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 From the mid 1950s onward, vacuum tube computers superseded relay logic.

Overview

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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

• vacuum tube computer

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