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Relay Computer

The Harvard Mark I, a relay-based computer, one of the earliest, made by Howard Aiken with the help of IBM in 1944.

A relay computer is a computer system built primarily using electromechanical relays and relay logic. Obsoleting most of the earlier mechanical computers, relay were a short-lived technology during the late 1930s to early 1950s. By the late 1950s technology has shifted almost exclusively to vacuum tubes. From the mid 1950s onward, vacuum tube computers superseded relay logic.


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The idea of using relays to realize logic circuits was not new even in the 1920s. However it was not until the late 1930s that actual full-scale computers and calculators were developed. It was not until the end of World War II that a large number of systems were built.

The choice of using relays instead of vacuum tube boiled down to cost and availability. Standard telephone relays were already in wide use and were considerably cheaper than any other alternative, albeit slow and power hungry. Since most of the early relay computers were developed by independent individuals such as George Stibitz and Konrad Zuse and without a large budget, using relays made the most sense.

Relay Systems

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System Developer Company/Institution Relay Count Year Complete Notes
Model K George Stibitz Bell Labs 1937
Model I George Stibitz Bell Labs 400 1939
Z2 Konrad Zuse 600 1939
Z3 Konrad Zuse 2,000 1941
Model II Bell Labs 440 1943
Z4 Konrad Zuse Zuse Apparatebau 2,500 1944
Model III Bell Labs 1,400 1944
Harvard Mark I Howard Aiken IBM 3,500 1944
Model IV Bell Labs 1400 1945
Model V Bell Labs 9,000 1946
Harvard Mark II Howard Aiken 1947
SSEC IBM 21,400 1948 vacuum tubes & relays hybrid
Baby Atlas CSAW 1949 Used for the debugging of the Atlas computer
Model VI Bell Labs 1950
BARK Matematikmaskinnämnden 8,000 1950
Harwell Computer (WITCH) Harwell 480 1951 Made with 480 relays, 828 Dekatron valves for math
Mark I Electrotechnical Laboratory 1952
Z5 Konrad Zuse Zuse KG 2,500 1953
FACOM 100 Fujitsu 1954
Z11 Konrad Zuse Zuse KG 1,665 1955
Mark II Electrotechnical Laboratory 22,253 1955

See also

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