A mechanical computer is built from mechanical components such as levers and gears, rather than electronic components. The most common examples are adding machines and mechanical counters, which use the turning of gears to increment output displays. More complex examples could carry out multiplication and division—Friden used a moving head which paused at each column—and even differential analysis. One model[which?] sold in the 1960s calculated square roots.
Mechanical computers reached their zenith during World War II, when they formed the basis of complex bombsights including the Norden, as well as the similar devices for ship computations such as the US Torpedo Data Computer or British Admiralty Fire Control Table. Noteworthy are mechanical flight instruments for early spacecraft, which provided their computed output not in the form of digits, but through the displacements of indicator surfaces. From Yuri Gagarin's first manned spaceflight until 2002, every manned Soviet and Russian spacecraft Vostok, Voskhod and Soyuz was equipped with a Globus instrument showing the apparent movement of the Earth under the spacecraft through the displacement of a miniature terrestrial globe, plus latitude and longitude indicators.
Mechanical computers continued to be used into the 1960s, but were quickly replaced by electronic calculators, which—with cathode-ray tube output—emerged in the mid-1960s. The evolution culminated in the 1970s with the introduction of inexpensive handheld electronic calculators. Mechanical computers were ailing in the 1970s and dead by the 1980s.
Early electrically powered computers constructed from switches and relay logic rather than vacuum tubes (thermionic valves) or transistors (from which later electronic computers were constructed) are classified as electro-mechanical computers. These varied greatly in design and capabilities, with some later units capable of floating point arithmetic. Some relay-based computers remained in service after the development of vacuum-tube computers, where their slower speed was compensated for by good reliability. Some models were built as duplicate processors to detect errors, or could detect errors and retry the instruction. A few models were sold commercially with multiple units produced, but many designs were experimental one-off productions.
|Automatic Relay Computer||UK||1948||The Booths, experimental|||
|Harwell computer||UK||1951||later known as WITCH|
|Harvard Mark I||USA||1944||(IBM Automatic Sequence Controlled Calculator)|
|Harvard Mark II||USA||1947|
|Imperial College Computing Engine (ICCE)||UK||1951||Electro-mechanical|||
|Office of Naval Research ONR Relay Computer||US||1949||6-bit, drum storage, but electro-mechanical relay ALU based on Atlas, formerly Navy cryptology computer ABEL|||
|OPREMA||East Germany||1955||Commercial use at Zeiss Optical in Jena|||
|RVM-1||Soviet Union||1957||Alexander Kronrod|||
|Simon||USA||1950||Hobbyist logic demonstrator magazine article|
|Bell Labs Model I||USA||1940||George Stibitz, "Complex Number Calculator",450 relays and cross bar switches, demonstrated remote access 1940, used until 1948|||
|Bell Labs Model II||USA||1943||"Relay Interpolator", used for wartime work, shut down 1962|||
|Bell Labs Model III||USA||1944||"Ballistic Computer", used until 1949|||
|Bell Labs Model IV||USA||1945||Navy "Mark 22 Error Detector", used until 1961|||
|Bell Labs Model V||USA||1946, 1947||Two units delivered, general purpose, built in trig functions, floating point|||
|Bell Labs Model VI||USA||1950||General purpose,|||
|Unnamed Cryptanalysis Multiplier||UK||1937||Turing|||
|Relay Computer||USA||2006||Harry Porter's Relay Computer, demonstrator/hobby, but integrated circuit memory.|||
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