Page 813 - Basic Electrical Engineering

simply modulates or changes the magnitude of some external source of
power. For example, a current flowing through a coil placed in a constant
magnetic field produces a torque. The magnitude of the torque can be used as

proportional to the current flowing provided the strength of the magnetic field
is constant.

A thermocouple used to measure temperature is a self-generating-type
instrument system, and falls under active instrumentation systems or simply

active instruments.
The output of a potentiometer having a fixed source of supply can be used

to indicate the level of a liquid and is termed as passive instrumentation
system.

11.7.2 Analog and Digital Instruments

Analog instruments provide output as a function of time, i.e. the output of the
instrument varies continuously as the magnitude of the quantity being

measured changes. A deflection-type instrument where the output is indicated
by a deflecting needle moving over a graduated scale is an analog instrument.
As the input changes, the needle or the pointer moves smoothly and

continuously till the final deflected position is reached. The range of
movement of the pointer can be divided through the scale into infinite

number of divisions. Thus, in an analog instrument the output is a continuous
function of time.
In digital instruments the output varies in discrete steps, and therefore can

have only a finite number of values. A digital revolution counter will count
the number of revolutions in discrete numbers and not as a fraction of a

revolution. Digital instruments and instrumentation system has a number of
advantages over the analog system. Digital instruments measuring the

magnitude of some signals can be transmitted over long distances without
much distortion. The output of a digital instrument can be fed directly into a

digital computer system. An instrument whose output is in digital form can
be directly interfaced with the computer for monitoring and control
operations.