The Homeostat is one of the first devices capable of adapting itself to the environment; it exhibited behaviours such as habituation, reinforcement and learning through its ability to maintain homeostasis in a changing environment. It was built by William Ross Ashby in 1948 at Barnwood House Hospital. It was an adaptive ultrastable system, consisting of four interconnected Royal Air Force bomb control units with inputs, feedback, and magnetically driven, water-filled potentiometers. It illustrated his law of requisite variety — automatically adapting its configuration to stabilize the effects of any disturbances introduced into the system.
In 1946, Ashby described the design of the units thus "Its principle is that it uses multiple coils in a milliammeter & uses the needle movement to dip in a trough carrying a current, so getting a potential which goes to the grid of a valve, the anode of which provides an output current." It was the realization of what he had described in 1946 as an "Isomorphism making machine".
When Alan Turing heard of Ashby's intention to build the Homeostat, he wrote to Ashby to suggest that he could run a simulation on Turing's Automatic Computing Engine (ACE) instead of building a special machine.
In 1949 Time described it as "the closest thing to a synthetic brain so far designed by man".
In 1952, Ashby demonstrated it at the ninth Macy conference on cybernetics. In the same year he published a description of the Homeostat in his influential book Design for a brain. In total, between 1946 and 1967, he wrote 38 entries about the Homeostat in his journal.
- Homeostat description and circuit diagram on pages 2431-2 of Ashby's journal, The W. Ross Ashby Digital Archive.
- The Homeostat as embodiment of adaptive control, Peter A. Cariani.
- Description of the Homeostat's unit from Ashby's journal page 2094 The W. Ross Ashby Digital Archive.
- Biography of W. Ross Ashby: The Homeostat and newspaper articles about it, by Jill Ashby, The W. Ross Ashby Digital Archive.
- Alan Turing letter The W. Ross Ashby Digital Archive.
- The thinking machine, Time, 24 January 1949.
- Decisions and noise: the scope of evolutionary synthesis and dynamical analysis, Ezequiel A. Di Paolo and Inman Harvey.
- Ashby's journal references to the Homeostat The W. Ross Ashby Digital Archive.
- Homeostat in the Principia Cybernetica Web.
- "The Homeostat simulation". Archived from the original on 2012-08-02. by Dr Horace Townsend.
- The W. Ross Ashby Digital Archive includes a biography, bibliography, letters, photographs, movies, and fully indexed images of all 7,400 pages of Ashby's 25 volume journal.
- W. Ross Ashby (1956): An Introduction to Cybernetics, (Chapman & Hall, London): available as a PDF, Principia Cybernetica Web, 1999