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

A relay computer is a computer system built primarily using relays and 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. However, 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. It's for those exact reasons that the earliest digital computers went with relays.

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	IBM		1947
SSEC		IBM	21,400	1948	vacuum tubes & relays hybrid
Model VI		Bell Labs		1950
BARK		Matematikmaskinnämnden	8,000	1950
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

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