It is possible to construct non-electronic digital mechanisms. In
principle, any technology capable of representing discrete states and
representing logic operations could be used to build mechanical logic.
MIT students Erlyne Gee, Edward Hardebeck, Danny Hillis (co-author of
The Connection Machine), Margaret Minsky and brothers Barry and Brian
Silverman, built two working computers from Tinker toys, string, a
brick, and a sharpened pencil. The Tinkertoy computer is supposed to
be in the Houston Museum of Natural Science.
Hydraulic, pneumatic and mechanical versions of logic gates exist and
are used in situations where electricity cannot be used. The first two
types are considered under the heading offluidics. One application of
fluidic logic is in military hardware that is likely to be exposed to
a nuclear electromagnetic pulse (nuclear EMP, or NEMP) that would
destroy electrical circuits.
Mechanical logic is frequently used in inexpensive controllers, such
as those in washing machines. Famously, the first computer design, by
Charles Babbage, was designed to use mechanical logic. Mechanical
logic might also be used in very small computers that could be built
by nanotechnology.
Another example is that if two particular enzymes are required to
prevent the construction of a particular protein, this is the
equivalent of a biological "NAND" gate.
