The Central Telegraph Office was serving spam 67 years before vikings sang about it on telly

Geek's Guide to Britain The BT Centre is an unremarkable-looking building just north of St Paul's Cathedral, nine storeys of Portland stone with straight unadorned 1980s lines softened by curved corners. The headquarters of the UK's largest telco could be mistaken for an apartment block if it wasn't for the company's logo.

Soon even that will go as in July 2019 BT sold the building for £210m, although it will continue to rent it while preparing to move to One Braham, a new 18-floor building above Aldgate East tube station on Braham St, by the end of 2021.

The sale looks likely to bring to an end nearly one-and-half centuries of telecoms history on this site. Most of this did not happen in the BT Centre, but the previous building on this site: the Central Telegraph Office (CTO).

Marconi demonstrated wireless telegraphy on its roof, but more importantly the CTO became the largest telegraph office in the world, connected to every large town and city in Britain, with links around the world and at one point employing 5,699 people.

Its technology included manual Morse machines, piano-style keyboards and miles of pneumatic tubing, inside and out under the streets of London. What is now an office was once a massive factory of communication.

The first viable electric telegraphy systems were developed in the 1830s by Samuel Morse in the US and by Charles Wheatstone and William Cooke in Britain. They combined the optical telegraphy systems first used in France in the late 18th century – lines of stations using visual signals to pass on messages – with the development of batteries and the realisation that electricity in a wire could be used to send signals a long way almost instantly.

Although time was required for operators to key in and decode messages, the telegram meant countries once weeks apart could communicate in hours or minutes.

Early commercial demand for telegraphy was particularly strong in financial services, where faster access to information means money. In 1845, Cooke teamed up with MP and financier John Lewis Ricardo to found the Electric Telegraph Company, which soon moved its main office from the Strand to Founders' Court, just north of the Bank of England.

By 1860, the company expanded to a bigger office on what the City of London renamed Telegraph Street, a couple of blocks north on Moorgate.

In 1868, the government decided that the General Post Office (GPO) should acquire the fast-growing telegraphy system. The GPO's main office was on the east side of St Martin's Le Grand, a few hundred yards north of St Paul's Cathedral, and it was given the ability to buy neighbouring land to expand operations.

A maze of houses and two pubs on the other side of the street were levelled to build GPO West, as the building was originally known. It opened and took over the work of the Telegraph Street office in 1874. Tom Standage explains in The Victorian Internet that messages continued to be marked "TS" after the former address.

Initially, telegraphy required only the upper floors of GPO West, with the service employing 1,445 staff and operating telegraphy 500 machines, handling some six million messages that year. Three-fifths of the "instrument clerks" operating these machines were women: "It is a cheerful scene of orderly industry, and it is, of course, not the less pleasing because the majority of the persons sitting here are young women, looking brisk and happy, not to say pretty," drooled the Illustrated London News in a November 1874 article. Women did not work nights, and got half an hour off from their eight-hour shifts in a segregated dining hall.

Ground floor? That's where we keep the batteries

The CTO had batteries on the ground floor – with 60 cells required to get a signal through to Edinburgh – along with three steam engines powering 26 lines of pneumatic tubes.

Tubes were used because telegraphy was highly efficient for sending messages over long distances but highly expensive for short ones, given both needed two skilled operators.

In 1891, tubes linked the CTO to post offices across central London, allowing messages wrapped in felts pocket to be blasted along tubes with compressed air. One tube line reached along the Strand to the House of Commons, while the City was covered by a mesh of tubes linking sites including the original Founders' Court and Telegraph Street offices, Lloyds, the stock exchange and the offices of foreign telegraph companies.

