page_title: Mechanical computer
section_title: Electro-mechanical computers
hierarchical_section_title: Mechanical computer / Electro-mechanical computers
caption_attribution_description: English: The WITCH (Harwell Dekatron) computer under restoration at the British National Museum of Computing, Bletchley Park, 2010-03-13. Deutsch: Der Harwell Computer (später Wolverhampton Instrument for Teaching Computing from Harwell (WITCH)) während seiner Restaurierung im British National Museum of Computing
context_page_description: 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 sold in the 1960s calculated square roots.
  Mechanical computers can be either analog, using smooth mechanisms such as curved plates or slide rules for computations; or digital, which use gears.
  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.
context_section_description: 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.
caption: Harwell Dekatron
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