The first pre-computers (large calculators) of the 1930's by George Stibitz (Bell Laboratories) and Howard Aiken (Harvard University) were built using mechanical switches and relays. Switches can easily be used in AND and OR circuits to create a current to a particular element such as a light. In these examples an "open" switch is equivalent to False and "closed" to True. (Click on a switch to change its position.)
A NOT circuit simply reverses the meanings of "open" and "closed". This is easiest demonstrated with a pair of mechanically linked switches where the input (leftmost) switch, which is not actually in the circuit, causes the second switch to change position in the opposite manner.
What is the "problem" with this circuit?
A combination of circuits each uses the previous one (that closest to the power source) as its power source. However, "multi-throw" switches are needed if the value of a variable is an element of several circuits.
Within these early machines a second level of switching was used that could also operate as a memory - the mechanical relay.
Last updated 2002/02/21
© J.A.N. Lee, 2000-2002.