one ion, or the charge of the atom of electricity. Such a determination was first made by J. S. Town send in 1897. The weight of the droplets in the cloud was found by observing the rate at which they fell through the air. Sir G. G. Stokes had worked out the force on a small sphere, of radius a, moving with uniform velocity, v, through a medium of viscosity µ, and found it equal to 6 irjs ay. In the case of a droplet of radius a and density p, falling with uniform velocity v, we have, there fore, 67rpav= -iraapg, where g is the acceleration of gravity. Townsend obtained a charged cloud in a gas, and determined its total charge, total mass and the mass of the individual droplets, and so was able to calculate the average charge per drop, which was about 3X10'° electrostatic units of electricity. He later found that some of the drops were positively and some negatively charged, and, allowing for this, found the average charge to be electrostatic units. The charged clouds used by Town send were obtained by passing the gases, evolved by the electrolysis of dilute sulphuric acid, over water. A similar investigation was carried out in 1898, by J. J. Thomson, on the clouds obtained by condensation of water on the ions produced in air by X-rays. He found the charge per drop to be about 6X I electrostatic units. A different method of finding the charge on the droplets was used by H. A. Wilson in 1903. The cloud of droplets was formed between two horizontal metal plates between which a vertical electric field could be maintained. The velocity with which the droplets fell was determined, both with and without the electric field. If is the velocity without any field, and that in a field F, then we have vl = ntg, where e is the charge on the droplet mg+Fe and m its mass ; m can be calculated from v,, and so the value of e can be obtained. In this way it was found that drops were present, having charges nearly as 1 : 2 :3, and this was explained by supposing that some drops carried one atom of electricity, and others two or three. Accurate results could not be obtained, mainly because the drops evaporate, so that m does not remain constant.
About 1910 an accurate determination of the ionic charge, e, was made by R. A. Millikan, who eliminated the error due to evaporation, by using droplets of oil. He was able to keep a single drop suspended in a vertical electric field for several hours, and to vary its charge by ionizing the air near it by X-rays. He showed that the charge on such a drop was always equal to a definite charge e multiplied by an integer which was varied from 1 to 15, or more. In this way Millikan finally found e=--4-774X10-1° elec trostatic units, and his result is believed to be correct to about one part in one thousand. Independent values of the ionic charge, e, have been deduced from the observed distribution of the energy in the spectrum of black body radiation, and from direct measure ments of the charge carried by a-rays, which are believed to be atoms of helium with a charge 2e. These determinations agree well with Millikan's result. (See ELECTRON.) Radioactivity.—A discovery of fundamental importance was made in 1896 by Henri Becquerel. He found that uranium and its compounds emit penetrating radiations, which can pass through considerable thicknesses of matter, and which affect a photo graphic plate, and produce conductivity in gases. This property of uranium is called radioactivity. This discovery excited great inter est, and other elements were soon found which, like uranium, emit rays. Schmidt and Madame Curie in 1898 found thorium to be radioactive, and in the same year Madame Curie discovered two new intensely radioactive elements, in the mineral pitchblende, which she named polonium and radium.
In 1899 E. Rutherford showed that the rays from uranium are not all of the same kind. Part, which he called the a-rays, are very easily absorbed, and the rest, which are much more pene trating, he called the /3-rays. Giesel, Becquerel and others found that the fl-rays are deflected, by a magnetic field, in the same way as cathode rays, and Curie showed that they carry a negative charge. Becquerel showed that they are also deflected by an electric field, and that the ratio of the charge they carry to their mass is of the same order as for cathode rays. It was thus clear that the /3-rays are high velocity electrons, a conclusion since abundantly confirmed. The properties of the a-rays have been investigated by Rutherford, W. H. Bragg and others, and it has been shown that they consist of helium atoms which have lost two electrons and so have a positive charge 2e. A third type of radiation from radioactive elements was discovered by Villard in 1900. These rays are called 7-rays and are found to be of the same nature as X-rays, i.e., they are electromagnetic waves of extremely small wave lengths. They are much more penetrating than the (-rays and than ordinary X-rays. The theory of radio activity which is now accepted was put forward by Rutherford and Soddy in 1902. According to this theory, the atoms of radio active elements decompose, with the emission of radiation, form ing new atoms having different chemical properties. The atoms originally present, therefore, gradually diminish in number, so that finally none of them remain. For example it is found that one half of any number of atoms of radium decompose in 1,690 years. (See RADIOACTIVITY.) When a narrow beam of a-rays is passed through a thin metal sheet, it is found that a few of the rays are deviated, or scattered, through large angles up to 18o°. Rutherford showed that this scattering through could be exactly explained by supposing that the rays were deviated by passing very close to fixed positive point charges which repelled them. He was thus led to propose his nucleus theory of atoms, according to which an atom con sists of a minute positively charged nucleus surrounded by a number of electrons describing orbits around it. This theory differs from that of J. J. Thomson, who supposed the positive electricity to be distributed over the whole volume of the atom. (See NUCLEUS.) Characteristic X-rays.