ROSS ASHBY - The Cybernetic Brain - page 104

circuit, with the different values of the twenty-five resistances being “deliber-
ately randomised, the actual numerical values being taken from a published
table of random numbers” (Ashby 1948, 381). Unlike the potentiometers and
commutators, these uniselectors were not set by hand. They were controlled
instead by the internal behavior of the homeostat. When the output current
of the unit rose beyond some preset limit, relay F in figure 4.4c would close,
driving the uniselector (via the coil marked G ) to its next setting, thus replac-
ing the resistor in the input circuit by another randomly related to it.
So what? The first point to bear in mind is that any single homeostat unit
was quite inert: it did nothing by itself. On the other hand, when two or more
units were interconnected, dynamic feedback interrelations were set up be-
tween them, as the outputs of each unit fed as input to the others and thence
returned, transformed, as input to the first, on and on, endlessly around the
loop. And to get to grips with the behavior of the whole ensemble it helps to
specialize the discussion a bit. Consider a four-homeostat setup as shown in
figure 4.4a, and suppose that for one of the units—call it homeostat 1—the
switch S brings a uniselector into the input circuit, while for the three remain-
ing homeostats the switches S are set to route the input currents through the
manually set potentiometers and commutators. These latter three, then, have
fixed properties, while the properties of homeostat 1 vary with its uniselector
setting.
When this combination is switched on, homeostat 1 can find itself in one
of two conditions. It might be, as Ashby would say, in a condition of stable
equilibrium, meaning that the vane on top of the unit would come to rest
in the middle of its range, corresponding by design to zero electrical output
from the unit, and return there whenever any of the vanes on any of the units
was given a small push. Or the unit might be unstable , meaning that its vane
would be driven toward the limits of its range. In that event, the key bit of
the homeostat's circuitry would come into play. As the electrical output of the
unit increased above some preset value, the relay would close and drive the
uniselector to its next position. This, in effect, would change the electrical
properties of homeostat 1, and then we can see how it goes. The unit might
again find itself in one of two conditions, either stable or unstable. If the lat-
ter, the relay would again drive the uniselector to its next position, inserting
a new resistance in the circuit, and so on and so on, until homeostat 1 found
a condition of stable equilibrium in which its vane gravitated to the center of
its range.
This is the key point about the homeostat: it was a real ultrastable machine
of the kind that Ashby had only imagined back in 1941. The uniselectors took