When most people use the term "computer" today, they are self-evidently referring to the digital computer. This statement was not always true. University courses on analog computing were still being taught in the 1970s. 

Analog computers are mechanical or electronic devices that operate by simulating the system they are being used to analyze or model. Probably the oldest analog computer is the hourglass. The grains of sand in an hourglass represent time; as time passes, the sand flows from one half of the glass into the other half through a small constriction. An hourglass is programmed by selecting the grade and quantity of the sand and the size of the constriction. The clock is another analog computer where the motion of the hands simulates the passage of time. Similarly, a mercury thermometer is an analog computer that represents temperature by the length of a column of mercury. 

An electronic analog computer represents a variable (e.g., time or distance) by an electrical quantity and then models the system being analyzed. For example, in order to analyze the trajectory of a shell fired from a field gun, you would construct a circuit using electronic components that mimic the effect of gravity and air resistance etc. The analog computer might be triggered by applying a voltage step and then the height and distance of the shell would be given by two voltages that change with time. The output of such an analog computer might be a trace on a CRT screen. Analog computers lack the accuracy of digital computers because their precision is determined by the nature of the components and the ability to measure voltages and currents. Analog computer are suited to the solution of scientific and engineering problems such as the calculation of the stress on a beam in a bridge, rather than, for example, financial or database problems. 

Although popular mythology regards the computer industry as having its origins in World War II, it is clear that engineers were thinking about computing machines within a few decades of the first practical applications of electricity. As early as 1872 the Society of Telegraph Engineers held their Fourth Ordinary General Meeting in London to discuss "Electrical calculations". 

Vannavar Bush is normally regarded as the father of the analog computer, although in 1876 the British mathematician Lord Kelvin devised a mechanical analog computer that could be used to predict the behavior of tides. Bush developed his electro-mechanical differential analyzer at MIT in the early 1930s. The Differential Analyzer was based on the use of interconnected mechanical integrators, torque amplifiers, drive belts, shafts, and gears. This 100-ton analog machine could solve equations with as many as 18 variables and was used to perform calculations in atomic physics and ballistics.

 Bush's Differential Analyzer

 The key to Bush's machine was his disk-and-wheel integrator that performed the operation of integration by mechanical means. When the horizontal disk in the figure below rotates, the smaller wheel resting on it also rotates because of the friction between the wheels. The ratio between the speeds at which the two wheels rotate is determined by the ratio of the diameter of the smaller wheel to the circumference of its track on the rotating disk. By moving the small wheel in or out, this ratio can be made a function of a variable (i.e., the radius). 

Disk-and-wheel integrator

In 1945 Bush wrote an essay in Atlantic Monthly proposing an information system he called Memex that would act as "…a personalized storehouse for books, records, correspondence, receipts…and employ a sophisticated indexing system to retrieve facts on demand." Such a system is not that far removed from today's World Wide Web. 

Later analog computers used electronic integrators and were used to simulate complex dynamic systems well into the 1970s. In many ways the electric organ was a very sophisticated analog computer that used analog techniques to model the processes used by real musical instruments to create sound. Modern electric organs now employ digital techniques.