vol 7, issue 12

Porthcurno's submarine telegraph system

17 December 2012
By Roger Dettmer
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View of Porthcurno beach from the coastal path

How a small Cornish town became a busy communications centre

Credit: Paul Melling / Alamy
Landing the cable at Porthcurnew Bay

An artist’s impression of the cable being landed at Porthcurno, Cornwall

Landing the Fayal cable at Porthcurno 1906

The golden sands of Porthcurno beach, where the cable was laid

Porthcurno telegraph museum, Cornwall, UK

Porthcurno's Cable & Wireless Museum

Credit: Steve Nichols / Alamy
Blast proof door at the Porthcurno cable station, 1941

Blast proof doors - just one measure to protect the Porthcurno cable station from World War Two

Cable operator using a siphon recorder

Cable operator using a siphon recorder

Beneath the sands of Porthcurno, Cornwall, lie the remains of a submarine telegraph system that helped pave the way for modern communications technology.

In the far west of Cornwall, beyond Penzance and just three miles east of Land's End, lies one of the most beautiful beaches in Britain. Porthcurno, the name is reputedly a derivation of 'bay of rocks' in old Cornish, is an idyllic cove, sheltered between towering granite cliffs, with a beach of pale soft sand sloping into the turquoise sea. It takes a bit of getting to, but on a summer's day the other-worldly beauty of the place is a veritable magnet for tourists.

But Porthcurno is much more than a 'hot' beach. In the last decades of the 19th Century, and the opening decades of the 20th Century, this isolated Cornish valley was home to the world's largest and most important telegraph cable station. This was the heyday of the British Empire, and it was through cables passing under Porthcurno's inviting sands, reaching out to the farthermost ends of the earth, that Britain was able to exercise sway over more than 450 million people and a quarter of the world's land area.

A safe haven

The British government was an early convert to the benefits of long-distance telegraphy. In 1857, during the Indian Mutiny, an emergency request for troops had taken 40 days to reach London from Lucknow. The following year, with the Mutiny effectively over, the government managed to get a message through the short-lived 1858 transatlantic cable, cancelling an embarkation order to troops in Canada and saving an estimated £50,000.

The first London-to-India telegraph link was opened in 1864. This ran overland through Europe to the Musandam Peninsula at the top of the Persian Gulf, then on to India via undersea cable. Unfortunately, problems with the overland section – it was prone to attacks by the locals – limited its usefulness, forcing the government to invest in a new link between Britain and India. This was all undersea, apart from a relatively short length across Egypt.

London to India via Porthcurno

The last section of this new cable, laid by the Falmouth, Gibraltar and Malta Telegraph Company, was brought ashore at Porthcurno on 8 June 1870. As the name suggests, the original intention was to bring the cable ashore at Falmouth, but late in the planning stage it was decided that the risk from ships' anchors in that area was too great, prompting the switch to Porthcurno.

Watching the landing of this cable was the one-time Manchester cotton merchant John Pender, who had been a director of the Atlantic Telegraph Company, responsible for the 1858 transatlantic cable. Four separate companies had been involved in laying the 1870 London-India cable, and in 1872, under Pender's chairmanship, they were all merged to form the Eastern Telegraph Company.

This company went on to develop a worldwide telegraph network – aptly called the 'Victorian Internet' – with Porthcurno as its principal station. When Pender died in 1896, he was the undisputed leader of the worldwide cable business, with the well-earned soubriquet 'The Cable King'.

At its height, 14 separate telegraph cables were landed at Porthcurno, necessitating the employment of a sizeable workforce. Teams of operators were required to read and re-key cable traffic – automatic signal regenerating equipment wasn't introduced until the mid 1920s – and the station also acted as a training centre for operators destined to work in overseas cable stations.

At the turn of the 20th Century, some 150 staff and trainees were based at Porthcurno. They must have been a rugged, independent lot, with the nearest sizeable settlement, Penzance, seven miles way along a potholed road. With intended irony, the Porthcurno staff social club was called 'The Exiles'.

Technology is ever changing, and by the 1920s, Marconi's radio telegraph was beginning to undermine the profitability of cable telegraphy. In 1928 the Imperial Wireless and Cable Conference, held in London, led to the merger of cable and wireless networks, to form Imperial and International Communications Limited – changed to Cable & Wireless in 1934.

Broadband communications

In the end, it was the submarine telephone cable that finally ended telegraphy's long reign. In 1952, Cable & Wireless's last major telegraph cable was laid from Porthcurno to Harbour Grace, Newfoundland. Four years later, TAT-1, the first submarine transatlantic telephony cable system began operating between Scotland and Newfoundland, initially carrying 36 telephone channels.

