Telegraph lines were essentially safe from eavesdropping from enemy powers - so long as the whole line ran along secure routes. Radio signals, however, went out into the ether and could be intercepted by anyone with an aerial, even hundreds of miles away.
This meant any messages could be intercepted - and deciphered if you had the skill. As the telegraph and radio became increasingly integral to operations of war from the 1860s onwards, so a new branch of military strategy evolved - the art of telecommunications interception and deception.
At the beginning of the 20th century, Britain had no structured intelligence-gathering unit and the government realised that it was dangerously exposed to espionage, or even attack, by enemy forces.
In 1909 the British Secret Service Bureau was established to counteract the threat. When war broke out in 1914, the service established a nerve centre in the Admiralty. Known simply as Room 40, it was responsible for gathering and decoding German messages.
Within no time messages were being intercepted and by the end of the year the team was successfully decoding nearly all significant signals transmitted by the German fleet, Naval Command and the new, rapidly expanding submarine section.
The team also focused on diplomatic signals and achieved notable success uncovering the secrets that persuaded the USA to join the war.
Room 40 continued its work during peacetime but relocated to Bletchley Park at the outbreak of the Second World War.
Bletchley Park was Britain's secret base for the 'Ultra' code-breaking team during the Second World War. Set in rural Buckinghamshire, 50 miles north-west of London, the government set up the site to house the Code and Cipher School during 1938.
Hundreds of the most gifted mathematicians, linguists, crossword experts and chess champions were recruited to Bletchley and worked tirelessly to crack the Germans' ingenious Enigma cipher - used by the German forces' encryption devices. Alan Turing, who dreamt up the concept of modern computing, worked there alongside Max Newman. Newman, with Tommy Flowers and Arnold Lynch worked together to develop the Colossus machine, the forerunner of the modern computer.
The Official Secrets Act cloaked the existence and achievements of Bletchley in secrecy for decades, but in the 1990s a successful campaign resulted in the house being restored as a museum, dedicated to the groundbreaking work of the Enigma team.
Gerry Milsted, joined the Post Office Research Station at Dollis Hill, in the thick of the Second World War in 1943. He remained working with the organisation until 1985, after it had become BT. Gerry was a signals expert with a high aptitude for Morse Code which he could operate at 34 words per minute.
He remembers putting his skills into good use intercepting the Enigma code messages of the German U-boats, which were immediately passed on to Bletchley Park for decryption.
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Colossus was the advanced code-breaking machine built by the Post Office for use at Bletchley Park, under the direction of Tommy Flowers, where it helped the British decode German messages during the Second World War. By the end of the war there were ten machines in operation, at which time it was decided that most of them should be dismantled and destroyed to protect their secrets from falling into unwanted hands.
A replica machine was reconstructed for Bletchley Park museum many years later, from photographs, surviving papers and memory, by some of the people who worked on the original project.
This transformer is one of the few parts of an original Colossus machine, and perhaps the largest, in existence today. It stands about 8 inches tall and is essentially a very heavy, solid lump of coiled wire. It provided the filament voltage for the valves at the heart of the machine. There were vast numbers of valves in each Colossus. Similar transformers in more domestic items - such as radio sets - were much smaller.
