Ζϊ,,,,ι), to be larger. This tends to make the output larger, thereby compensating for t h e initial effect. Thus, the feedback constitutes a built-in correction factor t h a t a u tomatically adjusts for variations in t h e amplifier's characteristics, and maintains an output t h a t is a faithful a n d constant enlargement of the input function.
The
Servomechanism
M a n y modern analytical instru ments arc capable of providing t h e chemist with a wealth of d a t a rapidly and continuously, in t h e main as a consequence of the ex ploitation of t h e servomechanism principle. I n fact, the servomecha nism is largely responsible for t h e embarrassment of riches under which scientists currently labor, since it lias given them the means to acquire data faster t h a n it can be assimilated. An illustration of t h e servomechanism principle is given in Figure 5, which shows, in a very schematic form, the basic features of a self-balancing potentiometer. If the potentiometer is unbalanced, there is an unbalance signal—i.e., a current—flowing between points A a n d B. T h e direction of this current depends upon the direction of the potentiometer unbalance— viz., if the slidewire contactor is above the balance point, t h e cur rent will be in one direction, if be low the balance point, t h e current will be in the opposite direction. This unbalance current is converted to a.c. b y the electromechanical chopper, a n d fed into t h e amplifier. The o u t p u t of the amplifier is a n a.c. voltage t h a t is applied t o one of t h e sets of coils—e.g., t h e stator —of a phase-sensitive, reversible motor. T h e other set of coils (e.g., the rotor) is connected to t h e line a . c , a n d always has a fluctuating magnetic field associated with it. Consequently, t h e rotor will t u r n only if both stator a n d rotor a r e energized b y alternating signals t h a t are out of phase with each other. I n t h e present instance, t h e rotor will turn only if there is an unbalance signal coming from t h e potentiometer. T h e direction in which t h e rotor t u r n s depends
-bem ι UMZU until ify toa
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