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Nature [August 5, 1920 NATURE [AUGUST 5, 1920 "indirect" group the variations agree in time, solar variations acting through the atmospheric but are opposite in character; in the third, the circulation as the main cause of meteorological "indifferent" group, there is no regular corre­ changes. To what extent the ocean helps by spondence. Sir Norman and Dr. W. J. S. regulating the air temperature and circulation the Lockyer have shown that a region may for years authors do not discuss in the present memoir; belong to the "direct" group, then suddenly that and other questions are to be dealt with after become " indirect," and later return to the further investigations in a series of memoirs to "direct" group. Drs. Helland-Hansen and which the present is introductory. The useful­ N ansen accept this frequent inversion, and also ness of the promised memoirs would be increased their explanation of the phenomenon. (should they have as many appendices and sup­ The authors' instructive study of North Atlantic plementary notes as the present) if each were temperatures therefore streng thens the case for provided with an index. The Thermionic Valve in Wireless Telegraphy and Telephony.1 By PROF. J. A. FLEMING, F.R.S. H E thermionic valve is an invention which has due to some material emission from the hot fila­ T vastly increased the powers and range of ment. wireless telegraphy. Like many other inventions, Another fact I observed very soon was that the the telephone, for instance, it is simple in its filament was giving off torrents of negative elec­ essential construction. It consists of a little elec­ tricity, and could discharge a positively electrified tric lamp comprising a glass bulb, very highly conductor connected to the plate, but not one exhausted of its air, containing a filament of negatively charged. Furthermore, I found that carbon, or better tungsten, which can be rendered the vacuous space between the fil ament and the incandescent by an electric current. W ithin the plate possessed a curious unilateral electric con­ bulb and around the filament are fixed certain ductivity for low-voltage direct electric currents, metal plates or cylinders, and, it may be, spirals and that even a single cell of a battery could pass of wire or metal networks calIed the grid. To a current from the hot filament to the collecting explain its origin in its simplest form I shall have plate if the negative pole of the battery was in to take you back in thought to the days when connection with the hot filament, but not in the the physical effects taking place in incandescent opposite direction. This fact had, however, becn electric lamps were first beginning to be con­ previously noticed in another manner by W. sidered carefulIy. In 1883 Mr. Edison for some Hittorf. These experiments were made in 1888 or purpose placed in the glass bulb of one of his 1889, and at that time were not satisfactorily ex­ carbon filament lamps a metal plate which was plained. carried on a platinum wire sealed through the It was not until nearly ten years later that your glass. ' \T hen the filament was rendered incan­ distinguished professor of natural philosophy, Sir descent by a current from a battery, he found Joseph Thomson, published accounts of his epoch­ that if the plate was connected by a wire, external making and important researches, in which he to the lamp, with the positive terminal of the proved that the agency we call negative electricity fil a ment, a small electric current flowed through is atomic in structure, and exists in indivisible it, but if connected to the negative terminal no units now named electrons, which carry a certain current, or at most a very feeble current, flowed. electric charge and have a certain mass. These This new and interesting effect became known as negative electrons are constituents of all chemical the "Edison effect" in glow la mps, but Mr. atoms. An electrically neutral atom which has Edison gave no explanation of it, and made no lost one or more electrons is called a positive ion, practical application of it. and neutral atoms which have lost or g ained elec· Edison supplied some lamps with plates in the trons are said to be ionised. There are a rguments bulb to the late Sir WilIiam Preece, and the latter in favour of the view that the · majority of the found that the current called the Edison effect atoms in metals and other good conductors of current increased very rapidly as the filament was electricity are in a state of intermittent ionisation, heated to higher and hig her t emperatures, and and that intermingled with the atoms or positive that the collecting plate could be placed a long ions, say in a wire of copper, tungsten, or car­ way from the filament, even at the end of a side bon, there are electrons which are jumping from tube, without altogether causing it to vanish. At atom to atom with great velocity. If we apply a li ttle later date I took up the subject, and one to the wire an electromotive force, this causes a of the first things discovered wa s that the Edison drift of these electrons at the instant they are effect wa s greatly reduced if that side of the free in the opposite direction to the force (on carbon loop filament in connection with the nega­ usual conventions), and this drift or unidirectional tive pole of the battery was enclosed in a glass motion is superimposed on the irregular motion, or metal tube, or if a sheet of mica was inter­ and constitutes an electric current. The drift posed between the filament and the collecting velocity may be very slow compared with the plate. This seemed to indicate that the effect was velocity of the irregular motion. The drift motion ) From a discourse delivered at the Royal Institution on Friday, May 21. of the electrons superimposed on the irregular NO. 2649, VOL. 10SJ ©1920 Nature Publishing Group AUGUST 5, J9 20] NATURE ____________________________ motion may be compared with that of a swarm of The thermionic current then becomes stationary and be.es in insect is flying hither and is said to be saturated. thither rapidly, whilst the whole sw arm is being It is remarkable that although this emission of blown by a gentle breeze slowly down a road. If electricity from incandescent substanoes had been electrons merely surge to and fro, it gives studied for more than a quarter of a oentury, none of them made any practical application of it prior to nse to a f.orm of current we describe as an 1904. At that date I was so fortunate as to discover and if they execute this a totally unexpected application of this thermionic very rapidly we call it an electric oscilla­ emission in wireless telegraphy. Before 1904 only tIon. three kinds of detector were in practical use in wire­ The reason a n electric current produces heat in a less telegraphy, viz. the coherer, or metallic filin¢s because the drift energy of the electrons detector, the magnetic-wire detector, and the elec­ then bem;; continually converted into additional trolytic detector. The coherer and the electrohltic Irregular-mobon energy in the free electrons and detectors were both rather troublesome to work with atoms by collisions of electrons with the atoms of the on account of the frequent adjustments required. c?nductor. .If, the temperature becomes very Themagnetic detector was far more satisfactory, and high-that IS, If the irregular electronic motion in the fGrm given to it by Senator Marconi is still becomes very great-certain electrons may acquire used. It is not, however, very sensitive, and it such velocities that they are fiung OUt from the surface requires attention at frequent intervals to wind up the of \yire even the attraction of the positive clockwork which drives the moving iron-wire band . IOns left behmd. If there is no electric force .In or about 1904 many wireless telegraphists were to make the electrons move away from the seeking for new and improved detectors. I was of the hot wire, these electrons con­ anxious to find one which, while more sensitive and stitute a chMge around it, and the repulsion less capricious tpan the coherer, could be used to they exercise on each other tends to keep other elec­ re cord the signals by optical means. Our electrical trons from out into the space. Suppose, how­ instruments for detecting feeble direct or unidirec­ ever, the IOcandescent wire is placed in the axis tional currents are vastly more sensitive than any we of a highly glass tube, and is surrounded have for detecting alternating curre nts. Hence it by a metal cylmder which is kept positively electrified seemed to me that we should gain a great advantage the electrons move to it, and othe rs then make thei; if we could convert the feeble alternatin¢ currents in from the wire. Such a tube with incandescent a wireless aerial into unidirectional currents which wire and cold metal plate anode .is now called could then affect a mirror galva nometer or the more a thermwm c tube. The steady emission of electrons sensitive Einthoven galvanometer. There were is called a thermionic CUtrrent. In the case of a already in existence applianoes for effecting this con­ tungsten wire brilliantly incandescent in vacuo and version when the alternations or frequency was low, under sufficient electric force, this current may nameJy, one hundred or a few hundred per second. amount to as much !Is an ampere per square oenti­ After trying numerous devioes myoid experiments on metre of surface.
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