STABILITY 4/12 



for if it is controlled by a capsule which by its swelling moves a 

 lever which controls the flow of gas to the heating flame, the 

 diagram of immediate effects would be : 



The reader should verify that each arrow represents a physical 

 action which can be demonstrated if all variables other than the 

 pair are kept constant. 



Another example is provided by ' reaction ' in a radio receiver. 

 We can represent the action by two variables linked in two ways : 



The lower arrow represents the grid-potential's effect within the 

 valve on the anode-current. The upper arrow represents some 

 arrangement of the circuit by which fluctuation in the anode- 

 potential affects the grid-potential. The effect represented by 

 the lower arrow is determined by the valve- designer, that of the 

 upper by the circuit-designer. 



Such systems whose variables affect one another in a circuit 

 possess what the radio-engineer calls ' feedback ' ; they are also 

 sometimes described as ' servo-mechanisms '. They are at least 

 as old as the Watt's governor and may be older. But only during 

 the last decade has it been realised that the possession of feedback 

 gives a machine potentialities that are not available to a machine 

 lacking it. The development occurred mainly during the last 

 war, stimulated by the demand for automatic methods of control 



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