'A new system of telegraphy without wires'

GPO West housed research as well as the CTO until this work moved to Dollis Hill in north-west London. On 30 March 1896, electrical engineer AA Campbell wrote a letter of introduction to the GPO's chief engineer William Preece: "I am taking the liberty of sending to you with this note a young Italian of the name of Marconi, who has come over to this country with the idea of getting taken up a new system of telegraphy without wires…"

Preece took up the offer, and on 27 July 1896, Guglielmo Marconi carried out the first public demonstration of wireless transmission from the roof of the CTO to GPO South, about 300 yards away on the other side of St Paul's Cathedral.

GPO South was later known as Faraday Building North and served as an international telephone exchange, but like most of the buildings in this article it no longer exists.

A grand doorway and a plaque have been incorporated into the hotel that replaced it. "Our first Experiment on the Roof of the General Post Office in July 1896 was to note if we got every signal on the receiver that was sent by the transmitter commutator, the tapping of the tube and working of the relay were the only point we could watch, and all went well," George Kemp, a technician on Preece's staff, wrote.

Preece supported further experiments by Marconi on Salisbury Plain in Wiltshire, but the Italian was keen to develop the technology's commercial potential. "They fell out big time," says David Hay, BT's head of heritage and archives. "Preece, a scientist and a public servant who had personally supported Marconi on his arrival in the UK, was quite hurt."

Marconi eventually moved his work to the Lizard peninsula in west Cornwall, where in December 1901 he pioneered transatlantic radio communication.

Telegraphy had its innovations, some less welcome than others. Over 16-17 December 1903, The Times sent 88,847 telegrams soliciting purchases of the Encyclopaedia Britannica, VM Dunford, deputy controller of CTO, explained in an article he wrote in March 1915.

It took until 1970 for Monty Python's Flying Circus to repeatedly sing the word "spam", leading to the canned meat product lending its name to junk mass communication. The CTO had been serving telegram spam 67 years earlier.

'The largest telegraph office in the world'

Meanwhile, the CTO kept setting new traffic records. Queen Victoria's death generated 201,886 telegrams on 1 February 1901, but this was topped by the 314,126 on 25 June 1902 following the last-minute postponement of Edward "Bertie" VII's coronation due to him needing an emergency operation.

The outbreak of the First World War on 5 August 1914 resulted in a relatively modest 265,339, and the war left the CTO largely untouched; a German aeroplane bombed the CTO on 7 July 1917 but only caused minor damage to a corner of the fourth floor.

In 1919, the CTO employed 5,699 staff who were paid a total of £728,300 – a mean average of £128 – or £6,500 in today's money, according to the Bank of England's inflation calculator.

Women made up 43 per cent of the workforce, and continued to enjoy special benefits: "Separate cloak and dining rooms for men and women telegraphists are provided in an adjoining building… a smoking room is set aside for the men and rest room for the women," according to an official guide.

This guide also boasted that the CTO was now the largest telegraph office in the world, handling nearly 45 million telegrams a year with more than 1,100 machines. It occupied all of GPO West, which had been extended upwards as well as into neighbouring buildings.

The ground floor now held the ends of most of the 52 external pneumatic tubes as well as 8km (five miles' worth) of tubing within the building, along with a public counter and facilities for the "phonogram" service first introduced in 1899, which allowed those with telephones to send and receive telegrams by voice.

Above the first floor stores, the second floor housed offices and international circuits, as well as tubes linked to the offices of foreign cable companies. Provincial circuits took up some of the second and all of the third floors, while the fourth was used for those connecting London and its surrounding counties. The fifth housed the school of telegraphy, where students were taught for four hours a day, and then spent four more as paid internal messengers.

The main telegraphy floors in 1922 looked like clothing factories, with tightly packed rows of hundreds of staff focused on their machines, wires snaking across ceilings and big railway station-style clocks. But this was skilled and specialised work. The CTO was using a wide range of equipment at this point, including old Morse equipment for communicating with the provinces and Hughes machines with 28-button keyboards that looked like pianos.

