THE ERROR-CONTROLLED REGULATOR 12/19 



The diagram of immediate effects is specially worth noting. It is 



or some equivalent form. In it, D, T, R and E are readily identi- 

 fied (though the distinctions between T and R and their parts are 

 somewhat arbitrary). The whole acts to block the passage of 

 variety from the Disturbances (whatever they are) to the eggs. If 

 the aim of the regulator is re-defined slightly as being to keep 

 constant the temperature of the incubator, then the regulator is 

 controlled by the error rather than by the disturbances themselves. 



In this form of regulator, the system must, of course, be stable 

 for any given disturbance, and the desired temperature must be the 

 system's state of equilibrium. The feedback around the circuit 

 must thus usually be negative. 



Many regulators in the living body are, of this simple form, and 

 Cannon's work has made them well known. Typical is that which 

 regulates the pR of the blood by the amount of carbon dioxide in it: 



Disturbances 



pH of the 

 blood 



pH of body's 

 tissues 



\ 



Concentration of 

 COt in blood 



Activity of 

 respiration 



i^ 



Rate of Ex- 

 cretion of CO2 



Again the system shows the features just mentioned. 



Among the innumerable examples of such mechanisms should 

 be included the economic. Tustin's Mechanism of Economic 



237 