—Very soon after Rontgen's discovery of X-rays it was shown by Perrin that they excite the emission of similar secondary rays when they fall on matter. These secondary X-rays were carefully investigated by Barkla, who showed that elements of small atomic weight merely scatter the incident rays, whereas elements of higher atomic weight emit rays, different from the incident rays, and having properties characteristic of the emitting element. These characteristic X-rays can be obtained also by bombarding the element with high speed electrons. In 1912 Laue made a suggestion of fundamental importance. He proposed to pass a narrow beam of X-rays through a crystal, and suggested that the regular arrangement of the atoms in the crystal would result in diffracted beams of X-rays coming out in different di rections, in much the same way as when light is diffracted by a grating. It was found that such is the case, and that, by studying the diffraction of X-rays by crystals, it is possible to determine both the wave lengths of the rays and the structure of the crys tal. The study of crystals by this method has led to a great advance in our knowledge of their structure, largely as the result of work carried out by W. H. Bragg and his son W. L. Bragg. (See X-RAY.) Since 1912 the wave lengths of the X-rays emitted by different elements have been measured accurately, and it is found that each element has a characteristic X-ray spectrum which may be re garded as a continuation of its spectrum in the visible and ultra violet regions. H. G. J. Moseley in 1913 first measured the wave lengths of the X-rays emitted by a series of elements, and found that the wave lengths of corresponding lines in the X-ray spectra varied in a regular way with the atomic weight. He found that the square root of the frequency of vibration of the X-rays was a linear function of the atomic number of the element, the atomic number being the number giving the position of the element in a list of the elements in the order of their atomic weights. It was clear that the atomic number represents some fundamental physical property of the atoms. On Rutherford's nucleus theory, a neutral atom having N electrons describing orbits round the nucleus must have a nucleus with a positive charge Ne, where e denotes the ionic charge, since —e is the charge on one electron. The only possible values of N are the integers I, 2, 3, 4, • • • and we should expect the atomic weight to increase with N, so that it was natural to conclude that the atomic number is equal to N. According to this N=1 for hydrogen, N=2 for helium, N=3 for lithium, and so on for all the other elements up to uranium for which N=92. This idea, suggested by Moseley's results on X-ray spectra, has proved of great value and is believed to be correct.
A. H. Compton, in 1922, showed that, when X-rays are scattered by light elements, the frequency of the scattered is slightly less than that of the incident rays. The change of wave length is the same for all frequencies. Compton has shown that this effect can be accurately explained by supposing that the rays consist of quanta having energy and momentum, and that, during the interaction between the quantum and electron, the energy and the momentum are conserved. The quantum thus loses some energy, and so its frequency is diminished since its energy is equal to hv. (See COMPTON EFFECT.) In recent years modifications of the quantum theory have been proposed, which promise eventually to make it intelligible. The idea that radiations such as light and X-rays, which obey all the laws of wave motion, may nevertheless consist of some sort of particles, or quanta, seems at first sight absurd. The electromag netic field of which radiation consists, however, possesses energy and momentum, and so has the essential properties of matter, which suggests that radiation and material particles are really of the same nature.
Early in the 19th century Hamilton showed that the path of a particle in the field of force is determined by laws which can be expressed in the same mathematical form as those which deter mine the path of a ray of light in a medium, the refractive index of which varies from point to point. L. de Broglie, in 1925, suggested that a train of waves is associated with an electron, and moves along with it. The group of waves moves with the electron, but the waves in the group move with a different velocity, appearing at one end of the group and dying away at the other end. The wave velocity is thus not the same as the group velocity, just as is the case for waves of any kind in a medium in which the wave velocity depends on the wave length.
An isolated electron is regarded as a group of waves, and its path is determined by the laws of geometrical optics. Just as these laws suffice for large scale optical phenomena, but fail to account for interference and diffraction, so in the wave mechanics of atoms it appears that the individual electrons, or groups of waves, can not be regarded as separated and describing orbits, but must be supposed to overlap, so that, instead of isolated electrons, we have a vibrating medium filling up all the space around the nu cleus. The results deduced from the wave mechanics theory of atoms appear to agree with the facts of spectroscopy as well, or better, than the results deduced from Bohr's quantum theory. Wave mechanics may be said to provide a rational interpretation of the assumptions of the quantum theory of spectra, and much may be expected from this new theory. A remarkable experi mental confirmation of the view that an electron may be regarded as a train of waves was carried out by C. J. Davisson in 1927. He found that a beam of cathode rays, falling on a crystal, gives diffracted beams in different definite directions, just as X-rays do.
(See QUANTUM THEORY. )