Introduction to Measurements

Measurements are the basic means of acquiring knowledge about the parameters and variables involved
in the operation of a physical system. Measurement generally involves using an instrument as a physical
means of determining a quantity or variable. An instrument or a measuring instrument is, therefore,
defined as a device for determining the value or magnitude of a quantity or variable. The electrical
measuring insturment, as its name implies, is based on electrical principles for its measurement function.
These days a number of measuring instruments, both analog as well as digital ones, are available
for the measurement of electrical quantities like voltage, current, power energy, frequency, power
factor, etc. The instruments considered in this book are analog devices in which the output or display is
a continuous-time signal and bears a fixed relationship to the input.
Analog instruments may be divided into three groups:
(a) Electromechanical instruments;
(b) Electronic instruments which are often constructed by the addition of electronic circuits to
electromechanical indicators thus increasing their sensitivity and input impedances; and
(c) Graphical instruments which are electromechanical and electronic instruments having a modified
display arrangement so that a graphical trace, that is, a display of instantaneous values against
time is obtained.
This chapter presents general concepts related to the working principles and construction and
certain features common to many electrical indicating instruments particularly of electromechanical
types. Important definitions relevant to instruments will be discussed first.