Essentially an analog computer must be able to accept inputs
which vary with respect to time, and directly apply these inputs to
various devices within the computer which performs the computing
operations of additions, subtraction, multiplication, division,
integration and function generation. ... The computing units of
analog computers are able to respond immediately to the changes which
they detect in the input variables, and the connexion of these various
units in a particular fashion can cause an analog computer to perform
very complex arithmetic functions at high speed while the actual
process under study is in operation. ...
Analog computers do not have the ability of digital computers to store
data in large quantities, nor do they have the comprehensive logical
facilities afforded by programming digital machines. And although the
arithmetic functions performed by the computing units are more complex
in analog machines than in the digital systems, the cost of the
hardware required to provide a high degree of accuracy in an analog
machine is often prohibitive.
Some analog machines are designed for specific applications, but most
electrical and electronic analog computers provide a number of
different computing devices which can be connected together via a
plugboard to provide different methods of operation for specified
problems.