4/19 STABILITY 



(b) Two systems, both unstable, may join to form a whole 

 which is stable. 



(c) Two systems may form a stable whole if joined in one w r ay, 

 and may form an unstable whole if joined in another way. 



(d) In a stable system the effect of fixing a variable may be to 

 render the remainder unstable. 



Such examples could be multiplied almost indefinitely. They 

 illustrate the rule that the stability (or instability) of a dynamic 

 system depends on the parts and their interrelations as a whole. 



4/19. The fact that the stability of a system is a property of the 

 system as a whole is related to the fact that the presence of stability 

 always implies some co-ordination of the actions between the parts. 

 In the thermostat the necessity for co-ordination is clear, for if 

 the components were assembled at random there would be only 

 an even chance that the assembly would be stable. But as the 

 system and the feedbacks become more complex, so does the 

 achievement of stability become more difficult and the likelihood 

 of instability greater. Radio engineers know only too well how 

 readily complex systems with feedback become unstable, and how 

 difficult is the discovery of just that combination of parts and 

 linkages which will give stability. 



The subject is discussed more fully in S. 20/10: here it is 

 sufficient to note that as the number of variables increases so 

 usually do the effects of variable on variable have to be co- 

 ordinated with more and more care if stability is to be achieved. 



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