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The Spanning of the Atlantic by Wireless Telegraphy

From The Radio Constructor, February, 1962.
    
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In December 1901 - Guglielmo Marconi became the first to send a wireless signal across the Atlantic. This remarkable achievement with such primitive equipment marked the birth of world-wide communication.

During the spring of 1900, Marconi had succeeded in sending reliable signals from St. Catherines in the Isle of Wight to The Lizard in Cornwall, a distance of 186 miles. This encouraged his belief that by using larger aerials and far more powerful transmitters he would be able to achieve transatlantic distances. Scientists were highly sceptical, many said it was impossible because of the curvature of the earth.

Marconi determined to make the attempt. A transmitting station nearly one hundred times more powerful than any previously constructed was built at Poldhu near Mullion in Cornwall. Enormous aerials were erected at Poldhu and at Cape Cod in Massachusetts but both were wrecked in severe gales. Another, less ambitious in design, was put up at Poldhu while Marconi and his two assistants sailed to Newfoundland where, from the top of Signal Hill, a receiving aerial was hoisted, at the third attempt, by means of a kite.

At 12.30 pm (Newfoundland time) on 12th December, 1901, Marconi and his assistant G S Kemp, using one of the primitive receivers of the period with a telephone earpiece heard a faint succession of Ss in Morse code. Signals from Poldhu, 2,200 miles away, had crossed the Atlantic.

To commemorate this historic achievement, a Special Exhibition was held at the Science Museum from 13th December 1961 - 25th January 1962. Among the many historic exhibits and original photographs, a notable feature was a recording of Marconis voice telling in his own words of how success was achieved.

During the early 1890s, many of the leading physicists were closely interested in the properties of 'Hertzian waves' but none expressed a thought that these waves would be of the slightest value for the purpose of communication.

In 1895, Guglielmo Marconi, working at his parents residence at Pontecchio in Italy, discovered the great increase in range which could be obtained by the use of an elevated aerial. It was this discovery which paved the way for the use of Hertzian waves in a practicable system of wireless telegraphy.

Early in the following year - 1896 - Marconi arrived in England and applied for the worlds first patent for wireless telegraphy. He had chosen to come to England partly because this country was then the most powerful maritime nation in the world and it seemed likely that wireless telegraphy would be of value to shipping, and partly because of a national affinity, his mother being Irish.

In I897 he founded the Wireless Telegraph & Signal Company (which in 1900 became MARCONIs WIRELESS TELEGRAPH COMPANY LIMITED), and this provided him with the money and technical resources necessary for his future developments.

Marconi spent the next four years in an almost continual round of experiment, development and demonstration, his object being continually to improve the reliability and range of his apparatus. At first only covering a mile or so on Salisbury Plain, he was soon communicating regularly from Alum Bay near the Needles in the Isle of Wight to Bournemouth and then to Sandbanks at the entrance to Poole Harbour, a distance of 18 miles. In March 1899 he spanned the English Channel and early in the following year he set up reliable communication from the Isle of Wight to The Lizard in Cornwall, a distance of 186 miles.

The shipping companies had shown mild interest but very little enthusiasm to install wireless equipment on their ships. It seemed, in fact, that far greater ranges and a chain of land stations would be required before wireless telegraphy would have a wide appeal. The scientists of the day, however, were almost united in believing that wireless waves, like light waves, would not follow the curvature of the earth. Therefore, they said, really long ranges were impossible.

Marconi thought otherwise. Experiments had led him to believe that the key to longer ranges lay in the employment of larger aerials and higher transmitter powers. He therefore determined to build two super-power transmitting stations, one on each side of the Atlantic, and to attempt two-way communication. Accordingly, a site was selected at Poldhu in Cornwall and the other at Cape Cod in Massachusetts.

It is difficult to visualise the stupendous problems which confronted him. The aerial system, at both Poldhu and Cape Cod, was of a size and complexity which had never been attempted before, for it consisted of twenty 200ft masts in a circle with an inverted cone of about 400 wires leading down to the transmitter. As to the transmitter itself, it was to be 100 times more powerful than any hitherto built, and no precedents whatever existed for the design. Marconi delegated the responsibility for this to his scientific adviser, Professor J A (later Sir Ambrose) Fleming, and Fleming carried it out brilliantly.

Some details of the transmitter may be of interest. The prime mover for the generation of power was a Hornsby-Ackroyd oil engine which drove a Mather and Platt 2,000V 50 Hz alternator. This was capable of delivering 25kW, although from a paper read by Fleming to the Royal Society of Arts in December 1921 it would appear that the plant was considerably under-run at the time of the transatlantic tests.

The transmitter proper, which embodied a form of the new syntonic tuning with all its advantages, employed two 20kW Berry transformers parallel-connected to step up the input voltage to 20,000 Volts. This was fed through RF chokes to a closed oscillatory circuit in which a capacitor discharged across a spark gap via the primary of a 'jigger' or RF transformer. The secondary of this transformer connected to a second spark gap and capacitor and the primary of a second RF transformer, the secondary winding of this transformer being in series with the aerial. Keying was effected by the short circuiting of the chokes in the alternator output.

The capacitors were made of 20 glass plates each 16 in square, backed on one side with one square foot of tinfoil. The plates were immersed in linseed oil contained in stoneware boxes; each box had a capacity of approximately 0.05μF.

Both the Poldhu and Cape Cod stations were all but ready when a double catastrophe struck; severe gales wrecked the aerial arrays and masts at both stations almost simultaneously.

