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a
b
c
Figure 4.4. the homeostat: a , four interconnected homeostats; b , detail of the
top of a homeostat unit, showing the rotating needle; c , circuit diagram. source:
w. r. ashby, “design for a brain,” Electronic Engineering, 20 (december 1948), 380,
figs. 1, 2. (with kind permission from springer science and business media.)
setting, the input current traveled to the magnet coil through a commuta-
tor, X , which reversed the polarity of the input according to its setting, and
through a potentiometer, P , which scaled the current according to its setting.
The settings for P and X were fixed by hand, using the upper and middle set
of knobs on the front of the homeostat in figure 4.4a. More interesting, the
switch S could also be set to route the input current through a “uniselector”
or “stepping switch”— U in figure 4.4c. Each of these uniselectors had twenty-
five positions, and each position inserted a specific resistor into the input
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