In 1983, TAT-7 was brought ashore at Porthcurno from New Jersey. With an initial capacity of 4,000 telephone circuits, it was one of the last transatlantic coaxial cables to be laid. Five years later, the fibre-optic TAT-8 came into service – the era of international broadband communications had arrived and telegraphy was fading into history.

The last telegraph circuit at Porthcurno was closed in 1970, but this didn't mean the end of the valley's link with international communications. After World War Two, Porthcurno's training function expanded with the opening of the Cable & Wireless Telegraph Engineering College. The college remained at Porthcurno until 1993, when it moved to Coventry. Four years later, on 28 March, 1997, the college buildings were re-opened as the Cable & Wireless Museum of Submarine Telegraphy, under the auspices of the PK Trust, an independent educational charity set up by Cable & Wireless. 'PK' had been Porthcurno's telegraphic code.

Telegraphy was the first major application of electricity, and the technical and commercial challenges faced by the pioneers of submarine cable systems were the inspiration for one of the great stories of human endeavour. In the communications age, it's good to remember how we got to where we are, and the engineers and scientists who made our journey possible. The Porthcurno museum does all that, and more. You can't miss it – it's the large white building just up from the beach. 

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The Kelvin Connection

William Thompson, later Lord Kelvin, was one of the outstanding scientific figures of the 19th Century, responsible for crucial theoretical and practical contributions to submarine telegraphy. In 1854, in a ground-breaking application of Fourier analysis, he showed that the cable was 'dispersive' – different frequency components travelled down the cable at different speeds – with the amplitude of the individual components decaying exponentially. This analysis provided an accurate explanation of the phenomenon of pulse retardation and spreading, evident in early cross-channel submarine cables.

Thompson's solution to the problem of detecting the highly attenuated pulses expected at the end of a 2,000-mile transatlantic cable was the mirror galvanometer – supposedly inspired by seeing the light reflected off a monocle dangling round his neck. He patented his design in 1858, receiving a payment of '500 from the Atlantic Telegraph Company.

The mirror galvanometer was crucial in detecting the micro amp signals in long undersea cables, but required two highly attentive operators to function – one to watch the movement of the light beam as it swung from side to side, following the transmitted dots and dashes, and the other to write down the characters received.

The siphon recorder, patented in 1867, was Thompsons's inventive response to the two-operator problem. Often described as an early ink jet printer, this featured a narrow glass siphon tube, filled with electro-statically charged ink, positioned just above the surface of a moving strip of paper. The tube was linked to a galvanometer coil (sitting between the poles of magnet) by a pair of silk threads. Signal currents in the galvanometer coil pulled the tube to the left and right, and the ink, forced out of the tube by electrostatic repulsion, traced a wavy line on the moving paper, providing a 'written' record of the incoming message. Thompson asked, and received, licensing fees of '1,000 a year from any cable company wanting to use his recorder.

In late July, early August 1870, Thompson made an extended visit to Porthcurno to adjust and install a siphon recorder. Of the eight recorders originally ordered for the new London-India cable, only two survive, one is in the Science Museum, London, and the other occupies pride of place in the Porthcurno museum.

Any device based on static electricity tends to work indifferently in a damp climate. The siphon recorder was no exception, and later versions of the device, believed to have been developed by Porthcurno engineers, used uncharged ink, with the siphon tube in light contact with the paper. A mechanical vibrator, known as the 'PK vibrator', was connected to the siphon tube by a third silk thread, reducing friction between the tip of the siphon and the paper, and 'flicking' a stream on fine ink dots on to the paper.

The PK vibrator version of the siphon recorder worked well in all climates, and became standard equipment throughout the Eastern Telegraph network.

Porthcurno at war

The operation of the Porthcurno cable station was vital to the Allies in World War Two, and following the declaration of war on 3 September 1939 immediate efforts were made to protect the station from attack. Three hundred infantrymen were deployed, and pillboxes constructed at various points overlooking the beach. A flame barrage fed by a large tank of petrol half way up the cliff was laid on the beach, and artillery pieces, disguised as bus shelters or haystacks, were sited close by.

In 1940, 200 Cornish tin miners were employed to excavate two tunnels to house the station's equipment. In ten months of digging they removed 15,000 tonnes of rock creating two bomb-proof tunnels, each 26ft wide, 23ft high and 150ft long. The equipment remained in the tunnels until the closure of the station, and this area now forms the working display section of the museum.

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