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The extraordinary developments in electronics, in which Sir Joseph Thomson has played so important a part, have had commercial consequences, of which the following are perhaps the most significant: In July, 1914, through the development of the DeForest audion into a distortionless telephone relay and amplifier, and the insertion of these amplifiers into suitably chosen places in the telephone line between San Francisco and New York, the research physicists of the Western Electric Company were able to give numerous demonstrations in which audiences in New York and Boston were able to hear with perfect distinctness the splashing of the waves in the San Francisco harbor. By the summer of 1915 the same group of men had succeeded in throwing telephonic speech up into the antennæ of the wireless station at Arlington with such intensity that it traveled without wires a third of the way around the world and was heard so distinctly at receiving stations in both Honolulu and Paris that even the voices of the speakers in Washington could be recognized. The illustration at the left is a cut (size) of one of the tubes with which this extraordinary scientific feat was performed. The simplified circuit of a thermionic amplifier is shown in the diagram above. The enfeebled incoming speech frequencies vary the potential of the grid G, and these variations produce like variations in the electronic currents flowing from the hot filament F to the plate P and thence into the circuit in which the amplified current is needed. By the use of these devices the enormous energy amplifications of 10,000,000,000,000-fold have been obtained


atom from which an electron has been detached. Any charged body in the gas therefore draws toward itself charges of sign opposite to its own, and thus becomes discharged.

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502. X-ray pictures. The most striking property of X rays is their ability to pass through many substances which are wholly opaque to light, for example, cardboard, wood, leather, and flesh. Thus, if the hand is held close to a photographic plate and then exposed to X rays, a shadow picture of the denser portions of the hand, that is, the bones, is formed upon the plate. Opposite page 359 is shown an X-ray picture of the thorax of a living human being.


503. Discovery of radioactivity. In 1896 Henri Becquerel (see opposite p. 446), in Paris, performed the following experiment. He wrapped a photographic plate in a piece of perfectly opaque black paper, laid a coin on top of the paper, and suspended above the coin a small quantity of the mineral uranium. He then set the whole away in a dark room and let it stand for several days. When he developed the photographic plate he found upon it a shadow picture of the coin similar to an X-ray picture. He concluded, therefore, that uranium possesses the property of spontaneously emitting rays of some sort which have the power of penetrating opaque objects and of affecting photographic plates, just as X rays do. He also found that these rays, which he called uranium rays, are like X rays in that they discharge electrically charged bodies on which they fall. He found also that the rays are emitted by all uranium compounds.

504. Radium. It was but a few months after Becquerel's discovery that Madame Curie (see opposite p. 446), in Paris, began an investigation of all the known elements, to find. whether any of the rest of them possessed the remarkable

property which had been found to be possessed by uranium. She found that one of the remaining known elements, namely, thorium, the chief constituent of Welsbach mantles, is capable, together with its compounds, of producing the same effect. After this discovery the rays from all this class of substances began to be called Becquerel rays, and all substances which emitted such rays were called radioactive substances.

But in connection with this investigation Madame Curie noticed that pitchblende, the crude ore from which uranium is extracted, and which consists largely of uranium oxide, would discharge her electroscope about four times as fast as pure uranium. She inferred, therefore, that the radioactivity of pitchblende could not be due solely to the uranium contained in it, and that pitchblende must therefore contain some hitherto unknown element which has the property of emitting Becquerel rays more powerfully than uranium or thorium. After a long and difficult search she succeeded in separating from several tons of pitchblende a few hundredths of a gram of a new element which was capable of discharging an electroscope more than a million times as rapidly as either uranium or thorium. She named this new element radium.

505. Nature of Becquerel rays. That these rays which are spontaneously emitted by radioactive substances are not X rays, in spite of their smilarity in affecting a photographic plate, in causing fluorescence, and in discharging electrified bodies, is proved by the fact that they are found to be deflected by both magnetic and electric fields, and by the further fact that they impart electric charges to bodies upon which they fall. These properties constitute strong evidence that radioactive substances project from themselves electrically charged particles.

But an experiment performed in 1899 by Rutherford (see opposite p. 446), then of McGill University, Montreal, showed that Becquerel rays are complex, consisting of three different types of radiation, which he named the alpha, beta, and

gamma rays. The beta rays are found to be identical in all respects with cathode rays; that is, they are streams of electrons projected with velocities varying from 60,000 to 180,000 miles per second. The alpha rays are distinguished from these by their very much smaller penetrating power, by their very much greater power of rendering gases conductors, by their very much smaller deflectibility in magnetic and electric fields, and by the fact that the direction of the deflection is opposite to that of the beta rays. From this last fact, discovered by Rutherford in 1903, the conclusion is drawn that the alpha rays consist of positively charged particles; and from the amount of their deflectibility their mass has been calculated to be about four times that of the hydrogen atom, that is, about 7400 times the mass of the electron, and their velocity to be about 20,000 miles per second. Rutherford and Boltwood have collected the alpha particles in sufficient amount to identify them definitely as positively charged atoms of helium.

The difference in the sizes of the alpha and beta particles explains why the latter are so much more penetrating than the former, and why the former are so much more efficient than the latter in knocking electrons out of the molecules of a gas and rendering it conducting. A sheet of aluminium foil .005 centimeter thick cuts off completely the alpha rays but offers practically no obstruction to the passage of the beta and gamma rays.

The gamma rays are very much more penetrating than even the beta rays, and are not at all deflected by magnetic or electric fields. They are regular waves in the ether, like X rays, only shorter; and they are commonly supposed to be produced by the impact of the beta particles on surrounding matter.

506. Crookes's spinthariscope. In 1903 Sir William Crookes (see opposite p. 358) devised a little instrument, called the spinthariscope, which furnishes very direct and striking evidence that particles are being continuously shot off from radium with enormous velocities. In the



spinthariscope a tiny speck of radium R (Fig. 476) is placed about a millimeter above a zinc-sulphide screen S, and the latter is then viewed through a lens L, which gives from ten to twenty diameters magnification. The continuous soft glow of the screen, which is all one sees with the naked eye, is resolved by the lens into hundreds of tiny flashes of light. The appearance is as though the screen were being fiercely bombarded by an incessant rain of projectiles, each impact being marked by a flash of light, just as sparks fly from a flint when struck with steel. The experiment is a very beautiful one, and it furnishes very direct and convincing evidence that radium is continually projecting particles from itself at stupendous speeds. The flashes are due to the impacts of the alpha, not the beta, particles against the zinc-sulphide screen.

FIG. 476. Crookes's spinthariscope

A mixture composed of a radium compound and zinc sulphide glows constantly and is used for the dials of airplane instruments, compasses, and watches, as well as on gun sights, making them visible for night use.

507. The disintegration of radioactive substances. Whatever be the cause of this ceaseless emission of particles exhibited by radioactive substances, it is certainly not due to any ordinary chemical reactions; for Madame Curie showed, when she discovered the activity of thorium, that the activity of all the radioactive substances is simply proportional to the amount of the active element present, and has nothing whatever to do with the nature of the chemical compound in which the element is found. Furthermore, radioactivity has been found to be independent of all physical as well as chemical conditions. The lowest cold or greatest heat does not appear to affect it in the least. Radioactivity, therefore, is as unalterable a property of the atoms of radioactive substances as is weight itself. It is now known that the atoms of radioactive substances are slowly disintegrating into simpler atoms. Uranium and thorium have the heaviest atoms of all the elements. For some unknown reason they seem not infrequently to become

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