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 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.
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