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The Mercury was an early 1950s commercial computer built by Ferranti. It was a development of the famous Manchester Mark I, adding a floating point unit for improved performance, and increased reliability by replacing the Williams tube memory with core memory and using more solid state components. Nineteen Mercuries were sold in total before Ferranti moved on to newer designs.

When the Mark I started running in 1951 reliability was poor. The primary concern was the drum memory system, which proved to break down all the time. Additionally, the machine used 4,200 vacuum tubes that had to be replaced constantly. The Williams tubes, used as random access memory and registers, were reliable but required constant maintenance. As soon as the system went into operation, teams started looking at solutions to these problems.

One team decided to produce a much smaller and more cost effective system built entirely of transistors. It first ran in November 1953 and is believed to be the first entirely transistor based machine. Metropolitan-Vickers later built this commercially as the Metrovick 950, delivering seven.

Another team, including the main designers of the Mark I, started with a design very similar to the Mark I but replacing any tubes used as diodes with solid state diodes. After watching use of the Mark I they also realized that the users of computers were almost all in the sciences, and they decided to add a floating point unit to greatly improve performance in this role. Additionally the machine was to run at 1 MHz, eight times faster than the Mark I's 125 kHz, leading to the design being referring to as the megacycle machine, and eventually Meg.

Meg first ran in May 1954. The use of solid state diodes reduced tube count by well over half, reducing the power requirements an equal amount from the Mark I's 25 kW to the Meg's 12 kW. Like the Mark I, Meg was based on a 10-bit "short word", combining two to form a 20-bit address and four to make a 40-bit integer. This was a result of the physical dimensions of the Williams tubes, which were used to make eight B-lines, or in modern terminology, accumulator/index registers. Meg could multiply two integers in about 60 microseconds. The floating point unit used three words for a 30-bit mantissa, and another as a 10-bit exponent. It could add two FP numbers in about 180 microseconds, and multiply them in about 360 µsec.

Ferranti, which had built the Mark I for the University, continued development of the Meg to produce the Mercury. The main change was to replace the Williams tubes with core memory. Although slower to access, at about 10 µsec for a 10-bit short word, the system required virtually no maintenance, considerably more important for commercial users. 1024×40-bits of core were provided, backed by four drums each holding 4096×40-bits.

The first of an eventual nineteen Mercury computers was delivered in August 1957. Manchester received one in February 1958, leasing half the time to commercial users via Ferranti's business unit.

The machine could run a high-level language, Mercury Autocode.

• MEG/Mercury