A mechanical computer is a computer 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, the Ascota 170 accounting machine 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 spaceflight until 2002, every crewed 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 had steadily been losing ground to digital computers since their advent. By the mid-1960s dedicated electronic calculators with cathode-ray tube output emerged. The next step in the evolution occurred in the 1970s, with the introduction of inexpensive handheld electronic calculators. The use of mechanical computers declined in the 1970s and was rare by the 1980s. In 2016, NASA announced that its Automaton Rover for Extreme Environments program would use a mechanical computer to operate in the harsh environmental conditions found on Venus.

Examples

Punch card data processing

Starting at the end of the nineteenth century, well before the advent of electronic computers, data processing was performed using electromechanical machines collectively referred to as unit record equipment, electric accounting machines (EAM) or tabulating machines. By 1887, Herman Hollerith had worked out the basis for a mechanical system of recording, compiling and tabulating census facts. "Unit record" data processing equipment uses punchcards to carry information on a one-item-per-card basis. Unit record machines came to be as ubiquitous in industry and government in the first two-thirds of the twentieth century as computers became in the last third. They allowed large volume, sophisticated data-processing tasks to be accomplished before electronic computers were invented and while they were still in their infancy. This data processing was accomplished by processing punched cards through various unit record machines in a carefully choreographed progression. Data on the cards could be added, subtracted and compared with other data and, later, multiplied as well. This progression, or flow, from machine to machine was often planned and documented with detailed flowcharts. All but the earliest machines had high-speed mechanical feeders to process cards at rates from around 100 to 2,000 per minute, sensing punched holes with mechanical, electrical, or, later, optical sensors. The operation of many machines was directed by the use of a removable plugboard, control panel, or connection box.

Electro-mechanical computers

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 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.