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(b.) The Are System. This system, invented by Poulsen in 1904, and developed by many other technicians, utilises continuous waves generated by an electric arc.
The Poulsen are is manufactured in sizes from 1 to 1,000 kilowatts, and from 100 kilowatts upwards is much more widely used to-day for long-range transmission than any other wireless system. Its construction and operation up to a power 250 kilowatts are well understood, and for ranges of at least 2,000 miles it can be relied upon with certainty to give useful service. One disadvantage from which it has suffered, namely, that during the spaces between the Morse dots and dashes of the signals the generated wave must be either thrown out of tune in the serial or deflected from it-that is, the electrical energy must be generated at the same cost alike when it is and when it is not transmitting useful signals-has, according to the statements of several inventors, been overcome.
Ares are installed at the Eiffel Tower Station, in Paris; at Lyons, communicating duplex regularly with Annapolis, United States of America, 3,993 miles; at Nantes; at Horsea; and at Rome; in America, at Annapolis, which cominunicates regularly not only with Lyons but also with Nauen, near Berlin, 4,109 miles at Sayville, Tuckerton, San Francisco, and San Diego; and in outlying American stations at Darien (Panamá), Cayey (Porto Rico), Honolulu (Sandwich Islands), and Cavite (Philippines). The two last-named intercommunicate, but with what efficiency we do not know, at a
The stations authorised by the Cabinet last distance apart of 5,305 miles.
for the year service between Leafield (near Oxford) and Abu Zabal (near Cairo), 2,239 miles, are being equipped by Post Office engineers with 250 kilowatt ares, of British manufacture, contracted for by Mr. C. F, Elwell.
In brief, the arc system is thoroughly well-established and proved for long-range wireless communication. It presents, in the powers at present used, no unsolved technical problems; it is covered by no valid patent; it is simple to design; it can be produced by any competent manufacturer of electrical machinery; it is easy to operate, and its results are known.
Taking all these qualities together, it is pre-eminent at the present moment among methods of long-range wireless transmission. But it must be added that the service of any of the above-mentioned French and American government stations, excellent and fairly regular as it is, and doubtless fulfilling the purposes for which it was designed, does not constitute a satisfactory commercial long-range service as we have defined
the term.
Ares of greater power than 250 kilowatts still present elements of uncertainty. We have observed, for example, that arcs of this size, of which there are several in America, do not deliver to the aerial a greater effective current than arcs rated at lower powers. An experiment of the greatest importance, however, is about to be made. The American army was erecting a wireless station at Croix d'Hina, near Bordeaux, to be equipped with an are of no less than 1,000 kilowatts, manufactured by the Federal Poulsen Company of America, when the armistice was signed. The station and the equipment were purchased by the French government, the buildings, and masts of great height, are completed, all the material is on the spot, and by the end of the present year transmission by this enormous power will be attempted. The results of this undertaking, which is under the direction of General Ferrié, C.M.G., Director-General of French Military Telegraphs, a wireless technician of pre-eminent experience and scientific qualifications, are eagerly awaited by all wireless engineers, and until these prove satisfactory it would be unwise for any other authority to incur the great expense of such an equipment.
The selection by the leading American wireless engineers of this power for a service merely across the Atlantic. conveys a lesson to which further allusion will be made later. (c) The High-frequency Alternator. The electrical generator in common use for lighting purposes produces alternating current of a frequency of, say, 50 complete cycles per second. A number of inventors have constructed machines similar in principle, but generating at frequencies of the order of 20,000 per second, that is, a wireless wave of the length of 15,000 metres, and at a power of 200 kilowatts or more.
The two chief high-frequency alternators are, first, the Alexanderson, of which the only large example, of 200 kilowatts. is installed in the American wireless station at New Brunswick, New Jersey. This machine is regarded with great approval by American authorities, and carries on fairly regular communication with Stavanger (3,554 miles), Lyons (3,845 miles), and Nauen (3,958 miles). It was by the use of this alternator that Mr. Daniels, Secretary of the American Navy, controlling its output at New Brunswick through 200 miles of land telephone line, spoke from his office in Washington by wireless telephony to President Wilson in mid-Atlantic. The second
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is the Bethenod-Latour, installed in the French military station at Lyons, which communicates regularly with Annapolis, U.S.A. (3,993 miles). This machine is highly esteemed by the French government and is about to be installed in five government stations.
