LEO III completed

April 1961
LEO III completed

The first LEO III was completed in 1961, becoming available to test in May 1961 and finally installed in the LEO bureau at Hartree House in May 1962.

LEO III was a solid-state machine, using transistors with a magnetic core memory. It was a parallel machine with 40 bit adders. Programs were loaded from magnetic tapes. LEO III allowed as many as 12 application programs to run concurrently through the Master program operating system. 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.

In 1963 LEO Computers Ltd was merged into English Electric Company. English Electric Company continued to build and sell the LEO III until 1967. Some were still in commercial use in British Telecom until 1981.

Related information:


  • The first LEO III bureau machine at Hartree House, April 1962.
    Credit: Anthony Blake, The Centre for Computing History item CH55714.

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LEO III completed

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