The benefit of adding high-gain feedback to many measurement systems is illustrated by considering the case of the voltage-measuring instrument whose block diagram is shown in Figure 3.3.
In this system, the unknown voltage Ei is applied to a motor of torque constant Km, and the induced torque turns a pointer against the restraining action of a spring with spring constant Ks.
The effect of environmental inputs on the motor and spring constants is represented by variables Dm and Ds.
In the absence of environmental inputs, the displacement of the pointer X0 is given by: X0 = KmKsEi.