8/8 DESIGN FOR A BRAIN 



the operator reversed the polarity of the output-input junction, 

 making the system unstable (S. 20/7). As a result, a runaway 

 developed, and the magnet passed the critical state (shown by 

 the dotted line). As a result the uniselector changed value. As 

 it happened, the first new value provided a field which was 

 stable, so the magnet returned to its central position. At D 2 , 

 a displacement showed that the system was now stable (though 

 the return after R x demonstrated it too). 



At R 2 the polarity of the join was reversed again. The value 

 on the uniselector was now no longer suitable, the field was 

 unstable, and a runaway occurred. This time three uniselector 

 positions provided three fields which were all unstable : all were 



Time 



Figure 8/8/5 : Two units (1 and 2) interacting. (Details as in Fig. 8/8/4.) 



rejected. But the fourth was stable, the magnet returned to 

 the centre, no further uniselector changes occurred, and the 

 single main variable had a stable field. At D 3 its stability was 

 again demonstrated. 



Figure 8/8/5 shows another experiment, this time with two 

 units interacting. The diagram of immediate effects was 1 <± 2 ; 

 the effect 1 — >■ 2 was hand-controlled, and 2 — > 1 was uniselector- 

 controlled. At first the step-function values combined to give 

 stability, shown by the responses to D v At R v reversal of the 

 commutator by hand rendered the system unstable, a runaway 

 occurred, and the variables transgressed the critical states. The 

 uniselector in Unit 1 changed position and, as it happened, gave 

 at its first trial a stable field. It will be noticed that whereas 

 before R x the upstroke of D 1 in 2 caused an upstroke in 1, it 



98 