Other high-frequency alternators are the Goldschmidt, at Eilvese, near Hanover, by which Germany communicated with America before the latter declared war; another German one, the Telefunken, at Nauen, near Berlin; and a new French machine, of which only a low-power example has yet been constructed.
are;
The high-frequency alternator is unquestionably one of the most interesting achievements of wireless engineering, and it may well prove, as its advocates anticipate, the best means of long-distance wireless transmission in the future. Apart, however, from the facts that its own claims can hardly yet be regarded as finally established, and that a rival method of transmission is developing with great rapidity, the difficulties and drawbacks attending the construction and operation of high-frequency alternators must not be overlooked. These machines demand extreme accuracy in manufacture, which means high initial cost, and they call for high skill to operate them safely and efficiently, which implies an expensive staff. Any good working electrician can run an a skilled engineer must be in charge of a high-frequency alternator. Much machinery auxiliary to the alternator itself is usually required. In the case of the Bethened-Latour, for example, the rotor, or revolving portion, must run in a vacuum, to avoid air-friction, and a pump is needed to maintain this. Further, the rotor and the main bearings must be kept cool by a forced oil circulation of as much as 22 gallons a minute, necessitating another pump. Again, the frequency, and therefore the radiated wave-length, depends upon the speed, which in this alternator remains admirably constant within one-tenth of 1 per cent. Special apparatus is required to maintain this unvarying speed. Change of wave-length can be secured only by change of speed of rotation, which implies change of efficiency. When, finally, it is added that in thia machine the rotor is about 4 feet in diameter, weighs over 4 tons, and revolves at 3,000 revolutions, per minute, with only the twenty-eighth part of an inch clearance (0.9 mm.) between its periphery (travelling at 600 feet a second) and the fixed portion, or stator, the constructional and operating difficulties will be readily appreciated, and, as has been said, the costs are of course proportionate.
Such a machine, in case of breakdown, can be dismantled and repaired only in a workshop equipped on a large scale and by highly competent workmen. be repaired in a remote station, unless this were also provided with expensive It could not engineering appliances and personnel. erection of such machines, which would have to be in duplicate, would be an undertaking In some stations the mere transport und of great difficulty.
For all these reasons, therefore, while fully appreciating the interest and value of the high-frequency alternator and its possible important future development, we do not recommend its adoption at the present time for any part of the Imperial scheme.
(d.) The Thermionic Valve. This is a very recent arrival in the field of wireless telegraphy, and, although but in its infancy, has revolutionised wireless practice during the past five years.
It consists, in its common form, of an exhausted glass bulb, containing three electrodes, one a metallic filament, the second a metal grid, and the third a metal plate. The filament, when incandescent, emits particles of negative electricity, known as electrons, which constitute, under the application of high electrical tension, a direct current between the plate and the filament. This electronic current can be employed to generate wireless waves of great purity of form.
At first only small transmission currents were produced from the small valves in use for reception, but valves have been constructed of ever larger dimensious, until to-day, by using a number of large valves grouped together, currents suitable for long- range wireless transmission are generated. Progress in this direction is so rapid that almost every month sees a distinct advance. A few of our battleships are equipped with long-range valve transmission; from an Admiralty station in the United Kingdom regular communication was held with a force 1,600 miles away; military communication between London and the Army of Occupation in Germany is maintained by transmitting valves at each end; the Post Office will shortly communicate by the use of valves with ships over a range of at least 1,200 miles by means of stations of small power; almost all long-distance wireless telephony depends upon valve transmission.
and
We desire here to express our appreciation of the work carried out by the officers and civilian experts of His Majesty's Signal School at Portsmouth in connection with the design and use of high-power valves. Evidence of great value has reached us