Homeostat
Introduction
The homeostat is an electromechanical device built by the psychiatrist and cybernetician W. Ross Ashby in 1948 to demonstrate that a machine could find its own stable state without being told what that state was. Ashby described it in Design for a Brain: The Origin of Adaptive Behaviour (Chapman and Hall, 1952) and demonstrated it to the Macy Conferences on cybernetics.
The machine consisted of four identical units. Each carried a pivoted magnet with a wire dipping into a trough of water, so that the position of the magnet set an electrical output, and each unit's output fed the inputs of the other three. The four units therefore formed a single coupled system with no separation between controller and controlled.
Ultrastability
If a magnet was pushed outside its acceptable range, a uniselector, a stepping switch, advanced to a new randomly determined set of parameter values. The system then either settled or, if it did not, stepped again. It searched its own configuration space until it found a set of internal parameters under which all four units returned to equilibrium.
Ashby called this property ultrastability: stability not of a state but of the search for a state. Each uniselector carried twenty-five positions, so four units gave 25 to the fourth power, or 390,625 possible configurations, and the machine explored them until one worked.
A homeostat does not need to:
- Be given a set point
- Be told which disturbance occurred
- Be reprogrammed after a change
- Report a fault to an operator
- The goal is not specified. Equilibrium is defined by the units' own tolerances, not by an external target.
- Adaptation is structural. The machine changes its own parameters rather than its outputs.
- Failure is survivable. An unsuccessful configuration is simply followed by another attempt.
Afterward
Ashby's later An Introduction to Cybernetics (1956) generalised the argument as the Law of Requisite Variety, which holds that a regulator can only absorb as much variety as it can itself produce. A system facing more distinct disturbances than it has distinct responses will fail on the surplus, and the surplus is what reaches the person using it.
The homeostat sits alongside Grey Walter's tortoises of the same period as an early demonstration that adaptive behaviour could be built rather than programmed. Its descendants include adaptive and self-tuning control, and the general design position that a device should resolve its own disturbances rather than escalate them.
Just as a machine can search for its own equilibrium without being told where it is, a well-designed system absorbs a change in conditions instead of passing it to a person.
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