THE use by Marconi of vertical reflecting wires, arranged in a parabola with the transmitting aerial in the focus line, has resulted in remarkable practical applications. In the case of a plain aerial, if IO,OOO units are required to reach a given distance, then with a two-wave aperture pariabolic reflector only 25 units are required, whereas with an eight-wave aperture only 1.56 units are required. Hence with power of :.8 kw. on the oscillator valves and with a wave-length of 92 metres, large distances, such as from Cornwall to Buenos Aires, 582o nautical miles, have been reached, and on this wave-length daylight signals have been reliable, and night signals stronger than were expected.
Another method which is now being employed is that of a network in a vertical plane resembling a gigantic tennis-net 3000 feet long and 3oo feet high. L-shaped conductors on this net are insulated and tuned to the desired short wave-length. Behind this, at a distance of a quarter of a wave-length, is another plane network used as a reflector. By this method, instead of using lofty towers 820 feet high with 25o kw. on the aerial, invoMng the use of expensive insulators, it is sufficient to put about 25 kw. on the network in order to obtain equally good results, and the engineering problem is also much easier.
It is somewhat remarkable that experiments indicate that waves of a hundred metres give bad results during the daylight hours, whereas waves of thirty metres give good results by day, but bad by night. With an intermediate wave-length of about 45 metres, results are equally good both by day and by night, so that this wave-length is likely to be employed for long-distance signalling, as from Canada to Australia. It is noteworthy, too, that Macmillan in Alaska is to communicate next July with Schnell on the Seattle, U. S. Navy, in the antipodes, off Australia, with a wave-length selected as eo metres. In fact, the day of the short wave-length has arrived! * Communicated by the Author. 327 ' March 7, I925, pp. 334-335. S They give a clear and forceful statement (pp. 215-234) in the Bell System Tech. Jour. for April, I925.