IN opening a discussion on the present state of the theory of wireless telegraphy and its outstanding problems, I am, to some extent, embarrassed by the wide field which presents itself for consideration.
Introductory remarks by Prof, J. A. Fleming, F.R.S., at a joint discussion by Sections A and G of the British Association at Dundee.
Numerous patents have been taken out for methods of using an antenna at the same time for sending and receiving. The inventions of Mr. Marconi in connection with this matter are both practical and important, and are being carefully developed by him.
The Marconi Co have recently introduced a call instrument, in which a signal equivalent to a prolonged dash on the Morse code deflects a galvanometer, which in turn closes a bell-battery circuit and rings a bell. The difficulty is, however, to prevent atmospheric discharges from making a false call, but render it sensitive only to a prearranged signal.
See H. M. Macdonald . Proc. Roy. Soc. London, vol. lxxi., A, p. 251, 1003; vol. lxxii., A, p. 59, 1904. The conclusions in the first paper were subjected to some criticism by Lord Rayleigh and Prof. Poincaré. See Prof H. M. Macdonald, On the Diffraction of Electric Waves Round a Perfectly Reflecting Obstacle, Trans. Roy. Soc. London, 1910, vol. ccx., A, p. 113.
See Prof. H. Poincaré, La Lumire lectrique, vol. iv., 1008, p. 323, December 12. Also Comptes rendus, April 29, 1909, and Jahrbuch der Drahtlosen Telegraphie, vol. iii., p. 445, 1910.
See Dr. J. W. Nicholson, Phil. Mag., 1910, 6th ser., vol. xix., pp. 276, 435, 516, 757.
See Dielektrizittskonstante und Leitfhigkeit der Gesteine, by Heinrich Léwy (see Annalen der Physik, vol. xxxvi., p. 125, 1911, for a number of measurements of the dielectric constants and conductivity of earth's crust materials). It has been shown recently in a paper by the present writer, assisted by Mr. Dyke, that the alternating current conductivity of insulators is a function of the frequency, and not by any means identical with the direct current conductivity. see Journ. Inst. Elect. Eng., 1912, On the power factor and conductivity of dielectrics for alternating electric currents. In the case of such substances as marble, slate, and probably others, the conductivity appears to increase with the frequency up to a certain point and then diminish again.
Ueber die Ausbreitung der Wellen in der drahtlosen Telegraphie, Annalen der Physik, vol. xxviii., p. 665, 1909.
See A. Blondel, Comptes Rendus du Congr's de Nantes, 1808; also E. Lecher, Physik. Zeitschr., vol. iii., p. 273, 1901, and Prof. F. G. Baily, Trans. Royal Scottish Society of Arts, February 9, 1903.
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Proceedings of the Institute of Radio Engineers (1913)