An analogue computer uses continuously variable voltages rather than limiting itself to 0 and 1, with the magnitude proportional to the real quantity. This can allow problems to be solved with a minimum of hardware- dividing by two just requires a potential divider, whereas making even the simplest digital circuit capable of division is a major undertaking. The downside is that accuracy is distinctly limited- in a digital computer you use variables of whatever length you like, given suitable software, but in the analogue domain you are dependent on the accuracy of the electrical components and readout devices.
Note that the PACE computer below uses a component oven and a digital voltmeter to achieve the required accuracy. Neither was a cheap option.
Left: A PACE 16-231R analogue computer. Photograph published in 1961.
SOLVING A DIFFERENTIAL EQUATION WITH AN ANALOGUE COMPUTER
In some ways this is simple and intuitive- if of limited accuracy- but it can get hellishly complicated. This is a very simple example- a first-order differential equation.
The diagram below shows the simulation of a mechanical shock-absorber or damper.
COMPUTERS & MANUFACTURERS
An incomplete list:
Berkeley Division of Beckman Instruments.
Goodyear Aircraft Corp
Electronic Associates Inc
Reeves Instrument Corp
The Design & Use of Electronic Analogue Computers.
C P Gilbert
& Hall 1964
Design Fundamentals of Analog Computer Components
R M Howe
Van Nostrand 1961