12/20 AN INTRODUCTION TO CYBERNETICS 



Systems shows how closely their properties are related to those dis- 

 cussed here. 



Ex. 1 : Draw the diagram of immediate effects of any regulator known to you. 

 Ex. 2: (Continued.) Think of some other parameters whose change would 

 affect the regulator's working ; add them to the diagram. 



12/20. A variant of this class, worth mention for the sake of 

 completeness, is that in which the regulating mechanism becomes 

 active only intermittently. 



A reservoir tank, for instance, may have the level of fluid in it 

 kept between two given levels by a siphon which has its inner opening 

 at the lower level and its bend at the upper level. If the supply is 

 usually greater than the demand, the siphon, by coming into action 

 when the fluid reaches the upper level and by stopping its action 

 when it reaches the lower, will keep the level within the desired range. 



Many physiological regulators act intermittently. The reaction 

 to cold by shivering is such a case. This particular reaction is of 

 special interest to us (compare S.12/4) in that activity in the regulator 

 can be evoked either by an actual fall in the bodily temperature 

 (error-control, from E) or, before the body has had time to cool, 

 by the sight of things that will bring cold (control from D). 



THE POWER AMPLIFIER 



12/21. The fact that the discussion in this chapter has usually 

 referred to the output E as being constant must not be allowed to 

 obscure the fact that this form can cover a very great number of cases 

 that, at first sight, have no element of constancy in them. The 

 subject was referred to in S. 11/15. Here we shall consider an 

 apphcation that is important in many ways already, and that will 

 be needed for reference when we come to Chapter 14. I refer to 

 those regulators and controllers that amplify power. 



Power amplifiers exist in many forms. Here I shall describe only 

 one, selecting a form that is simple and clear (Fig. 12/21/1). 



Compressed air is supplied freely at A and makes its way past the 

 constriction C before either going into the bellows B or escaping 

 at the valve V. The pressure at A is much higher than the usual 

 working pressure in B, and the aperture at C is small, so air flows 

 past C at a fairly constant rate. It must then either escape at V or 

 accumulate in B, driving up the pressure z. How fast the air escapes 

 at V, where a hole is obstructed to some degree by a cone, depends 

 on the movement up or down (x) of the cone, which is attached 



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