The LEO I – Lyons Electronic Office I – was the first computer used for commercial business applications.  J. Lyons and Co., one of the UK’s leading catering and food manufacturing companies in the first half of the 20th century, constructed this machine, based upon the pioneering University of Cambridge EDSAC computer.

LEO III GPO, Charles House

LEO I’s clock speed was 500 Hz, with most instructions taking about 1.5 ms to execute. To be useful for business applications, the computer had to be able to handle a number of data streams, input and output, simultaneously. Therefore the machine had to have multiple input/output buffers. In the first instance, these were linked to paper tape readers/punches and eventually to a fast punched card reader and punches, and a 100 line a minute tabulator. Later, other devices, including magnetic tape, were added. Its ultrasonic delay line memory based on tanks of mercury, with 2K (2048) 35-bit words (8¾ K bytes – compare that with  today’s laptops which  commonly have 6G Bytes), was four times as large as that of EDSAC.  The first test programs were run in September 1951.

The machine gradually evolved and the first LEO III was completed in 1961. This was a solid-state machine with a ferrite core memory. It was micro-programmed and was controlled by a multi-tasking operating system.

 In 1963 LEO Computers Ltd was merged into English Electric Company and this led to the breaking up of the team that had inspired LEO computers. English Electric Company continued to build the LEO III, and went on to build the faster LEO 360 and even faster LEO 326 models, which had been designed by the LEO team before the takeover.
All LEO IIIs allowed concurrent running of as many as 12 application programs through the Master program operating system. Some were still in commercial use in British Telecom until 1981. Many users fondly remember the LEO III and enthuse about some of its quirkier features, such as having a loudspeaker connected to the central processor which enabled operators to tell if a program was looping by the distinctive sound it made. [ Update: a colleague has reminded me  that operators used to write little routines to pass the time on night shift to make the LEO sing songs. He  remembers  Happy Birthday 🙂 ]
Software was written in Intercode (assembly level) or CLEO ( a COBOL equivalent).