4/13 DESIGN FOR A BRAIN 



of searchlight, anti-aircraft guns, rockets, and torpedoes, and 

 facilitated by the great advances that had occurred in electronics. 

 As a result, a host of new machines appeared which acted with 

 powers of self-adjustment and correction never before achieved. 

 Some of their main properties will be described in S. 4/14. 



The nature, degree, and polarity of the feedback has a decisive 

 effect on the stability or instability of the system. In the Watt's 

 governor or in the thermostat, for instance, the connection of a 

 part in reversed position, reversing the polarity of action of one 

 component on the next, may, and probably will, turn the system 

 from stable to unstable. In the reaction circuit of the radio set, 

 the stability or instability is determined by the quantitative rela- 

 tion between the two effects. 



Instability in such systems is shown by the development of a 

 ' runaway '. The least disturbance is magnified by its passage 

 round the circuit so that it is incessantly built up into a larger 



4^=^3 3^ 4 



Figure 4/12/1. 



and larger deviation from the resting state. The phenomenon 

 is identical with that referred to as a ' vicious circle '. 



The examples shown have only a simple circuit. But more 

 complex systems may have many interlacing circuits. If, for 

 instance, as in S. 8/8, four variables all act on each other, the 

 diagram of immediate effects would be that shown in Figure 

 4/12/1 (A). It is easy to verify that such a system contains 

 twenty interlaced circuits, two of which are shown at B and C. 



The further development of the theory of systems with feed- 

 back cannot be made without mathematics. But here it is 

 sufficient to note two facts : a system which possesses feedback 

 is usually actively stable or actively unstable ; and whether it is 

 stable or unstable depends on the quantitative details of the 

 particular arrangement. 



4/13. It will be noticed that stability, as denned, in no way 

 implies fixity or rigidity. It is true that stable systems may have 

 a resting state at which they will show no change ; but the lack 



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