With £50,000 already spent on the project, Marconi elected not to wait until both stations were repaired. Instead, a new aerial system was erected at Poldhu, consisting of 54 copper wires arranged in a fan-shape and upheld by a triatic slung between two 150ft masts. The current into the bottom of this aerial is stated by Fleming to have been 17 amperes and the radiated frequency is thought to have been between 100 -150 kHz. No one knows for certain, however, as no reliable means of measurement existed at the time and individual estimates made by those on the spot differ considerably.

With the encouraging news that Poldhus signals were being strongly received at Crookhaven in Ireland, 225 miles away, Marconi, with two assistants - Kemp and Paget - took passage to St. Johns, Newfoundland, the nearest landfall in the New World, taking with them large canvas kites and several small balloons with which Marconi proposed to raise the aerial. This latter course of action was decided upon for two reasons; to avoid the public speculation that the erection of tall masts would bring, and to save time.

At St. Johns all possible assistance was given them by the Governor of Newfoundland, Sir Cavendish Boyle, and the Prime Minister, Sir Robert Bond. Six hundred feet up on the clifftop of Signal Hill, overlooking St. Johns harbour, was the disused Barracks Hospital; a ground-floor room in this building was placed at Marconis disposal, and here he set up his instruments.

On 9th December a cable was sent to Poldhu instructing the engineers to begin transmissions on the 11th, between 3 pm and 7 pm GMT. The signals were to consist of repetitions of three dots (the Morse letter 'S'). This letter was chosen because - to quote Marconi himself - 'the switching arrangements at Poldhu were not constructed at the time to withstand long periods of operation - especially if letters containing dashes were sent - without considerable wear and tear, and if Ss were sent an automatic sender could be employed'.

Heavy gales were sweeping Newfoundland, however, and the next two days were spent in unsuccessful attempts to keep an aerial aloft. A balloon and a kite were lost in these endeavours.

On 12th December a full gale was still blowing, but despite this a kite was flown carrying an aerial to a height of 400 ft. Marconi began a listening watch, using his latest syntonic receiver, but could receive no signals because the erratic movements of the kite were continually altering the angle of the aerial to earth, and therefore its capacity. He decided, therefore, to revert to the older, untuned receiver, using a telephone earpiece in series with the coherer.

Various types of coherer were tried, one of which was the so-called 'Italian Navy' device. This is of particular technical interest in that it is described as consisting of a glass tube with a plug of iron at one end and another of carbon at the other, with a globule of mercury between them. The device was self-restoring and had to be used in conjunction with a telephone earpiece. It would seem, therefore, that what is described as a coherer was in fact a true semi-conductor rectifier with either the dissimilar plugs, or oxide film on the mercury, or possibly other surface impurities, performing the rectification process.

At 12.30 pm, Newfoundland time, on 12th December, 1901, Marconi heard, faintly but distinctly, the groups of three dots which could only have been emanating from Poldhu, 2,200 miles away. He passed the earpiece to Kemp, who confirmed that he had not been mistaken. Paget, to his lifelong regret, was ill on that day and was not present.

The feat was all the more remarkable when it is remembered that the onus was almost entirely on the transmitter, for no amplification was possible at the receiver, and so the received signal itself had to be strong enough to operate the earpiece.

The use of a telephone in place of a recording tape and the absence of any unbiased witness had unfortunate consequences for, immediately the news was made public, a stormy controversy arose as to whether Marconi and Kemp had been deceived into misinterpreting the noise of static as Morse signals. In this matter events conspired against Marconi in that the Anglo-American Telegraph Company, which had a message-carrying monopoly covering Newfoundland, threatened legal action if further experiments were carried out, and so there was no opportunity of giving a public demonstration. But two months later tests were carried out between Poldhu and the liner Philadelphia en route from Southampton to New York in which Ss were received on the ship at a distance of 2,099 miles and these were amply verified by witnesses. Ten months later - in December 1902 - two-way communication was effected between Poldhu and a new high-power transmitting station at Glace Bay, Canada - a circumstance made possible by the generous action of the Canadian Government in donating £16,000 towards the cost of the station.

There remained the problem of reconciling the theories of the scientists with the practical results achieved by Marconi. At that time no one knew of the existence in the upper atmosphere of an ionised layer which serves to reflect radio waves and so to make long-distance communication possible. ln 1902 Heaviside in England and Kennelly in America independently postulated the existence of such a belt to account for Marconis achievement, but its actual physical existence remained a matter for controversy until the 1920s.

There were, in fact, many unknowns at the time. Until the tests between Poldhu and the liner Philadelphia in February 1902 it had not been realised that much longer ranges were obtainable at night. Indeed, it was only then that it was realised that for the transatlantic experiment a listening watch had been kept at the worst possible time of the day! Again, the very success of the operation led to a universal acceptance of the rule 'the lower the frequency, the greater the range', and it was not until 1924 that the value of the short-waves for long distance communication was realised, largely as a result of the pioneering work of amateurs. The inauguration in 1924 of the Marconi-Franklin short-wave beam-radio service ushered in a completely new era in international radio communication. incidentally, it was at the Poldhu site that much of the experimental work in connection with short-wave beam transmission took place.

In the same way as Marconi by the introduction of the aerial/earth system had taken wireless waves out of the laboratory into the realm of practical communications, so by the 1901 transatlantic experiment did he introduce the concept of high-power radio engineering and world coverage. And although the spark telegraphy of that day was not electronically generated, it did lead directly to the invention of the thermionic valve and through this to the dawn of the electronics age.

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