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In spite of the confidence of the pioneer workers, the telegraphic service obtained was liable to frequent interruptions through natural causes, and still greater power stations were projected. This was necessary although the sensitiveness of the methods of reception of signals had been enormously enhanced since the original estimate of 25 kilowatts was made. In 1906 the Poulsen arc was introduced into practical wireless telegraphy, and completely new methods were advocated by its promoters. From their experience gained in trials across the North Sea between Denmark and England the Poulsen group anticipated complete success across the Atlantic with about 15 kilowatts: This estimate once more proved too low. Meanwhile the early pioneers and other inventors in various countries pushed the spark methods to higher and higher power, till in 1912 spark stations of more than 200 kilowatts existed for transmission across distances of about 2,300 miles. Even yet interruptions were frequent. Valuable observations were made about this period by engineers in the service of the American and other Governments on both are and spark stations, and it appeared that with the very best apparatus the telegraphic service over these ranges, or even smaller ranges in the tropics, could not be continuously maintained.
During the years 1913 to 1919 the methods of reception have, as has already been mentioned, increased vastly in sensitiveness. It has therefore been thought that surely now the large-power continuous wave stations erected in America and several countries of Europe just before and during the war will be able to maintain uninterrupted telegraphic service across distances of about 3,500 miles. Our enquiries have convinced us that arc stations employing 250 kilowatts are not sufficient to ensure an uninterrupted twenty-four hours' service over such distances throughout the year even in the temperate in spite of many elaborate methods for eliminating natural disturbances having been developed and applied. The demand is still for more power at the transmission stations. At present the largest transmitting station working across the Atlantic is the German station at Nauen, which is rated at 400 kilowatts, and which aims at communication across 4,000 miles, but it is understood that the German engineers have recently decided upon a great increase in the power of this station.
zone,
The problem of transmission of messages from England and their reception in India, a distance of 4,594 miles, is regarded by all experts as one of great difficulty, not only on account of the distance and the fact that this is so largely over land, but also on account of the prevalence and intensity of atmospheric electrical disturbances in India. No experience exists enabling a trustworthy estimate to be made of the power necessary to ensure a good commercial wireless telegraphic service under these conditions. Having regard to the most recent observations upon Transatlantic communication, the greatest caution must be exercised in assigning a figure for the necessary power of the transmitting station in England. We consider that a station of at least 500 kilowatta would be required, and that this would have to be provided with a very high and large aerial. Such a station could be expected to give a molerate but not an uninterrupted telegraphic service. It is to be remarked, however, that the service supplied by an equal station in India transmitting to England could be expected to be reasonably good, since the atmospheric disturbances prevalent in England can be largely eliminated.
We are convinced that consideration for commercial purposes of lines like those from England to India, from India to Australia, and from Australia to Canada, involves far-reaching speculation beyond existing experience. It is noteworthy, too, on the one hand, that mathematical theory tends to emphasise the need for great caution in embarking upon such speculation, and on the other, in practice, that when the leading American experts, as has been previously stated, desired in 1918 to ensure absolute regularity of service between France and the United States, they started to erect an are of no less than 1,000 kilowatts, and this for a distance not nearly that between England and India, and much less subject to atmospheric disturbance than India.
15. Further, a specific and demonstrable disadvantage attaches inevitably to trans- mission over very long ranges, where high-speed signalling, and therefore automatio reception, are essential factors of a satisfactory service. This can be explained only in technical terms, which we endeavour so far as possible to avoid. For low speeds of signalling (that is, few words a minute) the optimum wave-lengths to employ for communicating between two stations increase rapidly as the distance apart increases. This is an accepted principle in all wireless transmission. The ordinary short-range commercial wave-length is 600 metres; the latest long-range stations employ 20,000 metres. The optimum wave-length is, in accordance with accepted theory, proportional to the square of the distance. With short waves, the sensitiveness of the receiving apparatus is practically independent of the speed of signalling; but with long waves,
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especially with modern (retroactive triode) receiving circuits, the sensitiveness falls off as the speed increases.
This may be explained as follows: In the tuned receiving circuit an oscillation is gradually worked up under the continued stimulus of the incoming signal, and only reaches its sensibly full amplitude after the signal has been continued for a time proportional to the wave-length employed. Thus, the higher the speed of signalling- that is, the shorter the duration of an incoming signal corresponding to a Morse dot- the smaller the amplitude to which the receiver oscillation attains before the dot ends.
For a given height of transmitting aerial, even at low speeds (that is, before the above-described high-speed effect makes itself felt), the transmitter power required to communicate over a given distance is proportional to the sixth power of the distance; and when the speed is sufficient to make the high-speed effect fully felt, the trans- mitter
power ia proportional to a still higher power of the distance. the separation between two stations is increased, the possible speed of signalling falls It follows that as off in proportion to something higher than the third power of the distance.
Consequently, any important decrease in distance between two stations permits (a) an enormous reduction of transmitter power; or (b) a great increase in speed of signalling; or (c) a reduction of height of the transmitting aerial; or (d) a combination of these changes, giving a compromise between these economies. thus prescribed by theoretical reasoning, confirmed as it has been by experience, should The compromise form the basis of any practical proposals in which efficiency and economy are factors of equal importance.
With regard to wave-lengths, we would further point out that from the trend of inter-Allied conferences it is at least possible that international arrangements may compel this country to employ longer waves than would in principle be desirable. This is an additional argument, for what it may be worth, against the attempt to cover extreme distances by the use of great power.
16. Let us assume, however, that, notwithstanding the foregoing considerations, direct communication between England and India is demanded, and proceed to exhibit in this connection the third vital factor of a satisfactory commercial wireless service, namely, cost.
The arc system would unquestionably be chosen, and the smallest arc possible for the service would be one of 500 kilowatts. Each station would necessarily be equipped with the arc and power plant in duplicate, as it would naturally be hoped that after a time sufficient traffic would be forthcoming for a continuous 24-hour service, that is, for a load factor for one arc of 100 per cent. The following figures show the capital cost of such a station to generate its own energy*
Two 500-kilowatt are transmitters, complete with spares and auxiliaries.. Steam power station, namely, two 750-kilowatt D. turbo-generators, three water-tube boilers with superbeaters, economisers, &c., all pipe- work, switchgear, feed pumps and coal-handling plant; buildings and foundations; contingencies at 10 per cent.
30,000
63,525
Buildings for arc plant
Contingencies at 10 per cent. on above (except power plant. &c.) Receiving station and connecting land liue
Eight guyed steel masts, 820 feet, with aerial and earth connection Reads and fencing
··
7,500
130,000
3,000
17,000
10,250
261,275
To this must be added the cost of the site, unless government land were available. For the provision of a similar station in India an average of 25 per cent. must be added to the above costs, and it would be necessary to erect residences for the staff, making the cost of the Indian station as follows :—
Station as above, plus 25 per cent. Residences
Total
+
£
326,531
25,000
351,531
Total cost of the pair of 500-kilowatt are stations, erected in England and India, £612,756, gay, £615,000.
• We believe our calculations of cost to be as accurate as they can be made at the present time. Three firms have stated their willingness to tender for the supply of the arcs. The estimates for the masts are those of an experienced engineer. The cost of the power plants has been worked out for us by our colleague, Sir John Snell, Chief Electricity Commissioner, and includes a proper provision of reserve plant to ensure safe continuous working. The operating costs have been furnished to us by the Post Office.
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