Analog computer is a device that solves problems by working directly with a physical quantity, such as weight, voltage, or speed, rather than with digits that represent the quantity. The computers solve problems by measuring one quantity in terms of another quantity. In a problem involving water pressure and water flow, for example, electrical voltage might serve as an analogue (likeness) for the water pressure, and electric current for the water flow. Many familiar devices, including speedometers, thermometers, and thermostats, operate on the same basic principle as analog computers. For example, a thermometer measures temperature in terms of the length of a thin line of liquid in a tube. An analog computer presents output data in a continuous form, often as a position on a scale. In some cases, the data are displayed as electrical signals on an instrument called an oscilloscope.
Analog computers are not as accurate as digital computers mainly because they work with continuous, varying quantities that cannot be measured exactly. However, they can solve certain types of problems faster than digital computers. Analog computers also may be more convenient to use. They do not require detailed programs. An operator “tells” an analog computer how to solve a problem by simply connecting its electronic circuits or mechanical parts in a particular way.
Most analog computers are special-purpose computers. They solve engineering and scientific problems that can be described as systems of differential equations. These mathematical equations are expressions of natural laws that describe the rates of change of quantities. Electronic analog computers are especially well suited for designing and analysing electrical networks. They were used for the first simulators for aeroplanes, space vehicles, and oceangoing ships. These simulators reproduce the conditions under which such craft must operate. They are used either to check the performance of a craft or to train its crew in operating procedures.
Analog computers once performed much of the world’s calculation. During World War II (1939-1945), they were used to calculate the paths of bombs and bullets. The development of digital computers since the war has limited the use of analog computers. Hybrid computers were introduced during the 1960’s. They combine the features of analog and digital computers. These computers are faster than digital computers in solving certain problems involving differential equations. They are especially suited for studying problems in which researchers can alter parameters and observe the results directly.