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In November 1951 J. Lyons & Co. Ltd. received the "LEO I" computer (Lyons Electronic Office I), modeled closely on the earlier EDSAC, which the company had partly funded. The clock speed was 500 kHz, with most instructions taking about 1500 ms to execute, constrained by its paper tape I/O. Its ultrasonic delay line memory, based on tanks of mercury, was 4 times as large as EDSAC, with 1024 35-bit words.

The Lyons company used LEO I intially for valuation jobs, but its role was extended to include payroll, inventory, and so on; it was the first computer used for commercial calculations.

In 1954 with the decision to proceed with LEO II and interest from other commercial companies, Lyons formed LEO Computers Ltd. Leo Computers went on to become part of ICT eventually becoming ICL.