value changes, the moving system or pointer exhibits a smooth continuous motion. The
signals which vary in discrete steps and have only finite number of values in any given
range are digital signals and the associated devices are digital instruments. A digital
instrument has an output varying in discrete steps. An electronic counter is an example
of a digital instrument.
If the quantity is to be directly measured, then deflection methods are used. For e.g.,
ammeter, voltmeter, etc. acting as meters indicating the value of the measurand by the
deflection of a pointer over a graduated and calibrated scale. Alternatively, if the value
is measured based on the null balance conditions, then it is a null method. Null
methods are used only to detect the null condition of a measurand through a given path
or circuit. AC/DC Bridge measurements for measurement of resistance, inductance,
capacitance, frequency, etc. are null methods. They involve balance detection by using
null detectors, such as, Galvanometer, Vibration Galvanometers and Head Phones. Null
instruments are more accurate than the deflection instruments.
If the output of the instrument is entirely produced by the measurand, then it is an
active instrument. These are the power operated instruments requiring some source of
auxiliary power for their operation such as compressed air, electricity and hydraulic
supply. On the other hand, if the measurand modulates the magnitude of some external
power source, then it is a passive instrument. Passive instruments are self generating
instruments where the energy requirements are met entirely from the input signal.
The direct methods involve measuring the measurand by comparison against its own
standard. They are very common for measurement of physical quantities such as length,
mass and time. They are less sensitive and inaccurate since they involve human
operators. Thus direct methods are not usually preferred. On the other hand, indirect
methods use measuring systems, which are the systems having a transducer to convert
the measurand into its analogous form. This converted signal is processed, fed to the
end devices to obtain the results.
Absolute methods give the magnitude of the quantity under measurement in terms of
the physical constants of the instrument. They do not require calibration. They are used
only for calibration of other instruments. For e.g., Tangent Galvanometer, Rayleigh's
current balance and potentiometers. Secondary methods are so constructed that the
desired quantity is measured only by observing the output of the instrument, which
needs to be calibrated. Thus, they measure the quantity in terms of their deflection, for
which they are already calibrated. These are the ones which are the most commonly
used. For e.g., Voltmeters, Thermometer, Pressure gauge, etc. Secondary methods work
on either Analog mode or Digital mode and hence lead to analog or digital methods.
Contacting type instruments are those which are kept in the measuring medium itself.
For e.g., clinical thermometer. A non-contacting or proximity type instrument measures
the desired input even though it is not in close contact with the measuring medium. For
e.g., optical pyrometer measuring the temperature of a blast furnace, variable reluctance
tachometer measuring the speed of a rotating body, etc.
Mechanical instruments are very reliable for static conditions. Their parts are very
bulky, rigid and have a heavy mass. Hence they cannot respond rapidly to
measurements of dynamic and transient conditions. Besides, many of them are the
potential sources of noise. On the other hand, electrical instruments are very rapid in