More advanced were the Baudot machines used to talk to Berlin, where operators listened for clicks running at 180 beats per minute which told them when to press keys, allowing four machines to share one line. Modern Siemens machines, used to communicate with Oslo among other places, could send and receive 110 words a minute with messages coming out on gummed tape.

A modernisation programme completed in 1935 swept away the old kit in favour of teleprinters, which required much less training to operate and allowed customers with their own machines to send messages directly to the CTO, a system known as telex.

It also automated the pneumatic tube system, with its ground-floor room replaced by an air-conditioned lecture hall – although there were at this point 120km+ (75 miles) of tubes under London.

The CTO also introduced picture transmission by telegraph, which used a spot of light spiralling over an image and a photo-electric cell, connected to nine European countries. Staff numbers fell to around 3,000 but the CTO was busier than ever, setting a new daily record for telegram processing during the Munich crisis of September 1938 of 403,831.

The Second World War would generate vast amounts of work for the CTO, but would also destroy its home. On 29 December 1940, incendiary bombs fell on the building and were put out, but winds blew flames across from other buildings, reigniting it. Some equipment and records were salvaged and firefighters were tackling the blaze when water pressure failed. The CTO was gutted.

Work was rerouted to four "ring centres" in outer London at Acton, Addiscombe, Brent and Stratford and the bottom two storeys of the ruined CTO were rebuilt, allowing it to reopen in June 1943 – a demonstration of how vital telegraphy was to the country. In 1945/6, it processed a record 64.9 million telegrams.

But then the telephone started to take over, with traffic dropping away. In 1962, the GPO moved the CTO to the Fleet building on Farringdon Street, where it would employ just 600 staff, and the pneumatic tube system breathed its last. In 1967, the old GPO West building was declared unsafe and pulled down.

In November 2016, the Fleet building was itself demolished, revealing a time capsule buried in October 1958, which included telecoms kit of the era.

'The government's parting gift to BT'

After a number of false starts, planning permission for the St Martin's Le Grand site was granted in 1979, with the current BT Centre opening in June 1984. "In some ways, it was the government's parting gift to BT," says Hay, with the telecoms service floated on the stock market that year.

It is a very different building to GPO West. "Staff are able to enjoy an interesting and pleasant environment which is similar to many of the small courtyards and protected spaces that are a traditional feature of the City," said a 1985 corporate history of the site, summing it up quite well.

It added that workers could use four glass lifts travelling up and down the open sides of the glass-roofed atrium – but could stick to enclosed ones if they preferred. It also boasted about the BT Centre having 2,300 telephones for 1,700 staff, presumably to enable 1980s City dealing room-style conversations on two phones at once.

BT spruced up the building in 1997, commemorated by glossy "Countdown to BT Centre" newsletters for staff. This increased the number of workstations from 1,200 to 1,700 and launched the Connections Café, complete with tasteful neon signage and a "cyber bar" which laptop users could plug into. Each floor got a "buzz bar" with fax machines, a photocopier and vending machines serving tea, coffee and hot chocolate, the last apparently inspiring whoever owned the copy now in the archives to use it as a drinks coaster.

The neon café sign has gone and the buzz bars have buzzed off, but otherwise the BT Centre today looks much as it was described in the brochures, more like two linked courtyards of office blocks under a glass roof than a single building. There is a lot of natural light and the reception staff shelter under umbrellas on sunny days.

Traces of the site's history are visible. Panels and screens in reception cover BT's history including the 1896 Marconi demonstration, which is also commemorated by a plaque outside to the right of the main entrance. Inside, there are plaques for the CTO staff who died in the First and Second World Wars, including one added in 2018 naming 89 colleagues who died in service between 1914 and 1918. Recent history shows through as well, with BT's old italicised font visible on a few signs.

It will not be the BT Centre for much longer. But that will not change the fact that for more than nine decades it was the centre of British telegraphy, the workplace of tens of thousands of people who used a wide range of technology to process hundreds of millions of telegrams. ®