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FIGS. 9 (left hand) and ga (right hand). The wireless equipment of an aeroplane for wireless telegraphy and telephony. The generator is fixed to the outside of the hull of the aeroplane and driven by a wind screw. Fig. ga shows the general interior arrangement. (By permission of Marconi's Wireless Telegraph Co., Ltd.)

remain connected with the aerial. The arrangements will be understood from the diagram in fig. 9. The method is, however, not very successful and it cannot be said that two-way radio- telephony with a single aerial wire is a solved problem. On the other hand radio communication to and from aeroplanes up to 150 or 200 m. is now a thoroughly practical matter. The aerial is a wire about 230 ft. in length with a weight at the end which is unwound from a winch when the pilot wishes to com- municate. The pilot gene'rally wears a helmet with microphone transmitter opposite his mouth and two receivers over the ears. The transmission of speech from the aeroplane is more easy than reception owing to the great engine and propeller noises; never- theless it is of immense use in connexion with air traffic as by it aircraft can be guided through the clouds to their destination, and the pilot informed of the conditions as regards fog or cloud at the landing station.

In May 1921 Marconi's Wireless Telegraph Co. carried out very successful demonstrations of practical radiotelephony on the two-wave system between Southwold in England and Zand- voort in Holland a distance of 125 m. over the North Sea. The wave lengths used were 120 and 125 metres.

A very important use of the three-electrode valve is that of repeating speech currents from ordinary telephone wire circuits to wireless circuits and vice versa. Also this is perhaps the place to point out its extremely valuable qualities as a telephone re- peater or relay for long wire circuits. Since the characteristic curve of the triode is nearly flat at the central part it follows that any irregular variations of grid potential are exactly copied by the corresponding variations of plate current. Hence if we con- nect the secondary terminals of a telephone transformer to the grid and filament of a valve and the primary terminals of another transformer to the plate and filament with plate battery inserted we shall have an arrangement called a thermionic repeater, which repeats and amplifies telephonic speech currents. If one or more such repeaters are inserted in a telephone line at intervals they will operate to neutralize the attenuation of the speech currents due to the resistance of the line and enable telephonic speech to be transmitted over larger distances without greater expenditure on the copper line conductor. Thus in the United States the long transcontinental telephone line from New York to San Francisco and Los Angeles has 13 such repeater stations in it, and speech is thus rendered possible over 4,000 m. of line.

In Oct. 1920 a remarkable feat was carried out in telephonic transmission by the aid of thermionic repeaters. A ship four hours out in the Atlantic spoke by wireless telephony to the mainland of the United States. The speech was then repeated

into the transcontinental line from New York to Los Angeles and again repeated on to a radio circuit and delivered at Santa Catalina I. about 30 m. out in the Pacific. The speech trans- mission over this 4,000 m. was as perfect as over any exchange circuit in a large city. It is possible in this manner to speak to flying aeroplanes from the ordinary wire telephone of a town.

Long Distance Stations. The above improvements in generation and detection of electric waves have not only made radiotelegraphy from ship to ship and ship to shore a certain and indispensable aid to navigation, but have enabled a multitude of long distance radio stations to be established which can maintain communication over distances of several thousand miles. It is now generally agreed that this possibility is due not to true diffraction of these long electric waves round the earth but to the fact that the higher levels of the earth's atmosphere are in a state of permanent ioni- zation due to sunlight or extra-terrestrial causes. This creates a high level reflecting layer which guides the wave round the earth. There are, however, peculiar difficulties and effects at times of sunrise and sunset. In the United Kingdom the Marconi Co. have a large station at Carnarvon, Wales, near Snowdon, which is in correspondence with another at Marion, N.J., United States, for transatlantic working. The British receiving station is at Towyn, about 60 m. from Carnarvon, to enable reception and transmission to be carried out simultaneously. The direct effect of the Carnarvon radiation on the Towyn receiving aerial is neu- tralized by a balancing aerial (see British Patent Specification No. 13020 of 1911 of G. Marconi). The aerial at Carnarvon is a Marconi directional one, 3,600 ft. in length and 400 ft. vertical height supported on 10 tubular steel masts. The wave length of the radiation is 14,000 metres. The system of wave generation is the so-called timed-spark of Marconi. A direct current high voltage dynamo keeps two sets of condensers charged, and by means of a pair of rotating wheels with studs on their peripheries these con- densers are discharged alternately through the primary coil of a transformer, the secondary coil of which is inserted between the aerial and the earth. These two sets of oscillatory discharges are made to follow each other in step and in close sequence by means of a trigger disk discharger which times two discharges so as to constitute in effect a continuous oscillation. The signalling is con- ducted by switches worked by compressed air which are operated through a relay by efectric currents from Towyn. The same com- pany have also a radio station at Clifden in Ireland which corre- sponds with another at Glace Bay, Nova Scotia. The Clifden station employs large thermionic valves as generators of contin- uous waves.

The Imperial Wireless Telegraph Committee which reported to Parliament in June 1920 recommended the thermionic valve gener- ator for the imperial stations of the British Empire on the ground that the capital outlay would be less than for arc or alternator stations.

The high power radio stations in the United States comprise one at New Brunswick, N.J. which is equipped with Alexander- son alternators of 200 kilowatts capacity working on a wave length of 13,500 metres. The signals are made by means of a magnetic amplifier which is an independence coil, the impedance of which is varied by means of a small direct current which changes the per-