BT has launched a gallery of photos in telecommunications, showing the history of the industry when it started 150 years ago.
The Monarch 120b Compact Call Connect System, a small company exchange introduced by BT in December 1982. Based around the recently invented microprocessor, this was one of the first systems to bring standard computer techniques to low-cost telephony.
Colossus, the first reprogrammable electronic computer, was used to break German wartime codes and was built largely out of GPO parts at its Dollis Hill research centre in north London. After the war, the computers were broken down and the parts put back into general stores. With the last electro-mechanical telephone exchange retired in 1995, there's a chance you've had a phone conversation through a component that was once in this top-secret wartime codebreaker.
Jane Cain was the original voice of the Speaking Clock, pictured here in 1936. Her impeccable tones were recorded and played back optically on glass disks. Any mechanical contact in the playback system such as an electromagnetic head or phonographic needle would have generated far too much wear in the constantly-rotating system.
More sideboard than comms cabinet, this early trunk switchboard from Munich around 1900 connected callers to each other within an office or to the world outside.
This is a Hughes type-printing telegraph from the 1890s — quite a late model of this technology, which was invented in the mid-19th century. The piano-style keyboard was easier to operate than contemporary alternatives. Not commonly used within Britain, the GPO used it mostly for communicating with Europe.
Pneumatic messaging systems, such as this 1939 model, transferred the physical message — and sometimes small items such as money — by blowing it through a network of tubes in a snugly-fitting cylinder.
Videoconferencing has been the near-future of communications since the 1930s. This is a snapshot from a truly remarkable GPO promotional film made in 1969 called Connected Earth - how communication shapes the world.
The Lorimer automatic exchange, a Canadian system which dispensed with the need for telephone girls, is shown on trial at Hereford, England, in 1914. Although there were many automatic exchange systems, the winner was Strowger. The system was famously designed by an undertaker, Almon Brown Strowger, in reaction to his rival's wife intercepting calls to Strowger routed via the manual exchange where she worked.