ROSS ASHBY - The Cybernetic Brain - page 101

cated model for the adaptive and performative brain than anyone else at
that time.
The homeostat
The bead-and-elastic machine just discussed was imaginary, but on 19 No-
vember 1946 Ashby began a long journal entry with the words “I have been
trying to develope [ sic ] further principles for my machine to illustrate stabil-
ity, & to develope ultrastability.” There followed eight pages of notes, logic
diagrams and circuit diagrams for the machine that he subsequently called the
homeostat and that made him famous. The next entry was dated 25 November
1946 and began: “Started my first experiment! How I hate them! Started by
making a Unit of a very unsatisfactory type, merely to make a start.” 12 He then
proceeded to work his way through a series of possible designs, and the first
working homeostat was publicly demonstrated at Barnwood House in May
1947; a further variant was demonstrated at a meeting of the Electroencepha-
lographic Society at the Burden Neurological Institute in May 1948. 13 This
machine became the centerpiece of Ashby's cybernetics for the next few years.
His first published account of the homeostat appeared in the December 1948
issue of the journal Electronic Engineering under the memorable title “Design
for a Brain,” and the same machine went on to feature in the topic of the same
name in 1952. I therefore want to spend some time discussing it.
The homeostat was a somewhat baroque electromechanical device, but I
will try to bring out its key features. Figure 4.4a in fact shows four identical
homeostat units which are all electrically connected to one another. The in-
terconnections cannot be seen in the photograph, but they are indicated in
the circuit diagram of a single unit, figure 4.4c, where it is shown that each
unit was a device that converted electrical inputs (from other units, on the left
of the diagram, plus itself, at the bottom) into electrical outputs (on the right).
Ashby understood these currents as the homeostat's essential variables, elec-
trical analogues of blood temperature or acidity or whatever, which it sought
to keep within bounds—hence its name—in a way that I can now describe.
The inputs to each unit were fed into a set of coils ( A , B , C , D ), produc-
ing a magnetic field which caused a bar magnet ( M ) to pivot about a vertical
axis. Figure 4.4b is a detail of the top of a homeostat, and shows the coils as
a flattened oval within a Perspex housing, with the right-hand end of the bar
magnet just protruding from them into the light. Attached to the magnet and
rotating with it was a metal vane—the uppermost element in figures 4.4b
and 4.4c—which was bent at the tip so as to dip into a trough of water—the